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THE CEDAR INTERFEROMETRY PROGRAM
The GBOA sub-committee on Interferometry
John W. Meriwether, Jr., Chairman
Space Physics Research Laboratory
University of Michigan
Ann Arbor, Michigan 48105
Manfred A. Biondi, University of Pittsburgh
James H. Hecht, Aerospace Corporation
Craig A. Tepley, Arecibo Observatory
Roger W. Smith, University of Alaska
Fred L. Roesler, University of Wisconsin
Robert J. Sica, Utah State University
Gordon Shepherd, York University
IS, I9S6
Table of contents
1. Ground-based high resolution optical instriunentation: an overview
John W. Meriwether, Jr.
2. Coupling of energy and momentum in the thermosphere:
interferometric observations
Roger W. Smith
3. Collaboration of incoherent scatter radar and optical instrumentation
Craig A. Tepley
4. The WAMDn, Concept and preliminary results
Gordon Shepherd
5. Low resolution Fabry-Perot interferometer
James H. Hecht
6..Studies in equatorial dynamics
Manfred A. Biondi
7. Studies in geocoronal d3niamics
F.L Roesler
8. Improved Fabry-Perot interferometer- Design considerations
Manfred A. Biondi
9. The application of imaging detectors for Fabry-Perot spectroscopy
F.L. Roesler
10. Temperature,wind, and intensity analysis for an imaging Fabry-Perot
interferometer
Robert J. Sica
11. Implementation of CEDAR scientific objectives with
high resolution interferometry
John W. Meriwether, Jr.
1. Ground-based high resolution optical instrumentation: an overview
1.1 Introduction
Many examples exist in aeronomy that portray how dynamics plays an important or
even crucial role in atmospheric processes. Energy deposited in the upper atmosphere
through photo-ionization, plasma dynamics, and particle precipitation is redistributed
throughout the global system by transport. These pathways are reasonably well
understood for the thermosphere. Work remains, however, to cl£u*ify our understanding
further for the other atmospheric regions of the lower thermosphere, mesosphere, and the
geocorona. Furthermore, the need to include the dynamical coupling between atmospheric
regions in the analysis and modelling of observations has made current studies of upper
atmosphere dynamics considerably more complex. This is particularly true for the case of
the mesosphere. The temperature profile through the mesosphere is closely coupled to the
deposition of energy into the mesosphere by gravity waves that are dissipated within this
region. In addition, thermal tides, planetary waves, and joule heating (at high latitudes)
within the lower thermosphere all take part in the dynamical activity of the mesosphere.
For ground-based observations, high resolution doppler instrumentation is currently
restricted to the detection of the source motions averaged over the emission height profile.
Consequently, optical interferometry makes its most important contribution to aeronomy
studies through the global mapping of horizontal motions. This is especially true for those
areas where boundaries exist, whether between the polar cap and the auroral zone, or
between the equatorial and mid-latitude regions. However, the passage of the Earth's
shadow through atmospheric regions in twilight provides a possible but unexplored means
of determining the height profile of atmospheric dynamics. Little work has been devoted to
this question, largely because improved optical instrumentation with greater throughput
and better background discrimination is needed for the proper exploitation of the twilight
for dynamical studies.
The current state of detector technology determines how well certain dynamical
studies may be achieved. In some cases we may be able to detect the emission feature,
i.e., make a discovery, but the sensitivity needed to exploit the discovery properly may be
lacking. Progress in the last decade would have been considerably retarded had it not been
for the introduction of the GaAs photomultiplier increasing the sensitivity of the Fabry
Perot interferometer by factors of five or better for emission wavelengths above 600.0 nm.
The interferometry committee debated extensively how major increases
in
sensitivity may be attained. Solid state detectors have reached the state of development
where factors of 20 to 50 increase in signal counts may be feasible. An alternative
possibility lies in the application of optical design to collect more efficiently the light
existing in the outer rings, i.e., field widening. This was the concept proposed by
Hernandez[1982], but application proved to be demanding.
This section will summarize the application of high resolution interferometry,
provide examples of specific emissions where detector enhancements would greatly
improved the scope of possible exploitation, and then review the specific CEDAR scientific
goals that would be addressed through application of improved high resolution spectral
instrimientation. The other papers will extend the review of the CEDAR scientific
objectives in reference to the three atmosphere regions and also consider instrumental
improvements necessary to achieve these goals.
1.2 Fundamental aspects of high resolution interferometry
The application of the Fabry-Perot interferometer to the study of a specific spectral
line emission will determine the velocity distribution of emitting atoms as integrated along
the instrumental line of sight. For the case of thermalized distributions, the average
motion of the medium (doppler shift with respect to an imshifted reference), may be
inferred. We may also determine the doppler width (temperature) and the total population
of emitting atoms within the envelope of the spectral profile, i.e., the intensity. These
concepts are portrayed in Fig. 1.1 (taken from Hernandez and Roble, 1979).
For non-thermal populations, the interpretation of the spectral profile becomes more
difficult. The physics underlying the d3mamics of the emitting region has to be understood
and well modelled before the characteristics of the spectral profile can be utilized. This is
especially true for the case of intensity ratios of oxygen multiplets where the emissions
originate from optically thick parent states (see Section by Hecht in this report).
Application of the interferometer to atmospheric emissions, especially for metastable
molecular emissions, can provide important information regarding spectroscopic constants
that may be more precise than laboratory measurements can achieve due to wall
quenching collisions inherent in laboratory work. For molecular emissions, the FabryPerot interferometer can determine the rotational temperature from the measured
intensity ratios with great precision. The point here in comparison with other optical
instrumentation is the larger instrumental throughput capability of the Fabry-Perot
interferometer as compared with other spectroscopic instrumentation, i.e., the so-called
Jacquinot advantage.
1.3 Examples of interferometry achievements and failings
This section will provide an indication of possible achievements given improved
instrumental sensitivity. We will show cases where the potential rewards show promise of
being especially exciting and interesting.
The first example in Fig. 1.2 is the detection of 630.0 nm doppler shift in aurora
(from Hays and Roble, 1969]. The quality of 630.0 nm signal produced by the aurora is
evident. This discovery has been exploited by many investigators. Fig. 1.3 taken from
Hernandez and Roble [1984] shows the seasonal variations of thermospheric winds
throughout the year for solar activity at solar maximum and solar minimum. The
difference between the two is an indication of the importance of the coupling between the
auroral region and mid-latitudes for solar maximum. Improved instrumentation would
produce data of the quality shown in Fig. 1.2 for airglow signal levels.
Another example of discovery followed by exploitation is the 732.0 nm spectral
emission relating to O"'". Fig. 1.4 shows ion drift measurements made with the FabryPerot interferometer in the auroral zone for a period of unusually soft electron precipitation
(Meriwether et al., 1974). The introduction of the GaAs photomultiplier, which increased
the detector sensitivity a factor between 4 and 5, and the location of the instrimient at the
polar cusp where high electron temperatures and intense flux of soft electrons exist, made
this emission more useful for studies concerning the coupling between the plasma and the
neutral atmosphere. Section 2 discusses this issue concerning coupling in greater detail for
high latitude locations.
We turn now to the spectral emissions showing promise of application of important
scientific achievements but currently falling short of expectations due to the poor signal-tonoise ratios. An example of great interest today is the measurement of mesospheric
winds. Hernandez and Smith [1984] demonstrated in Fig. 1.5 that the Pj^(2) line of the 8-3
band of OH can be detected with sufficient precision for doppler shift measurements
provided that the instrument is sufficiently stable over the several hours of integration
needed to achieve spectra of this quality. Estimates suggested that major improvements
can be made by using OH emissions from a brighter OH vibrational band located further
to the red. Improved detectors sensitive to these near-infrared photons would open a new
area of atmospheric ground-based dynamical investigations.
Another example is the case of 620.0 nm emissions of atomic nitrogen emitted by
2
the N( D) atoms. Integration lasting one hour was needed to obtain the results shov/n in
Fig. 1.6. Observations at the higher resolution needed for thermal width and doppler
position determinations would require longer integration times. These nitrogen emissions
would be especially useful in providing F-region determinations of the kinetic temperature
and neutral winds at a slightly higher height than that corresponding to the oxygen line at
630.0 nm. Studies relating to F-region wind shear and perturbations of the thermospheric
temperature profile would be possible.
1.4 Summary of present FPI facilities
Fig. 1.7 simimarizes the present geographical distribution of FPI facilities that
currently exists (excluding two possible sites in the Soviet Union). Table 1.1 provides
further details concerning the distribution. Almost all stations operate fully automatically
during nighttime. Most stations are utilizing the atomic oxygen line at 630.0 nm for
thermospheric djmamics studies. The low spectral resolution instrument employed by
Hecht and Christiansen currently operates in campaign efforts only. Unfortunately, the
FPI station at Fritz Peak has been dismantled.
Practically all stations use the GaAs photomultiplier for detection.
Almost all
stations use etalon plates 10 or 15 cm in diameter. Piezoelectric and pressure scanning
techniques are commonly used for acquisition of the spectral profile, although several
stations use the imaging detector in place of these methods. Most stations are operated by
a mini-computer and run automatically. The Calgary facility can be reached by telephone
for interrogation as to data quality; a similar capability is plsinned for the Millstone Hill
facility. Because the optics in these instruments are very similar in size, and also because
of the widespread use of GaAs photomultipliers, the instrumental sensitivity is generally
about several c/s/R. The use of the multiple exit plate by Biondi at Laurel Ridge makes
that instrument sensitivity greater than any other by almost a factor of 5.
1.5 Airglow/auroral emissions lines commonly used for FPI studies
The committee discussed at length various source emissions that high resolution
instrimientation could observe in airglow or auroral emissions given improved sensitivity.
There is a large number of E-region and F-region emissions that is readily accessible from
the ground. These are listed in Table 1.2. Many other lines and molecular bands exist that
may be studied as well, but lack of adequate sensitivity has prohibited the undertaking of
this work. The committee was of unanimous opinion that a spectral atlas of much
improved quality as compared with the present atlas of Broadfoot and Kendall [1968] was
of considerable importance as an aid to improved future planning of new FPI studies.
Consideration should be given in future planning to the application of high resolution
instrimientation to other optical emissions lying either further to the blue or to the red end
of the visible spectrum Shiftmg to these spectral areas has the advantage that the
emissions from other atmospheric minor constituents become accessible providing further
opportumties for the exploration of the dynamics of the three atmospheric regions defined
by the CEDAR objectives.
1.6 Discussion of CEDAR objectives
CEDAR objective I
Mesosphere-Thermosphere
80 km < H < 300 km
The steering committee defmed three areas of concentration for this topic: A) quiet
conditions, B) active conditions, and C) small scale structures. Table 1.3 presents the
interferometry goals for these areas. The set of spectral features listed in Table 1.2
contains the major emissions that may be applied for specific studies relating to this
CEDAR objective.
Typically, the measurement times specified are those needed to achieve data
acquisition rates compatible with the rate of change of the underlying phenomenon,
whether twilight, aurora, nightglow, or dayglow. The error bars for measurements were
left unspecified but are taken to be that associated with high quality observations:
typically, doppler shift and temperature errors of 10 m/s and 30 K, respectively. In
twilight it is presumed the primary goal is to follow the change in the spectral profile
observed as a function of the shadow height for several directions. At other times,
although the observation time may be extended a bit, many more directions will be needed
to define the velocity and temperature fields to the level of precision wanted.
More directions will be needed at high latitudes, where small scale structures in both
temperature and velocity fields are known to exist. A reasonable goal would be a total of 9
directions for nightglow emissions and three times as many for auroral emissions. In the
case of small scale structures, imaging systems such as those based upon the Michelson
interferometer design (for example, the WAMDII) or the scanning Fabry-Perot
interferometer fitted with an area detector would define more suitably the velocity and
temperature fields with good precision. A bistatic observatory is required to determine the
components of the neutral wind vector within a common volume; it offers the best means
for gaining high temporal resolution of the neutral wind vector without any assumption
made concerning spatial gradients or temporal variations.
CEDAR objective n
Upper thermosphere-Exosphere/Plasmasphere H>300km
The second CEDAR objective concerns the geocoronal region, which is largely
populated by hydrogen and helium atoms with an occasional contribution by hot oxygen
atoms. The details of the CEDAR program are listed in Table 1.4. The central element is
the measurement of the atom's detailed velocity distribution by means of the high
resolution Fabry-Perot interferometer. Major emphasis must be placed upon the acquisition
of spectral profiles with high signal-to-noise ratios. The desired temporal resolution should
be of order of 16 minutes, so that short term variations in the velocity distribution may be
followed closely.
