177_189 adg v–5 n01 W.pdf
ANNALS OF GEOPHYSICS, VOL. 45, N. 1, February 2002
177
Mapping plasma structures in the
high-latitude ionosphere using beacon
satellite, incoherent scatter radar and
ground-based magnetometer observations
Jürgen Watermann (1), Gary S. Bust (2), Jeffrey P. Thayer (3), Torsten Neubert (4)
and Clayton Coker (2)
(1) Danish Meteorological Institute, Copenhagen, Denmark
(2) Applied Research Laboratories, The University of Texas at Austin, TX, U.S.A.
(3) SRI International, Menlo Park, CA, U.S.A.
(4) Danish Space Research Institute, Copenhagen, Denmark
Abstract
In the autumn of the year 2000, four radio receivers capable of tracking various beacon satellites were set up along the
southwestern coast of Greenland. They are used to reconstruct images of the ionospheric plasma density distribution
via the tomographic method. In order to test and validate tomographic imaging under the highly variable conditions
often prevailing in the high-latitude ionosphere, a time interval was selected when the Sondrestrom incoherent scatter
radar conducted measurements of the ionospheric plasma density while the radio receivers tracked a number of
beacon satellites. A comparison between two-dimensional images of the plasma density distribution obtained from
the radar and the satellite receivers revealed generally good agreement between radar measurements and tomographic
images. Observed discrepancies can be attributed to F region plasma patches moving through the field of view with a
speed of several hundred meters per second, thereby smearing out the tomographic image. A notable mismatch
occurred around local magnetic midnight when a magnetospheric substorm breakup occurred in the vicinity of southwest
Greenland (identified from ground-based magnetometer observations). The breakup was associated with a sudden
intensification of the westward auroral electrojet which was centered at about 69° and extended up to some 73°
corrected geomagnetic latitude. Ground-based magnetometer data may thus have the potential of indicating when the
tomographic method is at risk and may fail. We finally outline the application of tomographic imaging, when combined
with magnetic field data, to estimate ionospheric Joule heating rates.
Key words high-latitude ionosphere – ionospheric
tomography – magnetospheric substorm – auroral
electrojet – Joule heating
1. Introduction
The high-latitude ionosphere is strongly
coupled to the magnetosphere and influenced by
its dynamic variations. The magnetosphere
responds to variations of the solar wind (a stream
of ionized particles, predominantly protons and
electrons) and the embedded interplanetary
magnetic field and thus ultimately to solar activity.
The most important solar wind parameters which
have significant bearing on the state of the
Mailing address: Dr. Jürgen Watermann, Danish
Meteorological Institute, Solar-Terrestrial Physics Division,
Lyngbyvej 100, DK-2100 Copenhagen Ø, Denmark;
e-mail: jfw@dmi.dk
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Jürgen Watermann, Gary S. Bust, Jeffrey P. Thayer, Torsten Neubert and Clayton Coker
magnetosphere are plasma density, temperature,
bulk speed, and the magnitude and orientation of
the interplanetary magnetic field. Variations in
these parameters lead to variations of the state of
the magnetosphere through electrodynamic
interaction. They are eventually propagated to the
high-latitude ionosphere via electric fields, field-
aligned currents and energetic particle precip-
itation. The coupled solar wind-magnetosphere-
ionosphere system is generally very dynamic
during and just after solar maximum (the most
recent of which occurred in the years 2000-2001).
Since solar wind-magnetosphere-ionosphere
interaction is most direct in the auroral zone and
polar cap in a broad sense (i.e. at geomagnetic
latitudes exceeding some ± 60°) the high-latitude
ionosphere can be highly variable on time scales
of solar wind changes, which may be less than a
few minutes. Ionospheric effects at high latitudes
are particularly dramatic when the interplanetary
magnetic field is oriented southward (i.e. opposite
to the main geomagnetic field), in which case field
line merging can occur, or when its orientation
fluctuates a great deal.
