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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L18105, doi:10.1029/2006GL026581, 2006
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Identification of the temperature gradient instability as the source of
decameter-scale ionospheric irregularities on plasmapause field lines
Raymond A. Greenwald,1 Kjellmar Oksavik,1 Philip J. Erickson,2 Frank D. Lind,2
J. Michael Ruohoniemi,1 Joseph B. H. Baker,1 and Jesper W. Gjerloev1
Received 12 April 2006; revised 21 July 2006; accepted 31 July 2006; published 21 September 2006.
[ 1 ] Recent observations with the new mid-latitude
SuperDARN HF radar located at Wallops Island, Virginia
have identified a class of ionospheric irregularities that is
prevalent in the nightside sub-auroral ionosphere under lowto-moderate Kp conditions. These irregularities can be
observed for many hours and generally exhibit very low
Doppler velocities. A recent collaborative experiment using
the Wallops radar and the Millstone Hill incoherent scatter
radar has determined that these irregularities are located at the
ionospheric footprint of the plasmapause and in a region of
opposed electron density and electron temperature gradients.
We conclude that the irregularities are produced by the
temperature gradient instability (TGI) or by turbulent
cascade from primary irregularity structures produced from
this instability. This is the first experimental confirmation
that the TGI is effective in producing decameter-scale
ionospheric irregularities. Citation: Greenwald, R. A.,
K. Oksavik, P. J. Erickson, F. D. Lind, J. M. Ruohoniemi,
J. B. H. Baker, and J. W. Gjerloev (2006), Identification of the
temperature gradient instability as the source of decameter-scale
ionospheric irregularities on plasmapause field lines, Geophys.
Res. Lett., 33, L18105, doi:10.1029/2006GL026581.
1. Introduction
[2] In May 2005, the most recent addition to the
SuperDARN radar network was put into operation at the
NASA Wallops Flight Facility on Wallops Island, Virginia.
This radar, sited at L = 2.4, is the first SuperDARN radar
located below L = 3 and is ideally suited for the investigation
of storm-time ionospheric electric fields that cannot be seen
by the high-latitude SuperDARN radars due to the equatorward expansion of the auroral oval and high-latitude convection cells that occurs during geomagnetic storms. The
location also allows better determination of the low-latitude
extent of the high-latitude convection electric field under
weakly to moderately disturbed conditions and therefore
offers a new opportunity to study electric field penetration
from high to mid latitudes. During the first months of
operation, the radar observed a number of interesting
storm-time events and several other papers have been published or submitted for publication [Oksavik et al., 2006;
J. B. H. Baker et al., New views of ionospheric convection at
middle latitudes obtained from the Wallops high frequency
1
Johns Hopkins University, Applied Physics Laboratory, Laurel,
Maryland, USA.
2
Atmospheric Sciences Group, Haystack Observatory, Massachusetts
Institute of Technology, Westford, Massachusetts, USA.
Copyright 2006 by the American Geophysical Union.
0094-8276/06/2006GL026581$05.00
radar during high and low geomagnetic activity, submitted to
Journal of Geophysical Research, 2006]. However, the radar
has also detected a lower-latitude zone of ionospheric irregularities, which appears to be a persistent feature of the
nighttime sub-auroral ionosphere under low-to-moderate
Kp conditions. The occurrence rate of these irregularities is
very high. During the geomagnetically-quiet month of
February 2006, these irregularities were observed on 19 of
the 27 days that observations were made and lasted, on
average, more than 7 hours each night. A recent collaborative
experiment between the Wallops radar and the Millstone Hill
incoherent scatter radar (ISR) has shown that these irregularities are located at the ionospheric footprint of the plasmapause. This experiment and the nature and significance of
these irregularities are the subject of this letter.
[3] After the Wallops radar began operations, it was soon
obvious that most of the backscatter returns at night under
low-to-moderate Kp conditions exhibited Doppler velocities
ranging from 30 – 90 m s1. These low velocities were
initially misclassified as due to ground backscatter, i.e.,
backscatter from the ground or ocean following an oblique
reflection off of the ionosphere. However, as solar activity
levels declined in the winter of 2005 – 2006, the persistent
nature of this backscatter prompted the Doppler data to be
examined in greater detail. It was discovered that there
was a systematic azimuthal variation of the Doppler velocity
measurements similar to that observed from ionospheric
irregularities at higher latitudes. The only difference was that
the ionospheric motions implied by these Doppler measurements were typically 100 m s1 or less. The question then
was what type of plasma instability process could produce
ionospheric irregularities in a subauroral region characterized
by low plasma drift velocity?