Although the simplest measurement is in the zenith direction, it is also desirable to
look in various other directions as well. The critical test of existing theory in comparison
with these results is made with the comparison of predicted spectral profiles with
measurements for various directions. It will be especially desirable to combine the
observations of the spectral emissions of H and He in a study. This would allow the
composition of the geocorona as a function of shadow height be explored.
Other issues are the relative importance of Jeans escape versus charge exchange,
the fraction of the geocoronal population existing in satellite states, and the major question
of redistribution of hydrogen and helium atoms through coupling to the thermosphere and
via exospheric transport. Major improvements in instrumental sensitivity are needed for
these studies to be possible.
10
CEDAR objective III
Stratosphere-Mesosphere 30 km < H < 100 km
Interferometric goals for this CEDAR objective listed in Table 1.5 are primarily the
measurements of mesospheric winds and temperatures via airglow emissions of OH and
Og during nightglow, twilight, and dayglow. The measurement times should be short to
allow the acquisition of temperature and velocity field maps with excellent detail and good
temporal resolution. The complex nature of the dynamical processes in play within this
region requires that measurements of the winds and temperatures be complemented with
other data that extend the physical picture of the various processes. Possible auxiliary
observations are imaging of the OH or 01 intensities, vertical height profiles attained by
lidar or radar studies, and rocket experiments. Long term sequences of observations are
needed to assess seasonal and biennial variations.
11
Applied
Optics
So-
15 OCTOBER
1979
So-
Emitting
Loyer
Fritz Peak
Fig. 1.1
ATMOSPHERIC SPECTROSCOPY
Hatb and Eobld
INTEGRATION PERIOD « 2.0 SEC.
6300 A
H KojOiisa
g 400
111
2.0
PRESSURE(PSI)
Fig. 1.2 The X6300 fringe profiles obtained with a Fabi-y-Perot interferometer from 0240 to
0325 LT on May 14-15, 1969. Curve 1 shows the fringe profiles measured at a zenith angle of
70*N in the meridian plane of the Airglow Observatoty and curve 2 shows the fringe profiles
measured at a cenith angle of 70*8.
Hoi.N/kNDCZ AND Roau: iKEXXBrKDuc Winds
LT UT
T
r—r
SoU/ Minimum
SOm/»
itlll
00 07
2D 03
IS 01
J
J
Month
Fig 13aWind vector varUlton during the year over Friu Peak Observatory for wlar cycle miniiniun determined b>
avenging the zonal wind component measured east and west of Frii2 Peak Observatory and the mehdionaJ wind
componenu measured oortb and south of Friu Peak Observatory. The direaion is upward to the north, routine
dock wise to the east,south, and westdireaions, rcspeaively.
LT m
SOm's
Sotar Maximum (N)
Oi
11
02
09
00
07
2D 03
ie
01
Month
FigJ.3bWind veoor variation during the year over Fritz Peak Observatory for sotar cycle maximum determined b>
combining the wind components in the north and the avenged easi and west direaion from the sution The dircnion u
upward to the oonh. routing dock wise to the east, south, and westdirections,rcspectivdy.
RESE>U^CH NOTES
Mlchlgon drglow obstrvohry
E«t»r Oomt, Alotko
12 Fabnjory 1972 (AST)
Doppltr profll* of X73I9 0*
100m/»*c
i
©o-
T-0115 (AST)
60 L
rt'A-Otmct
T-0125 (AST)
4rw
rT-4-0 ••c
20-
l-O
2-0
PratHira,
s-o
4-0
psi
Fio.l^ Dorrtu. rKonus or On 7319 A CMmioNs obsuvtd dumno thz hioht or 12
pEBRUAxy 1972 ATEjtck Dome. AUksxA.
•The twoprofiJes were obtaioed by looking 45'Eajid 45*W, respcetivdy.
13664 40
Ar line
•Q (1)
Alternated by fi
^13663.27,
Xe-Aa
1*110
l»tIO
|«2J0
•9240
SrccTXUxi O/ TOJ S2CWK cnvTRCP »r -rot rM0iirLur<4o ntru uuvim wm
IMVUnSATIOH.
c-Acr
04
03
02
(cr - 13664) K
Speccra of Xenon (top) and OH X 'li (8-3)
Pj(2) lines (bnttom).
The frequency scale is in
SncnuM or ««
»toiOH *j F;o. i Ara Kiski* su- .ti'.'
Noieihe evedappinc or o.-d^.
dicated relative to the left side 1366A.4 Xe line.
Also, multiple orders are indicated.
Fig. 1.5
Fig. 1.6
IWETHER
SVAL0ARD
THULE
KIRUNA
SONDRESTROM
-ISASKATOOrf
CALGARY
MADISON
^r,
MILLSTONE HILL
FRITZ PEAK^
f
MT. ZAO
LAUREL RIDGE
ANN ARBOR-^
ARECIBO
AREQUIPA
SAO JOS^
MT. TORRENS
SA
BEVERI
VICTORIA
•/r
-LEY
AWSON
BAY
20
40
60
80
100
120
140
160
180
F P I
Polar cap stations: '
FACILITIES
Thule
Spitsbergen (2 stations)
Auroral zone stations
Sondre Stromfjord
College
Mawson, Antarctic (Australia)
Saskatoon (Canada)
Trixie Bay (Soviet Union)
Kiruna (Sweden)
Mid-latitude stations
Ann Arbor
Millstone Hill
Madison
Australia (2 stations; Dyson and Jacka)
Laurel Ridge
Calgary
Halley Bay, Antartica (U. K.)
Low-lat./equatorial;
Arecibo, P. R.
Arequipa, Peru
Sao Paulo (Brazil)
Dayglow instrumentation
Madison
Ann Arbor
Fritz Peak (Burnett, OH)
Portable FPI:
Millstone
Thule
Table 1.1
BASIC REPERTOIRE OF EMISSION LINES FOR FPI STUDIES
250-300 km aur./airglow
Tn' Vn
5 -
N
275-325
Tn' Vn
0.3 -
10 R
O-
250-300 km auroral zone/
Ti' Vi
0.5 -
100 R twilight
1.
630.0
nm O
2.
520.0
3.
732.0
km
1
-
200 R nightglow
10 R weak auora
100 R cleft/aurora
4.
557.7
97 km In nlghtglow
T
^n',
V
n
many kR in aurora,
250 R airglow
5.
OH
85
6.
Na
91 km
10 -
km
V
n
100 -
10 7.
8.
9.
10.
777.4
250-300 km equatorial
100 R
2000 R twilight
25 R nightglow
Ti, Vi
molecular bands of 02* N2/ N2+
atomic multiplet lines, 7990 of 01 and others
composition
metastable lines of N"*" and O"*"*"
(speculative)
T., V,
Table 1.2
P branch
CEDAR Topic I:
A) Quiet
Wesosphere-Thermosphere
80 km > h < 300 km
1. T^(h), T^(h), V^(h) possible in twilight, but concept remains
conditions;
^
.
to be confirmed.
Stable solar
Possible emission lines:
and magnetic
activity
i.
Molecular emission of N2+ and 0^
ii.
Basic repertoire
i i i . Ca"*"
Measurement time:
5-50 seconds
Intensity range;
1 - 400 Rayleighs, typically
2. Nightglow measurements define dynamic state of atmosphere
for centroid height of emission layer
Basic set of lines except Na
Measurement times:
1-10 minutes
Intensity range;
0.3 - 400 R
for brighter features, want more direc
tions for map definition of velocity field.
3.
Dayglow emission lines (same as nightglow with addition of
molecular emissions; signal enhanced by 2 - 3 orders of
magnitude; background enhanced by 6 - 9 orders of magni
tude) .
Very little work done in this area; mountain
station needed.
B)
Short term changes
Same as A1 and A2 but measurement time
- Major geomagnetic storms
available diminshed by need for much
- Substorms
improved doppler map quality.
-
(AE)
IMF
- Waves
C)
Small scale structures
Require bistatic observatory and upgraded
detector systems; very suitable for
imaging doppler system like WAMDII.
Table 1.3
CEDAR Objective II:
A)
Upper Thermosphere-Exosphere/Plasmasphere
1.
Global variations
of H,
He,
and hot 0
H>300 km
Measure spectral profiles of H, He» and
o''" emissions in twilight at various
places with FPI.
B)
Exospheric particle
Same as
above.
velocity disturbances
C)
Coupling with plasmasphere/
Magnetosphere
2.
Emissions excited by SAR arc and ring
current particles to be examined with
FPI.
Time resolution should be 15 minutes or less.
Twilight/nightglow, quiet aurora and polar cap, dayglow
sequence of priorities.
Quality of spectrum should be high as the details of line
shape represent principal aim of this objective.
Bare CCD detector and multiple etalons needed for the
1.04 V He emission.
CEDAR Objective III:
A)
Gravity wave,
Stratosphere-Mesosphere
1.
Measurements of mesosphere winds
eddy diffusion,
and temperatures with OH and OI/O2
transport, PCA
emissions in twilight and nightglow.
and rel.
Should be carried out as a part of
elec.,
LT variations.
campaign involving a cluster of
instruments.
B)
30 km < h < 100 km
Modelling and
related emission
measurements.
Table 1.3 Continued
2. Coupling of energy and momentum in ^
thermosphere:
Interferometric observations
(R. W. Smith, U. of Alaska)
2.1 Summary of coupling mechanisms
Coupling means the transfer of heat or momentum between different locations in the
thermosphere or between different components of the fluid. The t3rpes of physical coupling
may be identified by inspection of the relevant energy and momentum equations for the
components of the fluid. The momentum equation (Rishbeth and Garriot, 1969) shows
terms involving advection, viscosity, ion drag, and tides which may do work on the local
fluid particle and therefore transfer momentum.
Although there may be many
mechanisms at work, only those which are major effects are likely to be discernable from
ground-based measurements.
Advection can be considered as the coupling of stream momentum from some
upstream region. Here, work done at some distant point in the path of the fluid parcel
becomes evident locally due to the requirements of continuity.
In comparison, the viscous interaction can transfer momentum perpendicular to the
direction of flow. This may occur horizontally or vertically. The most important effect of
viscosity is in the vertical direction above 200 km altitude (Rishbeth, 1972; Killeen,
1985c). The viscous drag depends upon the scale length for velocity change. This may be
short in the horizontal direction under extreme disturbance at high altitudes. In such
cases (eg. convection chemnel ion drag) the viscous coupUng of momentum may be
substantial.
Tidal and gravity wave interactions are transitory unless the thermosphere is lossy.
Breaking waves at the height of the mesopause deposit their energy and heat the lower
boimdary of the thermosphere.
13
A very important and significant source of coupling originates in the interaction
between ions and neutrals. At high latitudes, this mechanism introduces energy from the
solar wind into the thermosphere via a complex route. The interplanetary plasma acts as
a power source generatmg a large potential difference between the dawn and the dusk
flanks of the magnetospheric tail. This is conducted to the polar cap ionosphere along
equipotential geomagnetic field lines. The resulting ionospheric electric field sets up ion
motion which is coupled to the neutral thermosphere through ion drag. Hence, ultimately
this mechanism depends upon the interaction of the solar wind with the boundary of the
magnetosphere. More immediately, the couplmg reveals the large scale features of ion
convection caused by the ExB drift.
Due to changes in the latitudinal extent of the polar
cap (the boundary of the polar cap is considered here to be delineated at the convection
reversal), this kind of coupling has profound effects on a global scale. The magnitude of
heat and momentimi deposited at high latitudes composes a considerable fraction of the
global thermospheric energy budget. Variations in the energy input at height latitudes and
the penetration of the magnetospheric electric field to lower latitudes cause substantial
modifications to the circulation in the midlatitudinal region (Hernandez and Roble, 1976).
The ion energy equation (Banks, 1981) can be used to identify coupling mechanisms
in which the neutral gas is heated. The frictional heating term applies to the thermal
energy generated as a result of the same drag process which was considered above as a
source of stream momentiun. The ions share the random kinetic energy equally with the
neutrals but rise in temperature more rapidly because oftheir much smaller density. Also
the ion temperature may be elevated due to the thermal transfer from the hot electron gas
which results from auroral precipitation. Thermal coupling may be considered in terms of
the difference in temperature between the ions and neutrals.