In recent years, with the advent of satellites
equipped with stable multi-frequency radio
transmitters (beacon satellites) the method of
ionospheric tomography has become an important
tool for imaging the ionospheric plasma density
distribution. Various measurement techniques,
based on satellite-to-ground or satellite-to-satellite
observations of the Total Electron Content (TEC),
and different inversion schemes have been
developed. Initially, a differential Doppler
technique was applied simultaneously to a meridian
chain of receivers in order to determine the
latitudinal variation of TEC profiles (Leitinger
et al., 1984). Subsequently, computer tomography
was developed to produce 2D images of the
ionospheric plasma density (Austen et al., 1988).
The image reconstruction algorithm was further
developed in various ways (e.g., Vasicek and
Kronschnabl, 1995) to improve its accuracy. The
tomographic method has successfully been applied
to the mid-latitude ionosphere (e.g., Bust et al.,
1997) where the ionosphere tends to be stationary
over the time interval needed for tomographic
inversion (typically up to 20 min, depending on
the satellites tracked). Application of the method
to high geomagnetic latitudes (auroral zone and
polar cap) has also been reported (e.g., Mitchell
et al., 1995; Pryse et al., 1997) though little
experience has yet been gained. Bernhardt et al.
(1998) present tomographic images of the iono-
spheric trough and of narrow auroral arcs obtained
at high latitudes under stable ionospheric
conditions.
Since the ionosphere tends to be very variable
at high geomagnetic latitudes, where its state may
change on time scales shorter than those needed
to collect reasonably complete satellite
observations for reconstructing tomographic
images, satisfactory performance of the
tomographic method is not yet confirmed. In order
to address some of the problems of tomographic
imaging at high latitudes, a chain consisting of
four satellite receivers was set up along the
southwestern coast of Greenland, colocated with
several ground-based magne-tometer stations and
the Sondrestrom Incoherent Scatter Radar (ISR)
facility. The ionospheric plasma density
distribution, among other parameters, can be
derived from ISR measurements on relatively
short time scales and be compared with
tomographic images obtained from beacon
satellite observations. We thus have a procedure
at hand to test the tomographic method at high
latitudes, validate it if possible, and try to
determine under which conditions it fails.
Similar verification concepts were employed
by Foster et al. (1994) and Mitchell et al. (1995).
The former examined storm-time observations of
the mid-latitude ionosphere and showed that large-
scale discrete F region ionization enhancements
and a deep ionospheric trough were recognized
in both, incoherent scatter radar data and
tomographic images. The latter demonstrated, by
comparing tomographic images with EISCAT
observations of the ionospheric plasma density,
that the tomographic method can render accurate
results in the auroral zone if the ionosphere
remains stable.
Next we give a description of the experiment,
specifically of the instruments used. This is
followed by an evaluation of the performance of
the tomographic method. We then outline the
estimation of Joule heating as a science application
well suited for the particular situation in
Greenland. We finally summarize the results and
elaborate on future work.
179
Imaging high-latitude ionospheric plasma structures
2. Experiment description
A map of Greenland is reproduced in fig. 1. It
shows the magnetometer sites of the Danish
Meteorological Institute (DMI), the satellite
receiver stations of the Applied Research Labo-
ratories at the University of Texas, Austin
(ARL:UT), and the Sondrestrom Incoherent Scatter
Fig. 1. Greenland with the magnetometer sites of
the Danish Meteorological Institute, DMI (full
black circles with three-letter station codes), the
satellite receiver stations of the Applied Research
Laboratories at the University of Texas, Austin,
ARL:UT (full gray diamonds with numbers), and
the Sondrestrom (STF) Incoherent Scatter Radar
facility operated by SRI International (open black
diamond). Geomagnetic latitude contours (dashed)
are plotted in addition to geographic coordinates
(dotted), and the ISR antenna scan trace at 350 km
altitude is marked by a heavy line.
Radar facility operated by SRI International.
