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Transcript
Alfven Waves and Poynting Flux Observed Simultaneously by Polar and FAST in the
Plasma Sheet Boundary Layer
J. Dombeck,1 C. Cattell,1 J. R. Wygant,1 A. Keiling,2 J. Scudder3
1
School of Physics and Astronomy, Univ. of Minnesota, Minneapolis, MN
2
Space Sciences Laboratory, Univ. of California, Berkeley, CA
3
Institute of Geophysics and Planetary Physics, Univ. of Iowa, Iowa City, IA
1
Abstract. We present the first simultaneous observations of Alfven waves at Polar and FAST
altitudes, ~7 RE geocentric and ~3500 km respectively, at ~23 MLT in the main phase of a major
geomagnetic storm on 22 October 1999. We compare the Poynting flux for these waves and the
electron energy flux at the two spacecraft. We also present a new method of Alfven wave
analysis, examining Poynting flux magnitude and directionality along with the perturbation
electric to magnetic field ratio of these waves as a function of wave temporal scale (frequency).
The results of this analysis are compared with those expected from kinetic Alfven wave models.
There is a mean net loss of ~2.1 ergs cm-2s-1 (mW m-2) in earthward Poynting flux over the
altitude region between Polar and FAST; a mean net increase in earthward electron energy flux
of ~1.2 ergs cm-2s-1 over the same region; frequency characteristics consistent with a mixture
Alfven waves obeying the kinetic Alfven wave dispersion relation mixed with some coupling to
the ionosphere; and high frequency kinetic Alfven wave generation between Polar and FAST.
Current models are found to be generally consistent with the study results but are not yet
sufficient well formulated to account for the details, including evidence for temporal and/or
spatial modulation of reflectivity.
2
1. Introduction
Alfven waves propagating earthward along the plasma sheet boundary layer (PSBL) at Polar
altitudes, 4-7 RE geocentric, in the earth’s magnetotail have been shown to carry more than
sufficient Poynting flux to account for the aurora observed simultaneously on the field lines
mapping to the PSBL [Wygant et al., 2000, 2002; Keiling et al., 2000, 2002]. Also evident in
these observations were concurrent small perpendicular scale waves consistent with kinetic
Alfven waves. Kinetic Alfven waves [Lysak and Lotko, 1996; Lysak, 1998] have an electric field
component parallel to the background magnetic field and are therefore a possible mechanism for
auroral electron acceleration.
The PSBL Alfven waves were initially detected by searching the Polar Electric Field
Instrument (EFI) data for the most intense plasma sheet electric field measurements which
occurred during 1997 [Wygant et al., 2000], and then May 1996 through April 1998 [Keiling et
al., 2001]. Subsequently Chaston et al. [2000, 2002, 2003a, 2003b] reported observations of
Alfven waves and energized auroral electrons using FAST data, at 350-4000 km altitude, on field
lines that map to the PSBL. These Alfven waves were observed in regions both with and without
inverted-V’s. Through simulations they showed that the FAST observations were consistent
with an Alfven wave propagating earthward from the “outer magnetosphere”, i.e. as observed at
Polar, to FAST altitudes. These simulations indicated that a significant portion (>50%) of the
wave energy flux was converted into electron flux at altitudes of 1-3 RE due to increased parallel
electric fields in the kinetic Alfven waves which are accompanied by increased reflection due to
the large increase is VA in this region.
3
The current study expands on the results of these previous studies in several ways. First, we
report the first direct comparison of Alfven waves observed simultaneously at Polar and FAST,
while they were on close to the same field lines. We also investigate the field aligned Poynting
flux (S) signatures and the wave electric to magnetic field ratio of these Alfven waves at various
temporal scales (frequencies), comparing these characteristics of the waves between the two
spacecraft and with the results expected from a model of absorption and reflection of Aflvenic
Poynting flux below Polar presented by Strelsov and Lotko [2003]. This model indicated that
larger amplitude and larger scale size (corresponding to lower frequency in our study) should be
preferentially absorbed in the anomalous resistive layer.