Coupling also occurs between height regimes by vertical winds. A steady upward
wind, possibly generated by local Joule heating in the lower thermosphere has the effectof
populating the F region of the ionosphere with molecules and thereby increasing the
14
recombination rate and adjusting the chemical balance. At higher altitudes, the outflow of
hydrogen and helium has a substantial impact on the physical structure of the exosphere
and the upper thermosphere.
Downward winds modify composition in the lower
thermosphere by moving air parcels of the thermosphere into the lower thermosphere and
by the resulting modifications of reactions rates caused by the higher temperature of these
air parcels. A summary of the coupling considered in this section is given in Table 2.1.
Each type of coupling is listed by row and various facets are identified by colimins.
2.2 Ground-based observations
Interferometric observations of an exploratory or synoptic type have been in
progress for many years (see, for example, the review by Meriwether, 1983). Studies of
coupling imply at least two measurements at selected points in space or referring to
different components of the fluid. This may involve either different wavelengths and/or
different instruments. Hence, it is necessary to establish comparability of calibration and
operating modes. Although temperature measurements, being scalar, require only one
instrument or one direction of view for a complete measurement, wind determinations
ideally require three independant observations to obtain the complete vector. Often the
vertical component is presumed small and a horizontal component is derived from two
measurements only (Smith, 1981, Tepley et al., 1984).
2.3 Existing coupling studies
The best studies of ion-neutral interactions have come from the in-situ and remote
observations from specially instnmiented satellites such as AEC and DE-2 (see, for
example Nagy et al, 1974; St. Maurice and Hanson, 1982; and Killeen et al., 1985b). In
the present dearth of a suitably instnmiented satellite, and in the context of GBOA, the
continuation of this work must be carried out from the ground. Investigations of the ionneutral interaction are in progress in Svalbard (Smith et al., 1982, McCormac and Smith,
15
1984, Smith, 1985) and have begun at other high latitude stations (Ponthieu et al., 1985;
Shih et al., 1985). The close correlation of ion and neutral directions of motions is shown
in Fig. 2.1 which represents measurements in the cusp from Svalbard. Fig. 2.2 shows
that the optical doppler technique may be applied to the studies of ion frictional heating.
The main restriction on progress is lack of sensitivity and therefore larger error bars
and inadequate time resolution. Application of the program to upgrade the sensitivity of
instrimients through the GBOA initiative will result in much better returns from this
work. Temporal and spatial resolution in the data will then represent the physical
situation more adequately.
Many investigators have collaborated to compile a northern hemisphere (middle and
high latitude) average thermosphere weather map shown in Fig. 2.3 for December 1981
(Killeen et al., 1985) showing the potential for investigating the large scale effects of
coupling through the application of the TCGM. The average effects of the separation of
geomagnetic and geographic poles, and the penetration of magnetospheric electric fields
down to the middle latitudes may then be deduced.
A strong disturbance in the wind field, such as that produced by a narrow convection
channel (St. Maurice and Schunk, 1981) will cause local increases in the horizontal viscous
coupling. Fig. 2.4 illustrates the horizontal coupling of momentum at high altitudes which
acts to couple the neutral momentum in the convection channel to adjacent regions. A
single station FPI may attempt to observe such features following the analysis of Tepley et
al.(1984), an example of which is shown in Fig. 2.5.
Thermospheric vertical winds in excess of 20 m/s have been found to be remarkably
commonplace (Hernandez, 1982; Meriwether, 1983; Rees et al., 1984a; Biondi, 1984;
Herrero et al.,1984; Biondi and Sipler, 1985). Although strong vertical components only
come out of large scale models for severe disturbances (Rees et al., 1984b), it is now
accepted that small scale vertical winds do exist and that their effect must be substantial
in the thermodynamics and chemistry of the thermosphere. Herrero et al.(1984) examined
16
the issue of the production of these vertical wind events by auroral heating sources and
concluded that the observations were consistent with a model for the propagation of a
gravity wave from a source located in the lower thermosphere.
2.4 Future coupling studies
Studies of coupling in the CEDAR context can be carried out as set out in Table
n. Existing FPI observatories located at high latitudes and operated at a fixed wavelength
(either 630.0 nm or 557.7 nm) are suitable for certain coupling studies. These are
restricted to a limited latitudinal and longitudinal description of the vector wind field and
temperature field (Tepley et al., 1984). At high latitudes, there are doubts about the height
of emission of the sources at each of the wavelengths. These introduce uncertainties which
are particularly worrying for temperature gradients and for winds when the source
wavelength is at 557.7 nm. Long tenm averaged data should demonstrate genered spatial
trends in temperature. On the other hand, 557.7 nm data is always difficult to interpret
when auroral contamination is present.
Clustering of instruments working at the same wavelength provides extra
information which can be used to better define the spatial distribution of wind vectors over
an extended region. Also, by bistatic operation, some uncertainty can be removed from the
derived wind vectors in the zone between the stations since both (or all) observations used
to compose the vector are measured at the same point. Advected momentum may be
deduced if several measurements of the wind are made along and perpendicular to the
stream direction. The passage of waves (but not their energy deposition) may also be
identified with greater certainty because of the removal of the spatial/temporal ambiguity.
The use of a fast (Class 1) instrument with a filter changer, or the colocation of two
or more single wavelength instrument opens up many other possibilities. Height resolution
may be obtained and also ion-neutral interactions may be investigated. Tables I and n
show that the potential for achievement of the CEDAR objectives is greatly increased for
17
this configuration of histruments. The source intensities which could be used in such work
are much lower than the 630.0 nm and 557.7 nm emissions now used. Also measurements
must be made more rapidly than at present. A very substantial improvement in sensitivity
is essential for a realistic exploitation of these research opportimities.
18
tSCu
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Fig. 2.1
FRICTIQNAI HEATING PLOT
D28NOV19&A
5
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APPROXIMATION
800
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CUSP AND
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3 ago
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Pig • 2.5 The wind field atCalgary on 30-31 March 1981
DEDUCED BY USING (a) THE 'COMBINED METHOD" AND (b) THE
"Calgary method".
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2.2
v/
3. Collaborative studies involving incoherent
scatter radar and interferometry
(Craig A. Tepley, Arecibo Observatory)
The simultaneous use of incoherent scatter radar and optical interferometer
instrimientation forms a powerful tool to study the ionosphere. Unique measurements can
be made from the mesosphere through the exosphere. However, the atmospheric region
that has received the most attention has been the thermosphere, particularly in the area of
dynamics.
An experiment performed by Cogger and Carlson [1977] exemplifies the multi-
instrument study of aeronomic problems. This work attempted to measure the neutral
concentration of the lower thermosphere through the combination of the Fabry-Perot
interferometer and incoherent scatter radar. In this altitude region, the effect that the
neutral density and temperature have on the radar spectral shape cannot be independently
determined.
Optical measurements of the width of the 557.7 nm line with an
interferometer, provided an independent estimate of the neutral temperature.
Such
measurements are shown in Fig. 3.1, where the bottom panel of the figure displays the
variation as a function of time. The temperatures are then used to extract the neutral
densities from the collistion frequencies that were previously determined from the radai'
spectra. In this manner, these two techniques of studying the temporal variation of the
neutral atmosphere near 100 km altitude complement each other.
Lineshape studies of the Ha emission from the hydrogen geocorona were made by
Kerr et al. [1985] at Arecibo. They used the incoherent scatter radar to measure the
temperature of the exobase, while the Ha line was observed with an interferometer. The
width of the line provides an effective temperature, and an escape flux of hydrogen is
indicated by a "cooling" or narrowing of the lineshape. The escaping flux should increase
19
with an incease in the temperature of the exobase. Fig. 3.2 compares the temperature
determined from the two methods.
Kerr and his coworkers also found evidence for a
satellite population of hydrogen as well as for escaping hydrogen.
Table 3.1 shows some basic geophysical parameters that can be obtained from radar
and interferometer studies of the thermosphere. This list is by no means exhaustive, as
other data c£ui be derived from these, such as conductivities and gradients in the
temperature and wind fields. An example of the comparison of the overhead vector wind
field is shown in Fig. 3.3 for both ionized and neutral atmosphere. The radar measures
directly the plasma motion while the interferometer determines the neutral wind from the
Doppler shift of the 630 nm emission originating in the thermosphere. Even though the
two seasonal patterns are significantly different, good agreement between the motions of
the plasma and neutral atmosphere is observed for the simmier and winter months. This
result mdicates the coupling between the F-region plasma and the thermosphere is usually
strong with a consequent development of a polarization electric field that is effective when
the coupling to the E region is weak.
Although optical and radar wind measurements have been made regularly at
Arecibo since 1980, comparisons of this type were made at Chatanika as early as a solar
cycle before (Nagy et al., 1974). Fig. 3.4 compares the average meridional component of
the neutral wind obtained by Wickwar et al. [1984], while individual experiments from
their work are illustrated in Fig. 3.5.
Incoherent scatter radar measurements are
sensitive to the neutrsil meridional flow provided that an accurate value for the plasma
diffusion velocity can be estimated. Finally, a direct comparison of the variation of the
vertical component of the neutral wind, shown in Fig. 3.6, illustrates the effects of gravity
waves on the measured data.
The work performed by Bumside et al. [1983] illustrates the effect of both the local
and the conjugate point polarization electric fields on the coupling between the neutral and
ionized atmosphere. They determined that the local E-region has little effect on the
20
nighttime electrodjmamics of the
F-region when the
height-integrated Pederson
conductivity of the F-region significantly exceeds that of the E-region.
Not only do the effects of the local E-region play a role in the plasma drifts, but it is
also important to consider how the ionosphere at the conjugate point influences these
drifts. The zonal ion and neutral velocities shown in Fig. 3.7a agree, for the most part.
However, for the winter data there is a significant departure of the ions from the variation
of the neutrals. This departure coincides remarkably well with the time of the F-region
sunrise in the conjugate hemisphere. Burnside et al. determined that the local ion velocity
is a function of both the local and conjugate point neutral winds, scaled by their
representive conductivities. This argument is based on the reasonable assumption that the
magnetic field line linking the two conjugate stations is an equipotential, as illustrated in
Fig. 3.8.
To fully understand measurements of this type would require the placement of
Fabry-Perot interferometers and radars, or simple ionosondes at the conjugate point. This
will enable us to determine the state of the neutral atmosphere and the plasma at stations
that are conjugate in the northern and southern hemisphere. Additionally, complementary
radar and optical instrumentation can be best utilized when formed into zonal or
meridional chains. In this manner, the conditions of the ionosphere, and other regions of
the atmosphere, can be studied on a global basis.
21
0130 mi 5.1973
t
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SPECTRAL CHANNEL
Observed and synthesized 5577A line profiles. Tht
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l-nch xpectrul channel is 6.94 x 10-» cm"' (2.16 x 10 » A
wide.
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Fig. 3.1 01 3577A Doppler temperatures for June 3/4 and June
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3000
JANUARY 6-7,1904
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Fig. 3.2
Two examples of'the eemldlumal variation of the effective
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zeros, with 2 standard deviation error bars. Exospherlc temperature
calculated by the MSIS mddel Is shown by a dashed line. loA temperatures
are also displayed for A50 km (•) and 530 km (X) altitude.
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Fig. 3.3 CocparlsoB of the *verage rector vlnd r*rl*tloo
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vlnter and Busjcr conditions. Also shovn In the lover
panels la the ootloo of the peak of the F-layer.
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Fig. 3./i Comparison of mean meridional winds from the radar data
and from ihc Fabry-Pcrol inlcrfcromctcr.
CHATANIKA
t march I«I3
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Fig. 3.5
Compnrison of simultaneous deierminaiions of ihc nicridt-
onal wind with Ihc radar nnd Fabry-Pcroi. A sniooih average cur\c
uoo
VIMC. UT
Fig. 3.6 Vcrlical wltul mr.tiurentcnii obtjm«J on Marclt I. I97.\ by limtiliaiieout Fabrx-Perol tnd raJjr
obtervoiiont li>c:U<«l a( Filer Dome imlCiinlnnlkn, re^pecilvcly, in Al:i*l;a [from ll'/rlHor *1 u/, I9S J.
from ihc curves in Figure ^ is included with c.ich sei of measurcmcnis
for reference.