Those instruments were used for this study and
are described below. The southern part of
Greenland is most often located in the auroral
zone and the central and northern parts in the
polar cap. The boundary between auroral zone
and polar cap is often found in the vicinity of
Sondrestrom but can also shift considerably in
latitude, depending on the local time and
interplanetary medium. We are thus in a situation
where we apply tomographic imaging to the
latitudes which cover both, auroral zone and
polar cap.
2.1. Satellite radio receivers
The Coherent Ionospheric Doppler Receiver
(CIDR) was developed at ARL:UT. It is capable
of tracking the coherent beacon from the Navy
Ionospheric Measurements Satellites (NIMS) as
well as other satellites including RADCAL,
ARGOS and GFO, and can receive three different
satellites simultaneously at data rates up to 1 kHz.
The measurements from an array of receivers are
converted into relative TEC profiles and ingested
into an inversion algorithm which renders a three-
dimensional image of the ionospheric plasma
density distribution. For an outline of the al-
gorithm consult a recent paper by Bust et al.
(2001) and references therein. For our paper, two-
dimensional maps in a specific vertical plane were
extracted from the 3D inversion results in order
to facilitate comparison with radar and
magnetometer measurements. The tomography
algorithm requires, in its present form, an
ionospheric plasma density pattern which is
stationary over the time of the radio reception,
typically between 10 and 20 min.
2.2. Incoherent scatter radar
The Sondrestrom Incoherent Scatter Radar
(Kelly, 1983) utilizes techniques which allow the
direct measurement of basic ionospheric plasma
parameters along the radar beam, including
electron number density, electron and ion
temperatures, and ion line-of-sight velocity. The
32-m parabolic antenna is fully steerable which
180
Jürgen Watermann, Gary S. Bust, Jeffrey P. Thayer, Torsten Neubert and Clayton Coker
allows, for instance, to determine the plasma
density distribution in a vertical plane oriented
along the magnetic meridian through Sondrestrom.
We operated the radar in a mode in which the
antenna performs a sequence of elevation scans
between geomagnetic north (at a declination of
27° from geographic north) and geomagnetic
south. Each scan started at an elevation of 30°
toward geomagnetic north, went through the zenith
to geomagnetic south, stopped at an elevation of
30°, and returned the same way but in opposite
sense, with a one-way scan taking about four
minutes to complete. The antenna speed changed
with elevation, using a lower velocity at lower
elevation and higher velocity overhead, in order
to provide equal ground distance increments
irrespective of elevation angle. This also com-
pensates for the greater signal power loss for
lower elevation angles (longer distance from
ionosphere).
We used a signal integration time of 20 s which
overhead is equivalent to about 100 km integration
width at 350 km. The 1.3 GHz two-channel radar
transmitter was operated using an alternating code
and a 320-µs long pulse the latter of which gives a
range resolution of about 48 km with sufficient
return signal strength from the F region. The
alternating code provides 3-km range resolution
but F region backscatter is usually too weak to
render reliable results. Tomographic images in the
antenna scan plane were then compared with two-
dimensional electron density maps produced from
individual 4-min ISR scans centered on the time
interval used for tomographic reconstruction.
2.3. Magnetometer chain
The Greenland west coast magnetometer chain
(Friis-Christensen et al., 1985) comprises 12
variometer stations along the west coast of
Greenland, complemented by three geomagnetic
observatories (colocated with variometer stations).
The stations are approximately lined up along the
same corrected geomagnetic (CGM) meridian so
that they order quite naturally according to CGM
latitude. The magnetometers sense the magnetic
field of electric currents in the ionospheric E region
(basically Hall currents, also known as auroral
electrojets). The magnetograms can be numerically
inverted to infer the temporal and spatial dis-
tribution of the equivalent ionospheric current (thin-
sheet ionospheric Hall current assumed to flow
at a mean E region height of between 110 and
115 km) crossing the magnetometer chain (Popov
et al., 2001). The current density depends linearly
on the product of electric field and Hall con-
ductance (height-integrated conductivity) the latter
of which depends primarily on the electron density
profile, c.f. Section 4. Significant variations in the
current strength indicate significant variations of
the electric field or the electron density or both.