[4] In order to understand the ionospheric conditions
leading to the formation of these irregularities, a 2-day
experiment was scheduled for 22– 23 February, 2006 using
the Wallops radar and the Millstone Hill ISR. Measurements
were made from 1600 – 2400 LT on each of these days. Here
we only report data from the second day of the campaign after
a multi-point elevation scan for the Millstone radar had been
implemented. The ISR data show that the irregularities are
excited on the equatorward wall of the mid-latitude ionospheric trough in a region of opposed density and temperature
gradients. Recent studies by Yizengaw and Moldwin [2005]
have shown that this wall of the trough is magnetically
conjugate to the plasmapause, while the opposed temperature
and density gradients are the precise conditions required for
the development of the F-region temperature gradient instability (TGI) proposed by Hudson and Kelley [1976].
[5] The TGI is an instability within the general class of
collisional drift wave instabilities. It occurs in plasmas
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having density gradients, which leads to opposed zerothorder diamagnetic drifts of electrons and ions. If perturbations to the boundary occur, they grow due to the associated
formation of electrostatic fields directed from the high to low
density regions of the perturbed boundary. These fields in
combination with the ambient magnetic field cause (E B)/
B2 drifts that lead to further perturbation of the boundary.
[6] Drift waves do not propagate exactly perpendicular to
the magnetic field. There is a small, but finite wavevector
along the magnetic field, which leads to potential structure
along the magnetic field lines. The highly mobile electrons
rapidly flow along field lines into regions of excess ions,
thereby maintaining macroscopic charge neutrality and
establishing a Boltzmann relation between the perturbed
densities and potentials. The phase velocity of drift waves
transverse to the magnetic field in the plasma rest frame is
equal to the electron diamagnetic drift velocity. For the
density gradients considered in this letter, this velocity is of
order 102 m s1 within the F-layer of the ionosphere.
[7] The TGI is a form of universal instability and derives
its free energy from the opposed temperature and density
gradients. Although such instabilities are said to be universally unstable, they can, in fact, be damped by a conducting
layer that shorts out the electrostatic fields. This can either be
a metal plate in a plasma chamber or the E-region of the
ionosphere. This is why these instabilities are only observed
at night when the E-region conductances have dropped to
extremely low values. Further discussion of drift wave
instabilities in plasmas can be found in the work of Chen
[1984].
[8] Hudson and Kelley [1976] noted that the TGI might
serve as an energy source for SAR arc generation and also
explain VLF and ELF waves that are observed on plasmapause field lines by low-altitude spacecraft [e.g., Gurnett et
al., 1969; Kelley, 1972]. We suspect that irregularities produced by this instability have been detected previously with
HF and higher frequency radars. To our knowledge, the
clearest identification has been by Oksman et al. [1979] that
compared measurements from an oblique ionosonde located
at Lindau, Germany with measurements from the STARE
VHF radar located in Hankasalmi, Finland. These authors
(one of whom is the principal author of this letter) reported
that the Lindau ionosonde was able to observe a band of
irregularities equatorward of the STARE field-of-view in the
latitude range from L = 3.2 – 3.7 that was commonly present
during the nighttime hours. Oksman et al. [1979] suggested
that these irregularities were produced by the TGI of Hudson
and Kelley [1976], but they were unable to provide measurements to support their conjecture. This letter shows that their
conjecture was correct and discusses some of the potential
impacts of the identification of these irregularities.
2. Instrumentation
[9] The data sets used in this study were obtained from the
Wallops HF radar, the Millstone Hill ISR, and the NOAA 17
spacecraft. The Wallops HF radar detects coherent scatter
from electron-density irregularities that are extended along
the magnetic field. This can only be accomplished, if the
ionospheric plasma has become unstable and sufficient ionospheric refraction exists for the radar signals to become
orthogonal to the magnetic field in the unstable plasma
volume.
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[10] For this study, the Wallops radar was operated in a
standard 16-position azimuth-scanning mode with a dwell
time of 3 seconds in each beam direction and a scan repetition
time of 60 seconds. The data were sampled at 110 range gates
extending from 180 km to 5130 km with a range resolution of
45 km. Short dwell times were used, because of the large
Doppler variability that is occasionally exhibited by these
nominally low-velocity irregularities. The observations on
the two nights were predominantly from F-region ionospheric irregularities extending from 500– 1400 km in range
from Wallops Island, corresponding to latitudes 52<L<59.