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Fig. 3.8 Schematic diagram used to Illustrate the coupling of Areclbo
and its conjugate point near Argentina along a cocsoon tLagnetlc field
line. As stated In the text. If the field line can be assuaed to be at
i conjugate neutral
driftvlnd
Masured
at Areclbo Is influenced
by both local and
variations.
Parameters that may be measured o r
from incoherent Scatter (radar) oo r
Optical (FPI) Observations
,
electron density
ion temperature
electron temperature
neutral temperature
meridional ion velocity
zonal ion velocity
meridional neutral velocity
zonal neutral velocity
vertical
N
radar
radar
"^i
radar
radar
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radar
radar
Tn
MSIS
FPI
radar
radart
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radar
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horizontal
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FPI
diffusion velocity,electron
which may
be derived from the
density profile and the
Table 3.1
4. WAJVIDII Concept and Preliminary Results
(Gordon Shepherd, York University)
4.1 Introduction
The purpose of this section is to give a brief description of the WAMDII concept, and
how its capabilities and characteristics differ from those of the Fabry-Perot spectrometer.
The two approaches are complimentary, and taken together, provide a powerful means of
applying the optical Doppler method. For some applications it may be useful to combine
both optical elements in tandem in a single system.
WAMDII (Wide Angle Michelson Doppler Imaging Interferometer) is an instrimient
scheduled for flight as an attached payload on the space shuttle in 1989. A developmental
model has been completed, tested in the laboratory and taken through thermal, vacuum
and vibration tests. In February, 1984, field observations of aurora were made and
reported upon (Shepherd, et al., 1984). However the published results are now known to
be influenced by intensity variations of the auroral source, and so require re-interpretation.
Much more extensive observations were made in February through April, 1985, using an
improved CCD that permitted the observation of airglow, and an all-sky fore-optics system
that yielded velocity images of the whole sky. Some preliminary results of these
measurements will be reported here.
/
WAMDII is a variant of WINDII that is now part of the UARS (Upper Atmosphere
Research SateUite Mission), to be launched in 1989. This WINDI Imaging Interferometer
is a combination of the original WINTERS (Wind and Temperature by Remote Sensmg)
UARS experiment of G. Thuillier, CNRS, France and the WAMDII instrument. Its
differences from WAMDII are imposed by the limitations of a long-duration free-flying
satellite, and by the addition of two orthogonal fields of view which generate two images,
side-by-side on the same CCD. For the remainder of this presentation, we shall refer only
to WAMDH.
23
4.2 Background
The output of a Michelson interferometer is called an interferogram; it is a recording
of signal as a function of optical path difference. Scanning Michelson interferometers
(Fourier spectrometers) make measurements from zero path difference up to some
maximimi value, and the Foiuier transform yields the spectrum, at a resolution
determined by that maximum value. Commercial scanning instruments are available (e.g.
from Bomen Inc.) with path differences up to one meter giving a resolution of 0.005 cm-1.
However, such instruments are used almost exculsively in the infrared because it is here
that the multiplex advantage is most clearly evident. However, they do work well in the
visible region also (Gault et al., 1981).
In the visible region one can take advantage of an additional capability of the
Michelson, and that is field-widening. Scanning field-widened instruments are notoriously
difBcult to build, but the ones developed at the Utah State University (Baker et al., 1981)
clearly demonstrate the power of the method as applied to the near infrared region. The
complexities of these instruments increases very rapidly with increasing path difference
and so the application has been limited to spectroscopy, at resolutions falling short of that
required for optical Doppler work.
For a spectrum that is known to be simple, in that it can be characterized by only a
\
few values, a simple configuration of the field-widened Michelson is adequate to extract
this limited information content. For a single purely Doppler-broadened Gaussian line the
interferogram is a cosine whose frequency is that of the line centre and whose envelope is
the Fourier transform of the spectral line shape — namely a Gaussian whose half-width is
inversely proportional to the line-width of the spectral line. Measurements of a single
fringe anywhere on the interferogram provides sufficient information to determine the line
intensity, and the line width. This approach was described by Hilliard and Shepherd
(1966a) and applied to the measurement of atmospheric temperatures by Hilliard and
24
Shepherd (1966b) and by Zwick and Shepherd (1973), using an instrument then called
WAMI (Wide Angle Michelson Interferometer).
4.3 WAMDII Concept
The WAMDII concept extends that of WAMI in two important ways. First there is
the recognition that a measurement of the fringe phase yields the Doppler shift; that is,
the wind velocity. Second, the field-widening is not exploited for enhanced total signal on a
single detector but is used instead to introduce an area detector for imaging. Velocity and
temperature imaging are thus possible.
The concept, which is described in more detail by Shepherd et al. (1984), is briefly
as follows.
A field-widened Michelson intereferometer of "fixed" path difference is
constructed by cementing dissimilar arm glasses to a solid beamsplitter. One arm carries
a piezoelectrically driven mirror permitting scanning over one fringe. The assembly is
placed in front of a CCD camera. Each pixel then generates its own sample of
interferogram, yielding a measurement of intensity, temperature and wind. In short,
images of these quantities are obtained.
4.4 Some Technical Considerations
Although the concept is extremely sunple, the implementation must meet exacting
requirements. A full description is given by Shepherd et al. (1985). To achieve field-
widening, dispersive materials must be used. The thermal expansion coefficient of glass,
and its refractive index change with temperature imply temperature control of
millidegrees. The wavelength dependence of refractive index means that an instrument
that is field-widened at one wavelength can be field-widened at another wavelenth only
with a change of mirror position that exceeds the capability of the piezoelectric drivers.
Happily, Title, and Ramsey (1980) found a solution to this problem, and by extending their
idea, Thuillier and Shepherd (1985) arrived at a configuration that is achromatic both in
its field-widening and in its thermal compensation. Other interesting configurations, such
25
as "superwidening" grow out of this, as has been shown by Gault et al. (1986), but that is
a digression.
One further problem is that the mirror position must be highly stable and accurate.
In WAMDn we use servo-control with capacitive sensing, using Queensgate technology.
Finally, one needs to pay careful attention to reflected light in wide-angle systems. Weird
et al. (1985) have described the effects of filter-reflected light re-entering the Michelson
interferometer.
4.5 Some Recent Results
The WAMDn developmental model was operated in Saskatoon during the new
moon periods of February, March and April, 1985, using the new RCA 50IE CCD. Good
signals were obtained from airglow with exposure times of 15 seconds for the 557.7 nm
emission gmd 30 sec for the 630.0 nm A emission. However, for adequate wind accuracy,
it is necessary to average several successive measurements
In Fig. 4.1 a set of four images relating to the 630.0 nm emission is shown. Fig.
4.11(c) shows the "raw sky phase" obtained by exposing four images in sequence, using
quarter-wave steps, and calculating phase for each pixel as described by Shepherd et
al. (1984). Fig. 4.1(a) shows the phase image for the Hg 546.1 nm A lamp that was used
to correct for thermal phase drifts — these are small but not so small that calibration can
be neglected. Fig. 4.1(b) is the phase image of the Ne 630.4 nm A lamp used to take out
the field-widening characteristic — this too is small but not negligible. Finally, the
corrected result is show in Fig. 4.1(d). One sees a linear gradient across the sky extending
from the NE to the SW. This is roughly consistant with a uniform wind field of
approximately 100 m/sec in the direction of the gradient. Our data processing plan is to
proceed by making a least squares fit to the data on the assumption of a uniform wind
field.
We will then subtract the uniform wind field from the data and examine the
differences from that.
Fig. 4.2 shows some different reduced images for a 557.7 nm observation. Fig.
26
4.2(c) shows the intensity image, with an aurora extending from the north past the
zenith. Fig. 4.2(a) shows the uncorrected visibility, which contains some residual rings not
removed by the processing. The raw sky phase is shown in Fig. 4.2(b) and the true phase
is shown in Fig. 4.2(d). Again the wind is from the NE to the SW. The results suggest a
roughly uniform wind field with a wind speed of about 100 m/sec.
4.6 Comparison of tiie Fabry-Perot spectrometer
and the Michelson interferometer
Using an area detector with the Fabry-Perot spectrometer provides a kind of field-
widening. If one uses an area detector then for both Fabry-Perot and Michelson systems
the responsivity is determined primarily by the camera, and the goal is to use the largest
detector with the fastest possible lens. However, one cannot work so far off axis with the
Fabry-Perot as with the Michelson. On the other hand, larger Fabry-Perot's can be
constructed. Real field-widening of the Fabry-Perot can be achieved with spherical plates,
but the gain in throughput so obtained is significant only at resolutions much larger than
are useful for Doppler work. With an area detector the Fabry-Perot can be used in either
of two ways: 1) viewing a single sky direction and integrating the data over the detector,
or 2) analysing each pixel as a separate sky direction and hence deriving an image. This
committee has adopted the former approach as consistant with exploiting the responsivity
advantage in order to study weaker emissions. The latter approach has been used by Rees
et al. (1984). We note however that unlike Michelson, the Fabry-Perot gives data only
where there are rings — to derive a complete image one would have to scan the rings in
steps through a full free spectral range. This gives a result equivalent to the WAMDII
method.
27
4.7 Future Prospects for Optical Doppler Measurements
The state-of-the-art Fabry-Perot spectrometer described elsewhere in this document
is based on certain specific requirements. High priority is given to the ability to work with
a complex spectrum, for example the components of a multiplet, or the luies of a molecular
band. For this, a double-etalon system is required, at least in some cases.
For WAMDn in orbit, emission lines can be isolated with interference filters, since
one does not need to worry about low altitude emission contaminating the 630.0 nm
emission, for example. Also it is not necessary to look through the troposphere, with all its
light-scattering effects. For the 0^ Atmospheric band it is planned to use a low resolution
solid Fabry-Perot etalon. This will restrict the useable portion of the image to the region of
the transmitting rings. This is a substantial limitation, but it does take advantage of the
imaging detector and it does allow altitude profiles to be obtained.
28
546
RftM SKY PHASE
Fig. A.l (a) Upper Left.
6 30
nm
"
rm
TRUE SKY PHASE
Phase image for the Mg 5461 A thermal drift reference.
(b) Upper Right. Phase image for the Ne 6304 A field widening reference.
Ic) Lower Left.
Uncorrected phase image from the sky at 6300 A.
(d) Lower Right. Corrected all-sky phase image combining (a), (b), and (c),
showing linear phase (velocity) gradients across the sky.
North is at
top, and east is on the left.
RftH SKY VISIBILITY
RAW SKY
Fig., A.2 (a) Upper Left.
INTENSITY
RAH SKY PHASE
TRUE SKY PHASE
Uncorrected sky visibility image for the 5577 A emission,
showing uniform visibility with residual rings resulting from internal
reflect light.
(b) Upper right. Uncorrected sky pliase for the 5577 A emission.
(c) Lower Left.
Intensity itnogc for the 5577 A emission showing aurora
across tl>e northern part of the sky.
Id) Lower RigJit. Corrected all-sky phase image showing a linear velocity
gradient.
5. Low resolution interferometric studies of line shapes
and line intensities in the aurora and airglow
(J. H. Hecht, Aerospace Corporation)
5.1 Introduction
Historically, auroral and airglow spectroscopy has been based on the use of grating
spectrometers,
tilting filter
photometers, or
interference
filters
[Vallance Jones,
1974]. That is because it was desired to measure, at low resolution (> 5 A), either isolated
bright auroral features or emissions over a large spectral region. However, as research
has progressed, problems have arisen which require that smaller regions of the spectrum
be examined at higher resolution. In this section we will discuss three exsunples where a
Fabry-Perot spectrometer (FPS) has been used in such a manner. The three resolution
regimes are near 4 A, lA, and 0.07 A. This bridges the gap between the conventional slit
spectrometers which operate at low resolution and the modern FPS which normally is set
to 0.01 A resolution. All of the FPS data were taken with the Aerospace Corporation's 70
mm diameter FPS which is fully described elsewhere [Hecht et al., 1985].