The magnetometer chain can thus serve as an
indicator of a changing ionosphere which poten-
tially poses problems to the tomographic method
by rendering its assumption of stationary plasma
density distribution invalid. On the other hand, can
the magnetometers serve to complement to-
mographic images by providing a means to
estimate the ionospheric electric field and Joule
heating rate.
3. Performance evaluation
In order to test the reliability of the tomo-
graphic method at high magnetic latitudes under
various ionospheric conditions, we compared
tomographic images obtained over southwest
Greenland with measurements from the
Sondrestrom ISR.
The time interval chosen for comparison covers
the night from September 29, 2000, 2145 UT
through September 30, 2000, 0500 UT during
which the radar operated in the mode described
above. This interval compares to 1820-0135 Solar
Local Time (SLT) and 1930-0245 Magnetic Local
Time (MLT). The ionosphere was practically
absent at E region altitudes, and only F region
measurements above some 170 km are compared.
It appears that the F region is sufficiently dynamic
to pose problems so that it can well serve to test
the tomographic method.
Figure 2 shows the F region plasma density
obtained from ISR measurements at a reference
altitude of 350 km in a latitude-time diagram. The
plasma density appears to vary significantly over
latitude and time. At 0400 UT, for instance, we
find 1.5 1011 m 3 around 72° and more than 1012 m 3
poleward of 75° geomagnetic latitude. More critical
181
Imaging high-latitude ionospheric plasma structures
to the tomographic method is the temporal variation
which becomes apparent through a sequence
of high-density ( 1012 m 3) plasma patches about
two degrees wide in latitude which move through
the ISR scan plane with an equatorward velocity
component between 200 and 600 m/s. Seven 3D
tomographic images were obtained during this time
period, and cross sections along the radar scan
plane were extracted. The satellite reception time
intervals which were used for image reconstruction
are marked by rectangles along the top and bot-
tom abscissae.
Five out of the seven tomographic images show
good agreement with ISR electron density maps
inferred from scans centered on the reconstruction
intervals. The 0247 UT case is shown as an example
of good agreement in fig. 3a. The tomographic
image taken around 0247 UT (contour lines) is laid
over the plasma density map obtained from the
0247-0251 UT ISR scan (color-coded). The display
is centered on Sondrestrom, with ground distance
projected from range and elevation on a spherical
earth, such that «0» refers to the Sondrestrom ISR
location. The two density peaks at about 350 km
height, one above the radar and the other some 500-
550 km to the north, are well reproduced in the
tomographic image as is the total extent of the
plasma structure. The fit would be even better if
the tomographic image were shifted down in
altitude by 20-30 km.
Fig. 2. F region (350 km altitude) plasma density in a latitude-time diagram, obtained from ISR measurements. Each
rectangle represents one 20-s integration cell expanded to the 8-min duration of the respective back-and-forth scan.
The full rectangles along the top and bottom abscissae represent the time intervals from which satellite radio signals
were used to reconstruct tomographic images.
182
Jürgen Watermann, Gary S. Bust, Jeffrey P. Thayer, Torsten Neubert and Clayton Coker
Fig. 3a,b. a) Electron density map from ISR measurements in the vertical plane of the antenna scan which lasted
from 024649 UT through 025052 UT, and tomographic image reconstructed in the same reference plane as the
antenna scan, centered on 0247 UT. 0 km ground distance and 0 km range refer to the Sondrestrom ISR site (STF).
b) as (a) but for the 010348-010751 UT radar scan and the 0106 UT tomographic image, respectively.
a
b
183
Imaging high-latitude ionospheric plasma structures
Fig. 4. Low-pass filtered magnetic field variations along the Greenland west coast (5 min cutoff), separated into
geomagnetic north, east and vertical components. Since all stations approximately line up along the same corrected
geomagnetic (CGM) meridian the magnetic field amplitude is plotted in a CGM latitude versus time diagram. The
stations are listed along the right-hand ordinate at their respective CGM latitude. The magnetic field intensity is color-
coded and white areas denote missing data.