The F-region is identified as the location of these irregularities based on two factors:
[11] 1. Wallops radar aspect angles at E-region altitudes
are significantly greater than 90 and will increase if there is
any refraction. At F-region altitudes, the aspect angles are
less than 90 and refraction will bring the SuperDARN
transmissions orthogonal to the magnetic field.
[12] 2. Millstone radar measurements at E-region altitudes
on this night (not shown) indicated electron densities significantly less than 1010 m3.
[13] Both of the 440 MHz antenna systems of the Millstone Hill ISR were used in this experiment. Overhead
measurements were obtained with the 68 m zenith-pointing
antenna and oblique measurements were obtained with the
46 m steerable dish. Beam 9 of the Wallops radar passes
overhead of the Millstone site and the steerable 440 MHz
antenna was directed along this beam at a geographic
azimuth of 33and elevation angles of 48.8, 28.8 and
18.8. Measurements were alternated between the zenith and
steerable antennas. The dwell time in each antenna position
was set to 240 s to assure adequate signal-to-noise ratio. This
was particularly important as the ionosphere currently decays
quite rapidly after sunset to densities below 1011 m3 even at
the F-region peak near 300 km altitude. The 300 km altitude
intersection of the four Millstone-radar positions with the
Wallops radar beam occurred at L = 53.9, 55.5, 56.9, and
58.2.
[14] Measurements of trapped and precipitating energetic
electrons and ions in the midnight sector were obtained from
the NOAA 17 and 18 spacecraft. In this paper, we show data
from one pass of NOAA 17.
3. Results
[15] The top three panels of Figure 1 show data from the
Millstone Hill radar for the time period 2200– 0500 UT on
22– 23 February 2006. The Kp indices covering this time
interval were 2-, 2- and 2, indicating fairly quiet conditions.
The topmost of the plots shows the electron densities at
300 km altitude along each of the four Millstone beam
orientations. It can be seen that the electron density decreased from 3 1011 m3 at the beginning of the experiment to as low as 3 1010 m3 at the end. Initially, the
meridional density gradient was rather modest since the Fregion was still illuminated by the sun, but it became much
steeper after 00 UT (F-region sunset) when the zenith antenna
continued to observe reasonable plasma densities at 300 km,
whereas the two most poleward directions of the steerable
antenna showed much lower values at the same altitude. The
48.8 elevation beam yielded densities that varied between
these two extreme values. The steepest part of the meridional
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Figure 1. Summary of Millstone Hill and Wallops radar
data from 22– 23 February 2006. The top three panels show
electron density, electron temperature, and scale length
measurements from the Millstone Hill radar. The pink
triangles mark the time that the positive gradient in electron
temperature reversed from poleward to equatorward. The
bottom four panels show profiles of backscattered power
from 4 beams of the Wallops radar.
gradient ranged from 54<L<57 from 0100 UT onwards. It
can also be noted that the steepest part of the zonal gradient
occurred from 0000 – 0130 UT in association with the postsunset decay of the E and F-region plasma. The zonal density
gradient was positive westward.
[16] The second panel of Millstone data shows the electron
temperatures measured at the 300 km intersection points. At
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and before sunset, the electron temperatures also had a
positive equatorward gradient consistent with solar EUV
heating. However, from 0030 – 0130 UT, there was a slow
change in the temperature distribution with the highest
temperatures shifting toward the north and significantly
lower temperatures being observed on the zenith antenna.
Several processes may contribute to the positive poleward
electron temperature gradient. One is Coulomb collisions
between thermal electrons and mirroring energetic electrons
or ions on the inner edge of the proton ring current. Another is
Landau damping of electromagnetic ion cyclotron waves
again located on the inner edge of the proton ring current
[Cornwall et al., 1971]. A third source that is not commonly
considered is cooling of thermal electrons on the plasmasphere side of the gradient through molecular conduction.
This third process becomes significant after sunset and is
indicated by the declining electron temperatures on the zenith
antenna of the Millstone Hill ISR. Most of the temperature
gradient was also observed from 54<L<57.