5.2 Mesopause OH intensity and temperature measurements
Measurements of OH Meinel band intensities, in the airglow, have been made by
many workers [Myrabo et al., 1983], In the infrared the instrument of choice is probably a
wide angle Michelson interferometer (see Shepherd section). An excellent discussion of this
technique is given by Baker (1981). Typically, in the visible or infrared regions either
spectrometers or interference filter photometers are used. However, if there is a problem
with possible contamination by unwanted emissions, such as is likely to occur in the
auroral zone, a scanning method must be employed. While a tilting filter photometer is a
good choice because of its high throughput, it suffers from the need to use, for example, 4A
wide filters if 4A resolution is desired. Besides the expense, these filters have a decreased
29
transmission, and are subject to shifts m the passband shape. A grating instrument has a
different problem; relatively low optical throughput. An FPS, however, has only slightly
reduced throughput from an interference filter and only needs to use relatively wide filters
(100 A FWHM) in order to obtain 4A resolution. An example of an OH spectrum obtained
from an FPS is given in Fig. 6.1. In that figure there are three lines each of the P^ and Pg
branch of the (8-3) OH Meinel band as well as one resolved component of the 011(732.0
nm) doublet. The signal to noise for this spectrum (± 5 K uncertainty in the OH rotational
temperature), which was obtained in 90 seconds, is comparable to that obtained by Myrabo
et al. with a grating instrument in 10 minutes. This is not surprising since it is known that
if the area of a Fabry-Perot mirror and a diffraction grating are the same, than the
product of the resolution and luminosity for an FPS is much greater than for a grating
instrument [Vallance Jones, 1974]. This results in an optical speed advantage which could
be utilized, for example, in measuring the effects of gravity waves on the OH
temperatures. Such a measurement would require that the OH temperature be obtained
from at least three positions in the sky every 10 minutes.
5.3 Auroral 01 (799.0 nm) line ratios
It has been proposed that the singlet (799.5 nm ) to doublet (798.7 nm) ratio of the
01 (799.0 nm) multiplet emission would depend on the oxygen atom density. Because of a
strong OH airglow line near 799.3 nm, a resolution of less than 1.5 A is needed in order to
measure this ratio. Moreover, because of auroral variability an optically fast instrument is
needed. Fig.5.2 shows a grating spectrometer spectrum, taken at about 7A resolution, of
auroral emissions from near 725.0 - 900.0 nm [Sivjee, private commimication]. At this
low resolution the 01 (799.0 nm) multiplet appears as the dominant emission near 800.0
nm. Fig. 5.3 shows a spectrum, taken with an FPS at 1 A resolution, of the region
between 797.9 nm and 799.9 nm. This is a sum of six 5-second scans. While the 01(799.0
nm) singlet component and the P^(3) OH (5-1) line are the dominant emissions a host of
30
other molecular and atomic emissions are present. In order to measure the doublet
emission it is necessary to accurately subtract the molecular back^ound. A fit to the
background was made using emission from the
Meinel.
first positive and
atmospheric systems. This resulted in a singlet to doublet ratio of 2.3. The errors in the fit
show the problems of auroral variability.
Thus an FPS set at 1 A resolution is especially useful for the measurement of line
intensities in a crowded auroral spectrum. In particular this technique might be required to
accurately measure rotational temperatures of molecular bands such as the
atmospheric system. For example, in Fig. 5.3 the emissions near 798.95 and 799.20 nm
indicate that the
atmospheric (4,4) band has a rotational temperature near 300
K. Previous to this FPS observation no rotational structure had been seen for this band.
In fact, even the identification of this band was tentative in the most recent observations of
this region with a grating instrument [Henriksen et al., 1984].
5.4 01(777.4 nm) and 01(844.6 nm) line shape measurements
The excitation mechanisms, in the aurora, for these two permitted oxygen multiplet
transitions have been subject to some speculation [Hecht et al., 1985]. It had been
proposed that perhaps dissociative excitation of Og molecules was a major source of these
emissions. A test of this hypothesis was recently made by observing these emissions at
between 60 and 80 mA resolution with an FPS. It was necessary to use this resolution
since each emission line could consist of two components; a narrow line, of between 20 and
40 mA FWHM, due to electron impact excitation of O atoms, and a broad component, of
about 0.25 A FWHM, due to dissociative excitation of
molecules. These multiplet
emissions were observed in both active and quiet auroras. The 01(844.6 nm) was found
only to have a narrow component whose width and intensity were consistent with direct
electron impact excitation. This was also true for 01(777.4 nm) emissions which occurred
in the mid to upper thermosphere (above 140 km). However, in active aurora where the
31
emission occurred in the lower thermosphere the line shape was best fit by a sum of both a
broad and a narrow component (Fig. 5.4). Each contributes approximately the same
intensity to the total emission.
This suggests that line shape studies of permitted oxygen and nitrogen lines would
provide information about the chemistry of their excitation. A list of some of the stronger
auroral emissions is given in Table 1. Since these are permitted emissions, once the
excitation process is understood, it would be possible to use these lines as probes of the
lower thermosphere. Thus, observations of emission from an isolated aurored form, as a
fimction of height, could provide atomic and molecular density information. Integrated
emissions could be used to test the accuracy of model atmospheres.
5.5 Conclusions
An FPS can be used as a powerful spectrometer to measure auroral and airglow
emissions from 10 mA to 5A resolution.
Such a spectrometer should exceed in
performance a slit grating instrument in this regime if the Fabry-Perot mirrors and the
diffraction grating have the same area. Even though the FPS is an optically fast
instnmient, there are still problems when measuring emissions from a rapidly changing
aurora. This could be overcome by the use of a modern imaging detector.
32
OH (8,3) Band (Nightglow)
7295-7375A
100
90
80
70
on
1
t—
5
60
50
40
111
q:
30
20
10
0
•
I.I.
20
40
60
80
100
120
140
160
180
BIN NO.
Fig. 5.1 Intensity measurements of the OH (8,3) band. Three
thirty-second scans were taken and co-added at Poker Flat,
Alaska, during March of 1985. The free spectral range of the
instrument was 80A and about 1.5 orders were scanned.
The P
and Q branch OH emissions are labeled, as is the 0 II atomic
auroral emission. This spectrum has not been corrected for
instrument response.
TCBRIURT
I?, 1985
AURORA, POCCK FU^T, ALA.SKA
^
° grating spectrometer of
in the wavelength region
-7250A- 900oA recorded at -ll resolution. N-, 1 P N+ M n
Intense emissions in this spectrum. 01 ^
and 01 hUUa multiplets are also clearly discernible The
wavelength scale is given in A.
scerniDie. The
01(7990)
DOUBLET
OH SINGLET
170
153
7979 7981 7983 7985 7987 7989 7991 7993 7995 7997 7999
WAVELENGTH langsiroms)
Fig. 5.3
A plot of intensity vs wavelength when the Fabry-Perot
is In a low resolution mode.
and the finesse is 17.7.
The free spectral range is 20.3A
The dotted line fit adds a background
of U ^ayleighs of continuum and emissions from N2 first positive
and N2 Meinel, O2 atmospheric and OH Meinel bands.
These are
used to fit the background from 7979 to 7986X, 7989 to 799lX and
7997 to 7998A. Their known molecular constants are then used to
calculate a background for the remaining wavelengths. A least
squares fit to the data is then made using this background and
the three main emissions at 7987.1^ (01(7990) doublet), 7993.AA
[OH molecular line), and 7995.lA (01(7990) singlet]. The un
certainty in the fit is probably due to the highly active nature'
of the auroral emissions.
.01(7774)
R/B RATIO = 0.6 WITH 5580 RY OF
nJ (4278)
fWHM 111 - 44.1 mA
fWHM (21 - 27S mA
FWHM • E3.3 mA
II11/1121 • 1.6
0117771.96 Al-.
0117774.18 Al
DII777S.4 AI
I
.
'
r~~
'
•
'
'
'
60
•
ICO
I
•
I
•
•
120 140 160 1B0
2U0 0
B!K NO.
BO
100 120
140 160
180 200
BIN' NO.
Fig. 5.4 A plot of emission intensity vs bin number. The FSR is 945mA
or about 155 bins. The 01 emissions at 7771.96.7774.18 and 7775.4oA are
indicated. The observation took place on 3/7/84 UT with a red/blue
ratio of 0.6 and a Nt(4278) flux of 5580 Rayleighs. The residuals
plotted below represent the difference between the data and the fit.
a) The fit assumes only one emission Is present.
b) The fit assumes both narrow and broad emissions are present.
6. Equatorial Studies with Fabry-Perot interferometers
(M.A. Biondi, U. of Pittsburgh)
Unlike the more mature FPI studies at mid- and high-latitudes, the equatorial
studies of the thermosphere are in some cases still in their exploratory phase, with quite a
number of important and exciting phenomena discovered in the past few years. The
interferometric studies have chiefly been concerned with thermospheric dynamics via
twilight and nightglow 630.0 nm line profile measurements.
The initial impetus for thermospheric wind and temperature measurements was
their perceived role in triggering or quenching F-region plasma instability growth; the
neutral dynamics was expected to be rather bland, since the equatorial thermosphere is
remote from the large, variable high latitude energy inputs. Instead, the studies to date
reveal a rich and varied set of phenomena which advance our understanding of the global
thermospheric circulation during quiet times and also reveal processes such as strong
gravity wave modulation of the thermospheric velocity field and deposition of large
amounts of energy by local processes, as well as by propagation from high
latitudes. Obtaining a fuller understanding of the occurrence and origins of these and
related phenomena represents excellent initial objectives in implementing the CEDAR
science program.
Some recent results (1977-1983) concerning the equatorial thermosphere are listed
in Table 6.1 and discussed in more detail in the following paragraphs. The measured
nighttime variations in T^ over Arequipa, Peru are shown by the X symbols in Fig. 6.1
from shortly after the autumnal equinox (April) to well after the winter solstice (August),
together with solar and geomagnetic indices. The solid lines on the T^ graphs are MSIS
predictions. Several features are evident: 1) Even during geomagnetically quiet periods,
the measured T^ values are usually substantially higher, by about 200K, than the
empirical MSIS values. 2) Following the onset of geomagnetic storms, the T
values are
n
33
elevated, with delays in the temperature rise ranging from zero to about 1 day; the latter
delay is consistent with energy propagation from high latitudes by gravity waves.
The seasonal changes in the horizontal winds are shown by the polar diagrams of
Fig. 6.2, which depicts the positions of the tip of the wind vector on various World Days
between 01
li
h
and 10 UT. Just after equinox (WD = 98-105) the eastward zonal wind
dies away during the night, while after solstice (WD > 175), the eastward zonal velocity
usually persists throughout the night. The feeble meridional component becomes somewhat
more poleward (southward) in the later night as winter approaches. These observations
are in general accord with TGCM predictions.
Changing effects of ion drag appear to be evident in the neutral wind data from
h
Vi
Kwajelein, Marshall Islands in Fig. 6.3. Shortly after sunset (08 UT = 20 LT) the zonal
wind increases as the ionosphere is lifted by ExB forces (thereby reducing the ion drag on
the neutrals), reaches a maximum at about 11^ UT and then decreases again as the ExB
reversal causes the ionosphere to sink, reimposing the drag.
An unexpected finding at equatorial latitudes, as at midlatitudes, is that on
relatively quiet days there occasionally is pronounced convergence (or divergence) in the
horizontal wind field. The Natal, Brazil data of Fig. 6.4 show tliat the strong convergence
in the meridional wind leads to a downward vertical velocity and an increase in T
measured overhead (the V symbols) as a result of the compressive flow.
Evidence for gravity wave modulation of the thermospheric wind field was found at
Natal, Brazil during a geomagnetic storm (ZKp = 62+), as shown in Fig. 6.5. The velocity
components oscillate with a period of 40-45 min and the horizontal/vertical velocity
modulation ratio is about 2; both findings are in accord with the theory of horizontally
propagating thermospheric gravity waves.
Finally, the very prompt response of the equatorial thermospheric temperature to a
geomagnetic storm observed at Arequipa is illustrated in Fig. 6.6. The very large rise in
T^ follows almost exactly the large negative excursion in Dst, which has been interpreted
34
as indicating precipitation of energetic charge exchange neutrals from the ring current into
the low latitude thermosphere. In support of this view is Tinsley Eind Sahai*s simultaneous
measurement at Cachoeira Paulista, Brazil indicating a corresponding sudden increase in
391.4 nm brightness (short-dash line in Fig. 6.6), indicative ofexcitation ofNg by particle
precipitation.