184
Jürgen Watermann, Gary S. Bust, Jeffrey P. Thayer, Torsten Neubert and Clayton Coker
The 0106 UT tomographic image shows
partial agreement with simultaneous ISR
observations (fig. 3b). The enhanced plasma
density structures at the poleward and equa-
torward edges of the ISR antenna scan field, again
at about 350 km height, are well reproduced in
the tomographic image. The latter, however,
shows an additional 350-km altitude peak
200-250 km south of STF, i.e. around 72°
geomagnetic latitude, which does not appear in
the 0104-0108 UT radar scan, and a minor peak
at 500 km altitude on a field line just north of the
radar. The major peak does appear, though, in
the preceding ISR scan, a few minutes earlier.
That scan falls within the satellite reception
interval (cf. fig. 2) so that the structure seen in
that scan but not later is likely to be representative
for the extra peak resolved in the tomographic
image.
The 0145 UT image (not shown) agrees poorly
with ISR measurements. This, too, can possibly
be explained by the particularly disturbed state of
the high-latitude ionosphere. The ISR scan
sequence coincident with the tomographic
reconstruction interval (0135-0153 UT) reveals first
the development and decay of mixed plasma
density enhancements lacking uniform propagation
characteristics, and then, from 0147 UT on, a new
well defined equatorward moving plasma patch.
The latter interval (after 0147 UT) is largely
(though not entirely satisfactorily) reproduced in
the tomographic image while the former, more
turbulent interval, is not.
The poorly matching 0145 UT case is
interesting because of the geophysical conditions
prevailing as inferred from ground-based magnetic
field observations. Figure 4 reveals that the
horizontal magnetic field in the equatorward
Fig. 5. Stacked high-pass filtered magnetic field variations along the Greenland west coast (15 min cutoff ), separated
into geomagnetic north, east and vertical components.
185
Imaging high-latitude ionospheric plasma structures
Fig. 6. E region (130 km altitude) plasma density distribution in a format similar to fig. 2.
section of the magnetometer chain was positive
(northward) in the beginning and decreased after
23 UT to values around zero. At about 0145 UT it
decreased suddenly to become strongly negative
(southward). The vertical component was negative
(upward) in the beginning, approached zero after
23 UT and at 0145 UT increased suddenly,
simultaneously with the horizontal field, thereby
exhibiting a steep latitudinal gradient from negative
(upward) below 70° to positive (downward) above
70° CGM latitude. This is equivalent to a westward
ionospheric current centered at about 70° and
extending up to some 73° CGM latitude. The elec-
trojet intensification is accompanied by the start
of magnetic Pi2 pulsations at 0145 UT with highest
amplitudes between 66° and 70° CGM latitude and
lasting for more than half an hour (fig. 5). Taken
together these observations are convincing
substorm onset indicators. ISR measurements of
the plasma density at E region height confirm our
view. Figure 6 shows, in a format similar to fig. 2,
that the plasma is tenuous at 130 km altitude prior
to 0145 UT but increases to some 3 1010 m 3 after
0145 UT (in the middle of the night, at 2220 SLT
and 2330 MLT). This is most likely a consequence
of auroral electron precipitation (electrons with a
characteristic energy of several keV). It may turn
out that the onset of a magnetospheric substorm
will frequently pose a problem to the tomographic
method, but a firm conclusion can only be drawn
once more cases have been investigated.
For the six cases of good and partial agreement
between ISR and tomography images we attempted
to determine whether the match is improved by
shifting the images against each other, in altitude
as well as horizontally. This was done in a semi-
186
Jürgen Watermann, Gary S. Bust, Jeffrey P. Thayer, Torsten Neubert and Clayton Coker
quantitative way by fitting the peaks and
certain contours visually. It appeared that no
substantial improvement can be gained by
shifting the images. Figure 7 shows the shifts
applied to the tomographic images against the
radar maps in order to obtain the best match.
They can amount to some 40 km in horizontal
and 30 km in vertical direction which is not
more than the inherent resolution of the radar
and radio beacon measurements in this
experiment. The figure further reveals that the
shift is probably randomly distributed. We are
thus confident that tomographic reconstruction
was not systematically biased with respect to
ISR observations.