[17] Finally, in the third panel, we plot the magnitude of
the density and temperature scale lengths defined as ne/rne
and Te/rTe, respectively. These values were obtained from a
linear least-square fit of the three lowest latitude sets of
Millstone data. Since the temperature gradient changes
direction in the time interval from 0030 –0130 UT, we have
plotted the temperature scale lengths prior to the direction
change as a solid red line and the temperature scale lengths
after the direction change as a dashed red line. At approximately the time of the reversal, the temperatures are comparable on several beams of the Millstone radar and the scale
length becomes quite large.
[18] We have plotted the electron temperature and density
data in terms of scale lengths, because we believe the TGI
[Hudson and Kelley, 1976] is responsible for much of the
irregularity backscatter that we observe. Hudson and Kelley
[1976] assumed values of 150 km and 580 km at an altitude of
1050 km for the density and temperature scale lengths,
respectively. The instability growth rate is proportional to
the inverse product of the density and temperature scale
lengths assuming that the gradients are oppositely directed.
If the gradients are in the same direction, the configuration is
stable. The density and temperature scale lengths obtained
from the Millstone data at 300 km altitude were 400 km and
800 km, respectively. These values reduce the growth rate
of the instability by a factor of 3 –4 from the case considered
by Hudson and Kelley [1976], but do not prevent it from
occurring. Moreover, the ISR values have been smoothed
over 3 of latitude and so are unlikely to reflect the true
steepness that may occur at the plasmapause boundary.
[19] The four lower panels in Figure 1 represent the
relative backscattered power observed by the Wallops radar
along 4 of the 16 beam directions. Beam 3 is directed about
11.5 to the east of the geographic meridian, while Beam 9
passes overhead of Millstone Hill. The backscatter returns in
each of these panels are due to several sources and we discuss
each one briefly. First, there are returns from higher magnetic
latitudes over the time interval 2200 – 0150 UT. These returns
are due to ground backscatter after an oblique reflection of the
radar signal by the ionosphere. Each of the two packets of
returns shows a poleward migration of the strongest
responses. The poleward migration is due to the post-sunset
depletion of the ionosphere causing the ground footprint to
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Figure 2. Overflight of NOAA 17 adjacent to the field of
view of the Wallops radar at 0256 UT on 23 February 2006.
The backscattered signals are observed from 54<L<59 in a
region where there is a positive poleward gradient in trapped
energetic particle fluxes (indicated by dot-dashed lines).
There are no discernible precipitating particle fluxes (solid
lines) in this region.
move to greater ranges. At 2310 UT the operating frequency
of the radar was lowered to bring the ground footprint into the
latitude range of the ISR measurements. It is as this ground
return again migrated poleward that the first ionospheric
backscatter was observed.
[20] At 2340 UT, backscatter from ionospheric irregularities began to appear between latitudes of 56<L<59 on
most of the Wallops radar beams shown in the figure. At this
time the electron density in the sub-auroral E-region is fully
depleted and can no longer short out electrostatic waves
generated in the overlying F-region. However, it is clear from
the Millstone data that conditions for activation of the TGI do
not yet exist. We suspect that the initial irregularities are
produced by the same processes that produce the dusk scatter
reported by Ruohoniemi et al. [1988]. Ruohoniemi et al.
[1988] suggested that this scatter was associated with the
gradient drift instability (GDI) [e.g., Reid, 1968] and postulated that the density gradient was either associated with
the poleward wall of the trough or the zonal gradient
associated with post-sunset plasma depletion. Whether either
of these gradients is sufficient to produce the observed
irregularities in the present case is open to debate. However,
all features of the dusk scatter phenomenon, including a
typical duration of 1– 2 hours near dusk, are totally consistent
with our observations.
[21] Approximately 1.5 hours after the onset of the initial
ionospheric backscatter, the Millstone temperature gradients
reversed and began to steepen (Note the pink arrows on the
Millstone scale-length data and Wallops Beam 9). At that
time, the backscattered signals on Beam 9 weakened. We
interpret this as the interval in which the instability driving
the dusk scatter was declining in importance, while the TGI
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was growing. After 0130 UT, the poleward temperature
gradient steepened rapidly and a very strong zone of irregularities appears in the region of short electron density and
temperature scale lengths extending from 54<L<57. We
attribute these backscattered signals to irregularities produced by the TGI. The signals remain strong on the lower
beam numbers but weaken on the higher beams. The weakening is particularly obvious on Beam 9. This reduction in the
backscattered signal is caused by continued post-sunset loss
of electrons in the F-layer, leading to a weakening of the
backscattered signal and insufficient refraction to bend the
radar signals to orthogonality with the geomagnetic field.