The foregoing examples of Fabry-Perot studies are intended to illustrate some of the
interesting and important thermospheric dynamics phenomena revealed by 630.0 nm
doppler
shift
and
width
measurements.
These
determinations
of
the
neutral
thermosphere^s behavior at 260-325 km altitude should be complemented by studies of the
lower thermosphere and mesospause at 85-95 km by 557.7 nm (01) and OH Doppler
measurements to obtain a more complete understanding of wave effects, local energy
deposition, etc. Needed correlative information on F-region plasma djmamics should also be
obtained using the next generation of FPIs to detect the feeble plasma radiation.
Topics such as propagation of energy from high latitudes via gravity waves and
neutral wind fields require coordinated measurements with mid- and high latitude
interferometer stations to obtain a more complete imderstanding of the processes
involved. Finally, simultaneous, colocated measurements by complementary instruments
such as photometers, radars, and imagers will provide the required, fuller characterization
of the phenomena under study.
35
200
1 •
•
••1
'
' -
' Arequlpi. PUIU (1903)
r.. ISO
d
Lu
Dlondl and
100
60
Mcrrlwcther
^..v-'v
Ai-equlpa, PKRU (1903)
CIIL 12, 26'1-no
(19057.
AIruv^'
v^\r
April
98
Q-lOO
1
S
10,
IS,
20,
30 10|
25
99
«a*
IS, •
\
\
•*
/
too
lOS
108
t03
154
ISS
IS6
157
o.
it:
»,
0
1500
is'
M
t-
• •
700
•
I . . . y ••.—.
• 1 • . • .4—
.L
ISO
I- '
1
V, /
> ,1
• **
*
mv
nPRIL
•
M9
lUt!
•"^tOOO
"T
H
1
1
....
1
•
'
1
'
1
*
*
1
'
'
»
•
r:o'
"3
I7S •
tB2
<Ai-
:50
•30
^
Y
f7,
Ml,
S,
/|
t(\
/v/
1^
50
Av -
N
fS
217
2IB
Jl9
M"*- 'V20,
2S,
30,
30,1,
j ^ -:oo
f
•—S-—k-:-E
5
^f\ji/yvv4
0
Auj^uat
1530
f
.
!i !Sli'
-
?» - i
.
•
.
i i(« (Wiii'mi •
1
i
"«»
J
1300
Hurlwuther, Hoody, Dlondl and lloble. JCn, acccptcd for publication, 1905.
700
JUNE
JULY
Fig. 5.1
RUGUST
Fig. 6.2
u* o
o o
—
o
o
ro
o
o
o
o
tn o
O
o
CO
infensity(rel. units) temp.(K)
00 •
vertical wind (m/s) zonal wind (m/s)
r
I
meridionol wind (m/s)
W
M
vn
Ul
j?
K
n
n
r
I?
W
latal. BRAZIL
. eastward
K«= 3 ,4-
7 ,0-,6:^5-
- 2000
2000
6
Sept
'n
1982
(K)
ArequJpa,
(391.^;
(K)
PEHU
1500
1500 -
upward
13 Jun
1000
looor
MSIS
V
18
22
23
time (UT)
Blondl and Slpler; PlAoet. Sjace Scl. ^
ri(. 6.S
817-823 (1985)
21
12
15
18
7
10
13 19
0
3
6
22
I
Dtoodl and Merlwetber, G.R.L. 12 267—270 (l96S)»
(Tlnaley and 8ahal, firlvate coonunlcailon)
Fig. 6.6
9
4
12 (UT)
7 (LT)
7. Studies of
exosphere
(F.L. Roesler, U. of Wisconsin)
Within approximately the past five years studies of the earth's exosphere through
observations of Ha line intensities and profiles with highly sensitive, large aperture FabryPerot spectrometers have provided important new data which have stimulated theoretical
work aimed at refining exospheric models. The field is presently in a state of fairly rapid
advance where the interplay between model development and observations from a small
number of well chosen sites can be expected to bring the field to a new plateau of
unerstanding within a few years. Accordingly efforts within the GBOA/CEDAR framework
should consist of an initial phase in which a relatively small number of individual
observers addresses specific gross problems in the developing models, followed by a global
effort adapting stations initially placed for global thermospheric studies. The second phase
would begin after the broad aspects of global variations appear relatively well
defined- approximately three to five years from now. Some of the questions to be
addressed in the first phase are the following:
A). Measure Ha intensities from high altitude to better understand the tropospheric
scattering problem. Such observations from low latitude would also give a large range of
solar depression angles to aid in model comparison.
B). Measure H|3 intensities and profiles to better understand the role of multiple
scattering and profile features such as the narrow core and the red wing (escaping
particles or fine stucture).
C). Continue the present initiative to accumulate a broader data base to help
establish trends and variations with various solar-terrestrial parameters. This initiative
should include the development of improved detection systems.
D). Conduct polar and equatorial studies with existing instruments to extend the
latitudinal range of measurements.
37
E). Conduct pilot studies of He 1083.0 nm and
732.0 nm to define techniques
and possibilities for a later effort using these lines.
From our present imderstanding of the variations seen in the exosphere it is clear
that a future global effort will be of enormous benefit in understanding the large scale
behavior of the exosphere. However, the definition of that phase will best be made with
the benefit of observations to be performed in phase I.
38
8. Improved Fabry-Perot interferometers; design considerations
(M.A. Biondi, U. of Pittsburgh)
To meet some of the CEDAR science objectives, Fabry-Perot interferometers of
considerably improved sensitivity are required to deal with measurements of feeble
radiation at very high resolution and/or rapidly fluctuating emissions on short time
scales. In addition, other improvements such as better instrument and reference source
stability, continuously variable free spectral range and broader wavelength coverage are
desirable. These features, together with reasonably attainable goals in five years or less,
are given in Fig. 8.1
To provide a basis for comparison, the sensitivity and performance of a
representative single etalon instrument are listed in Fig. 8.2. This refractive-index tuned
instrument employs a GaAs photocathode photomultiplier detector behind a multiple
aperture exit plate (MAEP) which gives a light-gain of 5X over a central aperture
instrument. The 10 c/s/R overall sensitivity at 630 nm yields adequate velocity and
temperature accuracies in integration time periods which vary with intensity, as
indicated. A minimum of a factor of 10 improvement over this performance is required to
accomplish some of the new CEDAR science objectives.
Attainable improvements which might reasonably be expected in five years or less
are listed in Fig. 8.3 (a GaAs photocathode photomultiplier-image intensifier- CCD array
detector combination should have properties comparable to the photomultiplier (PM) listed
in 2a). Since the dark counting rate per detection element of a GaAs PM with a position
sensing anode is the same as for a conventional GaAs PM detecting the central spot of the
F-P pattern, the improvement in both signal and signal/noise suggested in Fig. 8.3, part 2a
should be attainable. However, present bare CCD's with small, rectangular pixels, while
yielding a gain in signal of the magnitude listed in part 2b, would not provide a similar
improvement in signal/noise ratio because the equivalent readout "noise" of >3e's per
39
3
pixel leads to rather high noise levels when the 10 pixels per detection ring element are
summed. Rather, development of custom CCD's with concentric annular ring pixels suited
to FP applications would be required to limit the readout "noise" per pixel to more modest
*
levels. The timely development of the bare CCD is regarded as the most speculative of
the projections; however, with a 2a type detector, a 12x improvement over the current
"best" single etalon instruments seems quite feasible.
Some consequences for the CEDAR science program of achieving the improvements
listed in Fig. 8.3 are given for illustration in Fig. 8.4. Plasma dynamics studies at any FPI
location and exospheric transport investigations are but two of a number of upper
atmosphere science areas which would be available at relatively modest cost. Along these
lines, a cost-effective first step in expanding the upper atmosphere science objectives
attainable in the FPI interferometer program of CEDAR is suggested in Fig. 8.5. By
taking the most suitable and best located of current FPI instruments and slightly
modifying their design to accommodate the new detectors, the most critical of the
upgrading objectives would be achieved- that of greatly enhanced sensitivitj'. An added
advantage is that very similar detectors are a major need of the new spectrometer and
imager instruments, so that these detectors may merit top priority in the instrument
planning program.
*
There seem to be conflicting opinions as to whether readout "noise" varies directly with
pixel area or more slowly.
40
I
Class I Fabry-Perot Interferometer
Desired Features
1. Extreine Sensitivity
2.
State-of-Art
5 vrO
300 c/s/R
Great Stability
equiv. Av 1 1 m/s
3. Variable Free Spectral Range
Broad Wavelength Coverage
t = 0.1 cm - 4 cm
300 nm to > 1 pm
ng. 8.1
Current F-P Instrument Sensitivity
ExamEle: Single etalon, 150 mm aperture, t =1 cm,
f^ap = 15 (Ninstr - 12), 5x MAEP &PM(GaAs)
OVERALL SENSITIVITY: -v, IQ c/s/R
Consequences: For (Peak - Bkgnd) 2 1000 c, 50 points
[av -v 1 10 m/s, AT 'V t 30 K]
1(630 nm)
Time Required
lOOOR
5s
lOOR
SOs
lOR
•v 8m
IR
•V 1.4 h
Application
Requirement
Rapidly varying phenomena
(e.g., Aurora)
few seconds per
observation
Feeble (^ IR) emissions
(e.g., F-region plasma
recombination radiation)
Fig. 8.2
- 15 min per
observation
"Attainable" Improvements
(Time Scale:
1.
Throughput:
Few Years)
1.5 x (higher reflectivity mirrors, low-reflection
coatings on all windows/lenses)
2.
Detection Sensitivity;
(compared to central exit aperture
instr, Ngp = 15)
a) GaAs PM with "Position Sensing" Anode (40 wm spatial)
photon quantum efficiency Ix
multiplexing over 1 order 15x
four orders usable
total
(QE -v 162 P 630 nm)
6Qx
b) Bare CCD* (40 vm spatial resolution)
photon quantum effic.
(QE
3x
50% (a 630 nm)
multiplexing over 1 order 15x
four orders usable
Ax
180x
3.
Wavelength Coverage at Good Q»E.
Improved PK(6aAs)
K 900 urn
Extended range CCD ^1.5 ym
Fig. 8.3
Illustrative Consequences of Improvements
A.
Better Detection Sensitivity
Feeble Emission
New Investigation
1.
F-Region Plasma Dynamics
01(777.4 nm)
Determine
vi. Ti
(Equat./Mid-lat.)
2.
Exospheric Transport
Geocoronal H
(656.3nm)
hydrogen
escape
dynamics
B.
Broader X Capability
New Investigation
1.
Exospheric Transport
I.R. Emission
Geocoronal He
(1.083 wm)
Fig. 8.4
Determine
helium escape
dynamics
Cost-Effective Alternative to
Fabricating/Deploying Class 1 Instroments
Upgrade existing F-P's with new detectors.
Some
sacrifice in flexibility (free spectral range
changes, wavelength range capability}, perhaps in
ultimate stability.
Gain over current "best" single etalon instruments:
Detector
Gain over "pinhole"
Gain over
PM(GaAs)
60 X
12 X
180 X
36 X
w Position
Scaling Anode
Bare CCD*
*Gain in IR (> 900 nm) much larger.
Pig. 8.5
9. The application of imaging detectors for Fabry-Perot spectroscopy
(F.L. Roesler, U. of Wisconsin)
This report discusses the way in which imaging detectors can be used for the
purpose of obtaining spectra with Fabry-Perot interferometers, and the potential gains to
be expected from the application of such detectors.
Let us start by recalling how light filtered by a Fabry-Perot changes in wavelength
(or wavenumber a=l/X) as the angle of incidence 6 changes: we have acosd, or
2
approximately for small 6, La!a = 9 12. Fig. 9.1 illustrates how equal spectral intervals
Aa measured from the normal incidence wavenumber Oq define annular regions in the focal
plane of a lens behind the Fabry-Perot with radii proportional to the square root of
integers; the first ring has radius r^, the 5th ring radius r^/5, etc. In the familiar
scanning Fabry-Perot using a single element detector (most commonly a photomultiplier) a
mask is placed so that only light within the central element is passed to the detector. If a
large area imaging detector were placed in the focal plane, many resolution elements could
be recorded simultaneously, with an apparent large gain in efficiency. Apart from the
serious questions of detector noise and quantum efficiency, which we will discuss later, the
gain is limited by the number of annular elements that can be clearly defined within the
given detector airray size.