4. Joule heating inference
Inference of ionospheric Joule heating rates
constitutes one of our research objectives and one
reason for operating satellite receivers colocated
with magnetometers. Joule heating is among the
important high-latitude processes of transferring
energy from the magnetosphere to the upper
atmosphere, which ultimately means energy from
the sun via the solar wind into the atmosphere.
Joule heating is thus a process which governs an
important part of the effect the dynamic sun
exercises on the terrestrial environment. While
incoherent scatter radar measurements remain a
superb way to determine Joule heating rates, they
suffer from limited radar operation time, a
constrained radar field-of-view, and a multitude
of antenna and transmitter modes not all of which
are suitable for that purpose. A combination of
ground-based magnetometer observations
(basically measurements of the ionospheric Hall
current) and tomographic plasma density images
yields, under certain conditions, estimates of the
Joule heating rate on a fairly regular basis.
Magnetometer measurements are performed
continuously, and radio satellite reception occurs
about 20 times per day, though at irregular time
intervals.
Since most of the energy transfer from the
magnetosphere into the upper atmosphere occurs
in the lower ionosphere (below some 200 km
altitude), sufficient resolution of the ionospheric
E region is necessary for our method to work.
The capability of the tomographic method to
resolve an E region plasma density enhancement
was demonstrated by Mitchell et al. (1995) who
used almost simultaneous EISCAT incoherent
scatter radar and beacon satellite observations
above Northern Scandinavia. Our tomography
versus ISR comparison discussed in the previous
section is based on night time observations when
E region ionisation was practically absent.
However, prior to the start of the ISR operation,
tomographic images were reconstructed which
indeed show the presence of plasma in the E
region, see fig. 8. Note that the plasma density
increases to more than 10 · 1010 m 3 just below a
5 · 1010 m 3 valley observed at 150 km altitude in
the center, and also in a blob located at – 800 km
ground range and 100 km altitude. We lack,
however, quantitative confirmation from radar
observations.
If we impose the condition of zero neutral
wind speed in the ionosphere as a first ap-
proximation, we can derive Joule heating rates
40 20 0 20 40
40
20
0
20
40
Plasma Density Image Shift
Tomography vs. ISR
<
d
o
w
n
[k
m
]
u
p
>
< south [km] north >
Fig. 7. Horizontal and vertical shifts applied to the six
tomographic images which agreed well or partially with
Sondrestrom ISR observations, in order to achieve best
agreement with the ISR plasma density maps.
187
Imaging high-latitude ionospheric plasma structures
in the following way. The ionospheric Pedersen
and Hall current densities, j
P
and j
H
, read
(4.1)
where
P
and
H
denote the Pedersen and Hall
conductivities, respectively, and E the (horizontal)
electric field. Since the ionospheric electric field
does not change along the magnetic field line
because of the prevailing high parallel conductivity
which exceeds the Hall and Pedersen conductivities
by orders of magnitude, height-integrated conduc-
tivities and current densities can be used and are in
the following represented by capital letters. From
eq. (4.1) we obtain
(4.2)
and further
(4.3)
which leads to an estimate of the height-integrated
Joule heating rate, W, in the ionosphere
(4.4)
W = J E = JP
P
H
H .2
2
In order to determine electric field and Joule
heating in the ionosphere we require information
about the height-integrated Hall current density and
the Pedersen and Hall conductances. The height-
integrated Hall current, J
H
, can be approximated
by the equivalent ionospheric current (basically a
thin-sheet Hall current) which is derived from
magnetometer measurements. The technique is
well developed, and the algorithm which is applied
to the Greenland west coast magnetometer chain
is described in detail by Popov et al. (2001). The
ionospheric conductivities depend primarily on
theelectron number density, N
e
, and read (e.g.,
Watermann et al., 1993)
(4.5)
Here, the subscripts «e» and «i» stand for electrons
and various ion species with partial pressure
(relative abundance) p
i
, respectively. The ion
population consists primarily of atomic and
molecular oxygen and nitric oxide. The electron
and ion gyro frequencies,
e
and
i
, are determined
by the geomagnetic field, B0, which is accurately
modeled by the International Geomagnetic
Reference Field (IGRF), c.f. Mandea and
Macmillan (2000). The electron and ion collision
rates,
e
and
i
, reflect mainly collisions with neu-
tral atoms and molecules since the neutral number
density in the lower ionosphere is orders of
magnitude higher than the ion number density. The
neutral number density is well represented by the
MSIS-86 model (Hedin, 1987) and much less
variable than the electron number density. The main
unknown variable in these equations is therefore
the electron density, N
e
. Once the plasma density
Fig. 8. Tomographic image reconstructed from satellite
passes around 2000 UT, prior to the start of the in-
coherent scatter radar operation.