The orthogonality condition appears to be more constraining,
since a subsequent 1 MHz reduction of the operating frequency at 0412 UT led to intensification and latitudinal
spreading of the backscattered signals on Beams 5, 7, and 9 of
the Wallops radar. This frequency decrease enabled greater
refraction of the transmitted and backscattered signals and
improved sensitivity to the TGI irregularities.
[22] In Figure 2, we relate the observed electron temperature gradients and plasmapause irregularities to the energetic
particle environment observed during a near overflight of the
Wallops radar field of view by NOAA 17 at 0256 UT on the
evening of February 23, 2006. The data show significant
poleward gradients in the trapped fluxes of energetic electrons and ions equatorward of L = 60, indicating that the
high-latitude heat source is due to Coulomb collisions
between the mirroring energetic particles and cold ionospheric electrons.
[23] Finally, we consider whether plasma drift and the
GDI have a significant impact on irregularity generation
after 0145 UT. Measurements with the Wallops radar have
shown the Doppler velocities of plasmapause irregularities
to generally be quite small. To confirm that this implies that
the plasma drifts are also small, we have compared Wallops
and Millstone Doppler observations. The red-arrowed solid
line on Beam 9 of Figure 1 is the location of the comparison. It corresponds to an altitude of 300 km on the 48.8
elevation-angle Millstone beam. In Figure 3, the Wallops
Doppler measurements for this location were determined
every minute over a 5 hour period and are plotted as the
solid red line. The Millstone observations were made every
20 minutes and are plotted as asterisks. The two observations are not exactly parallel, but the impact of the misalignment is small. We see that the two data sets are in
substantial agreement confirming that the plasma drift
speed in the plasmapause is quite small (generally less than
50 m s1). With these small drifts and the observed density
gradients, it does not appear that the GDI has the primary
role in creating plasmapause irregularities. The primary
Figure 3. Comparison of irregularity Doppler velocities
(red line) with plasma Doppler velocities (asterisks) at
300 km altitude for 00– 05 UT on 23 February 2006.
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mechanism appears to be the TGI. However, we do not
preclude that other processes including the GDI may contribute to the subsequent turbulent cascade.
[24] We note finally that there are times when the plasma
velocity at the plasmapause changes dramatically in both
speed and direction. These changes occur over very short
periods and they last from minutes to tens of minutes. We
shall discuss this variability in greater detail in a future paper.
4. Summary
[25] The results presented in this letter may be summarized as follow:
[26] 1. The SuperDARN HF radar located at Wallops
Island, Virginia has detected a persistent zone of nightside decameter-scale electron-density irregularities on subauroral field lines under low-to-moderate Kp conditions.
[27] 2. A coordinated experiment using the Wallops HF
radar and the Millstone Hill ISR has determined that the
irregularities lie on the plasmapause in a region of opposed
electron density and electron temperature gradients. These
background plasma conditions are consistent with irregularity generation via the TGI as proposed by Hudson and
Kelley [1976]. This is the first experimental confirmation
that decameter-scale irregularities on plasmapause field
lines are produced by the TGI or a cascade product from it.
[28] 3. We suggest the sources of the electron temperature
gradient to be as follows: In the poleward portion of the
irregularity zone, ionospheric electrons are heated via Coulomb collisions with trapped energetic particles. In the
equatorward portion, electron temperatures are reduced in
the post-sunset period due to cooling through molecular
conduction. The two effects are of comparable importance
in establishing the temperature gradient.
[29] 4. Plasma drift velocities at the nightside plasmapause
are typically less than 50 m s1, indicating the GDI does not
have a significant role in primary irregularity generation.
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[30] Acknowledgments. The Wallops HF radar was developed using
internal funds from the Johns Hopkins University Applied Physics Laboratory and the NASA Wallops Flight Facility. Particular thanks are given to
Michael Hitch for acquiring the NASA/WFF portion of the funding. This
work was supported under NSF cooperative agreements ATM-0418101
(SuperDARN) and ATM-0233230 (Millstone Hill) and NASA grant
NNX06AB95G. We thank NOAA/NGDC for providing the NOAA 17
energetic particle data.
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