To calculate the number n of annuli that can be defined within a detector N pixels
wide and N pixels high (NxN array) we require that the last and finest annulus be at least
the width of two pixels. If a pixel width is e, then our requirement is
(Eq. 9.1)
(/n - /n-1) = 2e
But the diameter D of the n
th
annulus is
41
D = 2|/nr^
(Eq. 9.2)
= eN
Combining these two equations gives
N = 4/n/(/n - /n-1)
(Eq. 9.3)
= 8n
where the approximation is valid within 3% for n > 10.
For an array with 512x512 pixels for example, we can obtain 512/8 = 64 annuli,
with each annulus comprised of 1/64 the number of pixels contained within the outer
diameter of the largest annulus, or ?r(512) 2 /4x64 = 3217 pixels. Thus 64 data points
along a spectrimi can be recorded simultaneously giving a potential gain of 64. The actual
gain is, of course, limited by the instrumental finesse, by the physical number of annuli
that can be fit on a particular detector (64 in the illustration used), and by the actual
number of data points required (with, say, a scanning instrument) to obtain the desired
result. For many of the common problems in aeronomy the potential gains are impressive,
typically more than an order of magnitude.
The actual gain depends upon the time requu'ed to achieve a suitable signal to noise
ratio in a particular application, and that depends upon some detector characteristics other
than array size and pixel size. Oversimplifymg somewhat to get to the heart of the
problem, we can distinquish two basic classes of array detectors that appear useful for
visible aeronomy. One type is the CCD or charge coupled device, which achieves a
quantimi efficiency neai' 80% in the red, and accumulates photoelectrons in localized sites
defined by bias voltages in the circuity of the silicon chip from which the device is
made. After suitable accumulation, a clockwork of alternating biases moves the charge
packets in an orderly manner through the pixels to an on-chip amplifier which turns them
out like graduates going through university commencement. This amplifier adds what is
known as read-out noise to the output, and thus has a very important effect on the
achievable signal-to-noise ratio. There is also a dark rate of charge accumulation due to the
42
thermal generation of electron-hole pairs in the detector.
The second group consists of photoemissive devices in which a single photoelectron is
sufficiently amplified to cause an identifiable response in a position sensitive detector. A
typical example is a photocathode surface followed by microchannel plate intensifiers and a
phosphor output coupled (possibly with fiber optics) to a CCD or reticon. In another
common form the phosphor is replaced by a position sensitive charge detector. These
devices are designed to identify and locate the splash (count) for each photoelectron
collected from the photocathode.
The photoemissive imaging detectors have quantum efficiencies limited to about
10%, or a little more at best, through most of the visible spectrum, a value considerably
less than for CCD's. However they are free of the readout noise problem of the CCD. For
readily available CCD's a readout noise of between ±20e
and ±50e
rms is typical; but
developments in the near future are expected to bring this value down to about ±(a few).
For either of these types of detectors we have found that a suitable estimate of the
signal-to-noise ratio (S/N) for a particular data point is given by the equation
Qstkj
S/N =
(Eq. 9.4)
v/[Q(s+b)+d]tkj+kr^
where the symbols are as follows:
Q = quantum efficiency
t = integration time
s = signal rate per pixel (photons per second)
b = background light rate per pixel (photons per second)
d = dark electron generation rate per pixel
r = rms readout noise (equivalent photoelectrons)
j = number of pixels binned before readout
k = number of binned pixel units coadded after readout.
Compared to an axial fringe scanning instrument the gain from using the imager is
43
equal to the number of simultaneously recorded data points multiplied by the ratio of the
integration time needed to achieve the same (S/N) with the imager. A qualitative look at
the S/N equation shows that for a given problem the quantum efficiency and readout noise
have the largest influence. In general the higher the quantum efficiency the better;
however with the high quantum efficiency CCD's the photoelectron equivalent of the
readout gets added thousands of times per annulus (3217 times in the example given
above) and drastically reduces the S/N for low signal rates. Thus even a low Q amplified
photoelectron imager might surpass the high Q CCD. For studies of the daylight sky,
however, the background of scattered simlight would dominate the readout noise, yielding
the full area and quantimi efficiency advantage of the CCD.
Obviously the gain depends on details of the specific problem to be studied. As an
example we have chosen the condition we encounter in measurements of geocoronal Ha
intensities with S/N - 7 at 1 Rayleigh including a 50 data point baseline for the accurate
determination of galactic Ha, zodiacal light, and starlight backgrounds needed to extract
pure geocoronal emission. Fig. 9.2 compares the gain relative to a scanner with a GaAs
photomultiplier expected from intensified imagers with various photocathode quantum
efficiences (== 10% at Ha is about the best we can expect from GaAs on an intensifier, and
about 4% is achieved for an S-20 photocathode), and conventional CCD's with various
readout noise levels ( we have used a conservative Q=64%). The drastic affect of the
readout noise is obvious, and for this problem the CCd's which are readily available offer
no gain. However CCD's with r=:±3e
are expected within a few years or less, and this
would alter the picture. The last column ofthe figure shows what might be achieved using
a CCD with thin annular pixels with eight segments per annulus. Since far fewer pixel
would be added after readout (small value ofk in the S/N equation), the benefit ofthe high
Q for the CCD is restored. It appears technologically feasible to develop such a detector,
and this possibility should be investigated.
44
loo
r» o
50
r= ±S"o
zo7.
r* tJe"
y
\0
-
;r:l7c"
(ys V'/.
r=2'S"c.~
/n-ien^t-fierj
CCD
Corv\/e«N'V«oi*d\
"nH^''ccrj
cxfrctg ,Cv>.,(.M7.
Fig. 9.2 Comparison of gains for various Imagcrs.
Unit gain is for a scanning axial- fringe Fabry-
Fig.
Division of an area detector into annulii
of equal spectral Intervals. (The upper half of
Perot spectrometer observing Dalmer alpha (6563^)
the pattern is Implied.)
with a GaAs photomultlplier. The gains achieved
depends strongly on the observing parameters, as
explained in the text.
10. Temperature, wind, and intensity analysis
for an imaging Fabry-Perot interferometer
(Robert J. Sica, Utah State University)
Thirty years ago most Fabry-Perot interferometers (FPI) used film as an imaging
detector. The output is then a series of annular rings, each closer to the next by the square
root of its order number from the zero order in the center. When high gain, low noise
photomultipliers became readily available, the ring pattern was scanned, usually with an
aperture passing the center order. The proposed Class 1 FPI would substitute for the film
substrate an area detector such as the charged-coupled device (CCD). The advantages of
such a detector include the possibility of a static system, field-widening to increase the
etendue, and higher quantum efficiency (but without the low noise and high gain of a
photomultiplier tube). However, the data acquisition and reduction requirements for such
a system have not been treated in any detail.
The present data analysis for single Ime spectra uses the noise inherent in the signal
to determine the Fourier coefficients of the line profile. These coefficients can then be
related to the temperature, wind, and intensity of the source. The data analysis is
reasonably straight forward and not demanding upon computer resources.
Two-dimensional analysis, however, is considerably more complex. The image first
needs to be corrected for sky continuum, star background, detector noise, and differences in
sensitivity between pixels. These important corrections are perhaps best done in the data
acquisition phase.
The sampling criterion for the imaged spectrum is important. A typical scanning
FPI used in observing the 630.0 nm airglow requires 14 samples per free spectral range to
retrieve the geophysical parameters of interest without aliasing. This requirement must
also be satisfied for array detectors in the radial direction. This limit constrains the
number of useful rings that can be imaged on the presently available CCD's.
45
Furthermore there are sampling considerations in the azimuthal direction. These
relate to the structure of the source region under examination. If a structured region of
the sky is undersampled, aliasuig may effect the retrieved coefficients. An additional
constraint on the number of pixels along an inner ring is required. One of the advantages
of an imaging system is the possibility of mapping large regions of the sky
simultaneously. It is then important to determine the amount of directional information
needed in the image.
Rectangular CCD's have a varying number of pixels in different radial
directions. There are also a different number of samples per free spectral range in each
higher order, since the spacing between orders decreases by the square root of the order
difference from the center. This complicates the harmonic analysis of the image. It is
highly desirable to have samples at equal wavenumber intervals. A radially-clocking CCD
with large pixels in the center which decrease in radial size and increase in circumference
as the distance from the center increases would allow equally spaced sampling during data
acquisition. Manufacturers, however, are not likely to build such a specific device for a
small number of users, though university research groups may be interested. The
measurements would also benefit from the lower read-out noise of such a detector.
The image obtained by the rectangular CCD must next be processed by appropriate
analysis software. Currently there are two possible approaches to handling the images. A
flow chart of the first method is shown in Fig. 10.1. Here the image is re-sampled at equal
intervals in 16 x 16 pixel sections. A two-dimensional Fourier transform is then
performed. The difficulty here is preserving the noise in the data during the interpolation,
since it is the noise that ultimately determines the useful information in the image.
A flow chart for an alternative method of analysis that avoids interpolation is
presented in Fig. 10.2. There are transform pairs that allow the spectrum to be sampled
at unequal intervals. A specific example of a transform pair that allows the sample points
to be spread arbitrarily on the unit ch-cle, the Cheybshev transform, is shown. The
46
coefficients can then be found by searching the image for the periods associated with each
free spectral range.
Besides
these
analysis
considerations,
there
are
practical computational
constraints. Storage of the images is not unreasonable. For (512) pixels recorded at 3200
bpi, about 6150 frames can be written on 3600 feet of magnetic tape. For 1 second
integrations that isj 1 3/4 hours of data, and for more "routine" 1 minute frames, a tape
would last about a week.
The computational time that could be required for the data analysis may be a
problem. The estimated time for a (256)
2
pixel discrete Fourier transform on a micro-VAX
(assumed to be five times faster than a VAX 11/730) is about 3 minutes. Therefore, even
at 1 minute frames the data from the proposed Class 1 instrument will be acquired 3 times
faster than it can be analyzed. Perhaps the need to transform each image can be relaxed if
a library of processed images can be used for comparison.
The design plan for the Class 1 instrument must address the data acquisition and
processing limitations, some of which are outlined in this report. Much theoretical and
computational work remains to be done in the analysis of the images. A complete two-
dimensional analysis must first be demonstrated for synthesized fringes. The construction
and demonstration of a rigorous analysis of the FPI images is an important step in the
design of the Class 1 instrument.
47
V
Cticbyshcv polynomials
Take imago
and Fourier series
N
Guess at peaks
-Fix" FSR (stretch by geometric factor)
I
X| = cosO j
Take imago
Interpolate to 16 * 16 * (N-l) points
(for N rings)
Perform 2-0 Fourier transform
y-
f
f(x.y) exp(-2-7t i (ux»vy)l dxdy
r "
-OO
r
•
-
Find Fourier
The f(cos"'xi ) can then be
expanded over an orthogonal
set of polynomials whose
coefficients
oo
F(u.v) exp(2-Tt i (ux»vy)l dudv
f(K.Ij)= 1
J -od
The Xj are otherwise
arbitrary between (-1.1).
OO
F(u.v) = 1
J -oo
2-d harmonic analysis
(CheOyshev?)
coefficients can be related to
-OO
the Fourier coefficients.
flelate these lo
Oo coefficients change?
p
yyyyvy/itv^
T. V and I
Soarch for mulLlpIc periodicities
Guess periods
I
Estimate means
Relale these to T. v an(J I
For k * n calculate
I^ ( Continue^
Fig. 10.1
,A|( and Oj^
Change in
cocfficienls
Anow chort lor the annlyoia o( FPI Imaget re<vnng InltfpolatJon lo obtain
i
eqijoJiy opaced eamptcs.
Fig. 10.2
A flow chad for th« analyeis ofFPI Inoagos udng une<fj.illy spaced data. A
poaotbto transform lor this case uecs Chebyehev polynominla.
11. Implementation of CEDAR scientific objectives with
high resolution interferometry
(John W. Meriwether, U. of Michigan)
Examining the several areas of aeronomy involving application of the Fabry-Perot
interferometer , we see a common theme throughout. The CEDAR objectives defined in
the Introduction can not be met by the instruments in use today. The science that may be
achieved with high resolution interferometry would be more extensive and more far
reaching, if the sensitivity of the detector system were improved by at least one order of
magnitude. The key problem is not the lack of photons but how the interferometer could
make more efficient use of the photons captured by the collecting optics of the
instrument. Hence, the application of the array detector has been given highest priority in
the consideration of the means by which this goal may be achieved.