P, H P, Hj = E
H H
H
H
= E E = J
J
P P
P
H
HJ = E = J
P e
e e
e e
=
e
B
N
+
+
0
(
2 2
i i
i i
i+
+
p
ions
2 2
)
H e
e
e e
=
e
B
N
+ 0
(
2
2 2
i
i i
i
+
p
ions
2
2 2
) .
188
Jürgen Watermann, Gary S. Bust, Jeffrey P. Thayer, Torsten Neubert and Clayton Coker
profile along the magnetic field line is known, e.g.,
from tomographic images, the height-integrated
conductivities can be modeled via eq. (4.5), and
the Joule heating rate can be inferred with the help
of the equivalent ionospheric current via eq. (4.4).
Work on this topic is currently in progress.
5. Summary and outlook
We compared seven cases of simultaneous
tomographic images and ISR electron density maps
from the high-latitude ionosphere and obtained in five
cases good and in one case partial agreement. The
lack of agreement could be traced back to the presence
of plasma patches moving with a few hundred met-
ers per second through the field-of-view. The
tomographic method employed tends to reproduce
the integrated effect of the plasma structures moving
through the imaging plane during the satellite
reception interval. But the method did not appear to
produce results confused by the plasma motion and
entirely misleading except for observations during
the onset of a moderate substorm which constituted
the worst agreement. Since a substorm can easily be
detected in ground magnetic field measurements,
such observations may possibly serve as a means to
predict the performance of the tomographic method
in specific cases. However, this needs to be further
investigated using a larger observational data base.
The result has potentially significant bearing on the
application of the tomographic method at high
latitudes not only to ionospheric imaging but also to
investigations of the lower atmosphere.
We have further outlined a method to combine
tomographic imaging with ground-based magn-
etometer observations to derive the ionospheric
electric field and Joule heating rate. This requires
accurate mapping of the ionospheric E region, and it
has yet to be confirmed that high-latitude tomography
can reliably resolve the E region plasma density
distribution. A test and validation of E region imaging
will be performed in a way similar to the one
described in this paper, namely by comparing
tomographic images with ISR maps of E region
plasma density, electric field strength and Joule
heating rates.
The assumption of negligible neutral wind speed
is not always justified. From an analysis of
Sondrestrom ISR measurements, Thayer (2000)
deduced that neutral winds can reduce the transfer
of electrical energy from the magnetosphere to the
upper atmosphere by some 20% for moderate
energy deposition rates (< 10 mW/m) and between
30% and 50% for higher deposition rates.
Refinements to the method outlined here are
needed in order to obtain a quantitatively more
realistic assessment of the Joule heating rate.
Acknowledgements
The tomographic receivers on the Greenland
west coast are operated by the Applied Research
Laboratories, The University of Texas at Austin
(ARL:UT), with financial support from the
National Science Foundation (NSF) through
grant ATM-9813864, and in collaboration with
the Danish Meteorological Institute (DMI). The
Sondrestrom incoherent scatter radar is operated
by SRI International under the NSF Cooperative
Agreement ATM-9813556 and in cooperation
with DMI. The Greenland ground-based
magnetometers are operated by DMI.
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