The results of this investment of resources would be improved mapping of the neutral
wind and temperature fields with excellent temporal and spatial resolution. Other benefits
include: 1) the detection of weak spectral sources in nightglow, aurora, ring current
induced particle precipitation, and the polar cusp, 2) the observation, with higher signal-tonoise ratios, of the spectral emissions of twilight and dayglow with improved background
rejection, and 3) the improvement of the precision by which line shape studies of hydrogen
and helium coronal emissions and oxygen multiplet line emissions may be carried out.
However, the direction that should be taken in future detector development is still
unclear. If the promise of low noise CCD chips now being heralded by the solid state
detector manufacturers can be achieved, especially if the devices could be fabricated with
improved geometrical arrangement that more closely match the requirement for
application in the Fabry-Perot interferometer, the goals hoped for can be reached through
the combination of the field-widening of the Fabry-Perot, the utilization of the multiplexing
gain, and the
higher
quantum efficiency of these devices.
49
It is
therefore
the
recommendation of this committee that substantial funds be devoted to the support of
development of the solid state detector, not just for the bare CCD detector but also for the
other forms including image intensifier CCD, and the image plsme detector such as the
wedge and strip or resistive anodes with a GaAs cathode surface. Once a satisfactory area
detector has been identified, the committee recommends that all FPI interferometers be
upgraded to achieve the improved level of sensitivity needed to satisfy the CEDAR science
objectives.
The committee also recommends that optical means for improving the Fabry-Perot
sensitivity be examined with an eye to the practicality of this alternative approach. Optical
techniques are likely to be less expensive to implement than the application of solid state
technology, and the investigation of possible optical designs should be supported.
While the research and development effort needed to upgrade FPI detectors is
imderway, it is desirable that the present distribution of FPI stations be augmented by
several additional facilities. Examination of the current coverage of the upper atmosphere
dynamics shows a major gap exists with the lack of any automatic station operating at the
polar cusp of the Southern Hemisphere. The theorists have strongly indicated that a major
need exists for simultaneous observations of the thermospheric dynaunics in both
hemispheres to study the effects of variations in the IMF parameters in the opposite
hemispheres and to examine the different degrees of coupling between the plasma
dynamics and the neutral atmosphere. The South Pole station is an obvious candidate for
an automatic facility, because it passes within range of the polar cusp each day, it
possesses very clear skies during the winter months allowing continuous observations over
many weeks, it is conjugate to the FPI and radar stations at Sondrestrom, and technical
support and excellent radio communications exist at the station. This facility should
include an instrument of tested design and high reliability.
The committee also recommends a Class 1 facility be installed at Mt. Haleakala in
Hawaii. To exploit the natural advantage of the high elevation of this location, i.e., the
50
reduction of the Rayleigh and aerosol scattering, this site should be capable of dayglow
observations. Another advantage of this site is the capability of observing the ionospheric
plasma optical emissions found in the Appleton anomaly located south of the Hawaii
station. Furthermore, the measurements of thermospheric dynamics made at this station
would supplement the observations made at Arequipa, Peru, with data from a different
longitude sector. In addition, this proposed location would be another good location for the
study of low-latitude ring current induced particle precipitation. The Hawaii site has good
weather, is readily accessible for reseachers, and could be operated inexpensively via
telephone.
The committee gave detailed consideration to the instrumental upgrading of the
facilities at the existing sites. It is recommended that additional etalons be installed for
multiple wavelength operation needed for coupling studies. These etalons should be
designed for ease of plate spacing selection without disassembly. This latter feature would
enable instrumental optimization for the chosen spectral feature and provide a range of
spectral resolutions for application of this instrument. These additional etalons may utilize
more conventional detector systems for low resolution applications. Moreover, optical
means of enhancing instrumental sensitivity would require an low resolution etalon for the
purpose of an optical mask (Hernandez, private communication).
The committee recommended that all existing FPI facilities be automated fully for
extended operations. It also recommended that the progress in the WAMDII program
under the direction of G. Shepherd be monitored. This instrument is a major advance in
optical doppler technology. Consideration should be given to the construction and support
of a portable WAMDII instrument for use in special campaign efforts in addition to
continual operations in an observatory.
A major new area of research involving FPI stations at high latitudes involves small
scale dynamics for dimensions of the order of a few kilometers. The committee felt that a
bistatic observatory was an excellent means for exploring this new field of atmospheric
51
dynamics. The geometry of such an observatory allows the neutral wind vector to be
measured within the common volume established by the two intersecting lines of sight. The
WAMDn and the area detector field-widened Fabry-Perot interferometer represent
alternative means of examining small scale djrnamics as both instruments possess the
capability of examining many more directions within the sky than the typical current FPI
instrimient.
Once these instruments are constructed and operated, the demands upon analysis and
interpretation will be taxing. The present means of communicating results within the
community will not be adequate. A clear need for a network exists, whereby data and
messages may be passed from one investigator to another. In this way planning and
interpretation would be markedly improved as a result of the improved communications
between workers. A workshop program is needed to bring together the principal
investigators on a regular basis. Electronic mail to establish improved interactions among
the principal investigators can be initiated immediately. Satellite communications with
remote stations may be feasible within the next few years.
It is difficult to project precisely the costs for the program as outlined. Table 11.1
sets out prospective budget costs for these various options; over a period of five years,
some 3 million dollars looks to be necessary to carry out these plans. Whether five years is
enough time is dependent upon how successful aeronomy can be in attracting additional
researchers into the field.
52
IfTippo vemen ts»
upsradinst
and additions needed
to interferotneter capabilities to
implement CEDAR scientific soals
!•
Mawior
needed
increase in sensitivity
to
improve and enhance FPI
capabilities
forJ
1.
Improved time and spatial mappins
2.
Improved detection of weak sources
auroral ring
polar cleftf
3.
Improved
in
4.
and
line shape studies of
line components
Improved sensitivity for
usins
bac^csround
dayslow work
Improved sensitivity for
studies
in twilisht?
current induced emissionst
polar cap» and dayslow
rejection of white lisht
twilisht
multiplet
5.
resolution
molecular
rotational
temperature
emissions
Implementation:
1.
Fund
parallel
bare CCD
wedse and
2.
development of solid state detectors
200 K
in prosress
strip
Consider alternatives:
100 K
imasins i n t e n s i f i e r CCD
imase plane detector with
GaAs cathode surface
3.
Implement
FPI
2.
upsradins
detector
of
400 K
systems
Complement present distribution of FPI
stations*
automate stations where necessary*
install additional eta 1ons/detector systems
for multi-emission studies*
A.
Antarctica South Pole station
-southern hemispherica1 coverage
-continuous 24h coverage in winter
—excellent clear weather
-2800 m. elevation
-conJucacy to Sondrestrom
-sees c l e f t emissions at noon so
both neutral and ion motions
can be readily studied
—technical support available
Table 11.1
150 K
B.
Construct state of
the art
to
Haleakala
be
installed
at
instrument
350 K
-sees e«\uatorial anomaly emissions in
South direction (sive access to
ionospheric parameters)
-complement present coverase of
e«^uatorial l a t i t u d e s
-sood location for studv of low-latitude
rins current induced particle preciPitation
to complement Aresuipa station? also take
advantage
of
to
weak
detect
-clear
weather
-mountain
WAMDII
Bistatic
FPI
Variable
facility
prevalent
ideal for davslow work
support available
Portable
located
Additional
spectroscopic
emissions
site
-technical
C.
GBOA
instrument
350
observatories
in auroral
etalon and
sap
desisn
K
300 K
zone
detector
for
etalon
channels
200 K
chamber
100 K
-hish temporal resolution
-observe small scale structures in dynamics
of atmospheric resions
-study coupling effects in momentum t r a n s f e r
via multiple emissions
-study composition chanses within auroral arcs
-study l i n e shape of weak auroral emissions
that responds to production mechanisms? variable
sap desisn of etalon assembly important
feature to allow optimization of spectral
r e s o l u t i on
D.
Portable FPI
observatories
150 K
-round out FPI station distribution
in auroral zone for slobal campaisns
for
winter
months
-provide FPI observatory for special campaisns
such as CRESS releases* rocket launches*
BARSt s a t e l l i t e overflishts (UARSt Vikins)
- f a c i l i t y to be current instrumentation
with detector upsradins addition
E.
Complete automation
stations
(Laurel
where
Ridse»
of
present
FPI
150 K
necessary
Madison)
it
T
n r%« t a/4
Manasenie-nt
and
-verv
Coord i nai i on
important
within
to
of
improve
interferometrv
purposes
of
i n t e r f e-rofTie-ter
observations
825 K
c onimuni c a t i o n s
communitv
collaboratiori and
for
plarinins
-exchanse dates of observationSt archive
data» share software concernins data an&lvsis
and srarhics^
provide .user-friendlv TGCM
-plan interferofTietrv campaisns relating to CEDAR 3oals?
study transient events with slobal distribution;
concentrate upon specific soals such as F-resion
E-resion*. ms'sosphere optical emissions
-need project scientist to
and coordination activity
oversee manasenient
Implementation:
—workshop
prosram featurins
two meetinss/vear
30 K/vear
-electronic mail for improved interactions amons'PIs 50 K/vear
-satellite communications with remote FPI stations
35 K/vear
-project manasement
50 K/vear
Sc hedules
1986/1987
Fund CCD proof of
and
concept
200 K
compare with wedse and strip
Start worlishop prosram
30 K
for coordinatins activities
amons different doppler stations;
elec tron i c
ma i1,.
Start construction of Antartica FPI station 150 K
1987/1988
Fund construction Mt.
Haleakala station?
Start upsradins efforts for
improved
arrav
detector
automation of approp.
various FPls
svstems
FPI
stations
relocation of FPI stations where needed
Fund construction of WAKDII instrument
project manasement and workshop prosram
FPI communication support
1988/1989
workshop and FPI communication support
support bistatic observatories
support additional
350 K
500 K
etalons at
Itev sites
350 K
80 K
85 K
165 K
300 K
150 K
1989/1990
complete FPI upsradins efforts
150 K
workshop> communication* manasement support 165 K
1990/1991
workshop
and FPI
communication support
Table 11.1 Continued
165
K
References
Baker, D., A. Steed, and A. T. Stair, Jr., "Development of Infrared interferometry
for upper atmospheric emission studies, Appl. Opt.,
1734, 1981
Banks, P.M., Energy sources of high latitude upper atmosphere, "Exploration of the
Polar Upper Atmosphere" Eds. Deehr and Holtet, Reidel 1981.
Burnside, R.G., J.C.G. Walker, R.A. Behnke, and C.A. Gonzales, Polarization
electric fields in the nighttime F layer at Arecibo, J. Geophys. Res., 88, 6259, 1983.
Cogger, L.L. and H.C. Carlson, An experiment to measure variations in nighttime
E-region neutral concentration. Radio Sci., 12, 261, 1977.
Cocks, T.D., and F. Jacka, Daytime thermospheric temperatures, wind velocities
and emission intensities from ground-based observations of the 01 630 nm airglow line
profile. Jour. Atmos. Terr. Phys. 41, 401-415, 1979.
Hecht, J.H., A.B. Christensen, J.B. Pranke, W. T. Chater, C. K. Howey, R. L. Lott,
and M. G. Sivjee, An auroral and airglow Fabry-Perot spectrometer. Rev. of
Sci. Instr., in press
Henricken, K., g. G. Sivjee, C. S. Deehr, and H. K. Myrabo, Ground-based
observations of OgCb 1 a + -X3 a — ) Atmospheric bands in high latitude auroras.
g
g
J. Geophys. Res., in press
Hernandez, G., and R.G. Roble, Direct measurements of nighttime thermospheric
winds and temperatures 2, Geomagnetic storms. Jour. Geophys. Res. 81, 5173, 1976.
Kerr, R.B., S.K. Atreya, J.W. Meriwether, R.G. Burnside, and C.A.
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Simultaneous Ha line profile and radar measurements at Arecibo, J. Geophys. Res., in
press, 1985.
53
Killeen, T.L., R.G. Roble, R.W. Smith, N.W. Spencer, J.W. Meriwether, Jr.,
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and L.H. Brace, Ion-neutral Coupling in the high latitude F-region: Evaluation of ion
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