2. Data
The primary subject of this study is the simultaneous observation of Alfven waves by both
Polar and FAST on 22 October 1999, ~02:10 UT, during a period of decreasing AE in the main
phase of a major geomagnetic storm. During the event both satellites traversed the northern
PSBL region field lines at ~23.2 magnetic local time (MLT). The situation is depicted in Figure
1. FAST transited the region during a northward pass at ~3500 km altitude and observed Alfven
waves and significant earthward Poynting flux from ~66.7º to 67.5º invariant latitude (ILAT),
traversing the region in <20 seconds, ~02:09:20-02:09:50 UT. Polar was moving outbound and
northward when it encountered the region of Alfven waves and spent ~15 minutes, ~02:05-02:20
UT, in the region at distances of ~7.1-7.3 RE geocentric. Due to the nature of the measurements,
including spacecraft and PSBL motion and the structure of the Alfven waves, the data from both
spacecraft contain a mix of temporal and spatial effects that cannot be readily separated. Also,
4
while the similarity in MLT, the proximity of the transition to the lobe field lines and the
latitudinal width of the region of observed Alfven waves in the data from each spacecraft
indicate a very close mapping of field lines, the differences in orbital velocity, region thickness
and motion of the field lines at the different radial distances of FAST and Polar result in different
measurement contexts at each spacecraft. This difference is apparent in Figure 1 where the bold,
dark blue portion of the orbit trajectories represent ~30 seconds for each spacecraft. The FAST
data represent more of a “snapshot” of the region while the Polar data represent a cross between
a snapshot and a temporal sampling of the region. Therefore, while there may be some spatial
structure correspondence between the observations at the two spacecraft, direct mapping of
individual structures is not straight forward, and the results presented in this paper will be limited
to a comparison of general characteristics of the regions.
Figure 2 shows the region of interest with FAST (Polar) data on the left (right). Panels a and b
shows electron and ion spectra respectively, while panels c-e depict the electron density, field
aligned wave Poynting flux (S) in the frequency range of interest (5 mHz to 4 Hz) and the total
net earthward electron energy flux. Both types of energy flux measurements are mapped to the
ionosphere, and positive is earthward for all plots in this paper. The electron energy flux plots,
panel e, show both the measured flux at the sampling rate, 0.63 (13.8) seconds for FAST (Polar),
in red, along with a thick dark line which shows the data smoothed by 5 (69) seconds. The
passes show each spacecraft moving from the plasma sheet (PS) field lines though the PSBL to
the first open lobe field lines. Three regions that topologically map between the spacecraft
passes are shaded in orange, gray and brown. The orange region is on the PS side of the PSBL
and is the region of comparison in this study. It is characterized by significant earthward
Poynting flux and, at FAST, broadband electrons. The region is ~0.45º in latitudinal extent at
5
both spacecraft and has relatively stable plasma conditions. The gray region is the central region
of the PSBL and is characterized by variability in plasma conditions and a higher ion
characteristic energy than the orange region. The brown region is on the lobe side of the PSBL,
characterized by low density with closed field lines that contain some higher energy particles.
This region ends at the last closed field line and the transition to the lobe field lines with polar
rain. The time span of the FAST orange region is depicted on the Polar data by the darker
orange band.
The differences of the gray region, particularly the existence of field aligned Poynting flux and
electron energy flux at Polar but not FAST, may be due to either an actual difference in this
region at the two spacecraft altitudes or a temporal effect, since the Polar pass through the gray
region is >5 minutes after the FAST pass through the corresponding region. Cluster observations
(Marklund et al., 2001) have shown that significant changes in the region mapping to auroral
acceleration can occur over time scales on the order of ~200 seconds. An interpretation of a
temporal variation in this region at Polar is further supported by the significant variability in
density and temperature (not shown) as well as the region being ~0.40º thick at FAST but only
~0.25º thick in latitudinal extent at Polar. For this reason, only the average characteristics in the
orange regions are compared in this study.
3. Methodology
Alfven waves have their electric and magnetic field perturbations, E and B respectively,
perpendicular to the background magnetic field, B0, and also perpendicular to each other. For
sheer Alfven waves traveling along the PSBL, E is generally aligned perpendicular to the PSBL
6
surface. This direction is denoted 1, while the direction perpendicular to B0 and the 1
direction, the general direction for δB, is denoted 2. This alignment makes both Polar and
FAST, whose spin axes are generally in the azimuthal direction relative to earth as depicted in
Figure 1, well suited to study these waves as both spacecraft provide vector electric fields
measurements in their spin plane and three dimensional magnetic fields measurements. Since
Polar provides full three dimensional electric field measurements, although the spin axis
component has substantially shorter booms, E2 and B1 on Polar were also examined to
compare the full parallel Poynting flux with the component calculated solely from E1 and B2
and to confirm the assumption of E being predominantly in the 1 direction. Including the
E2 and B1 signal was found to not significantly alter the results at Polar. Therefore only the
E1 and B2 signals were used so as to allow comparison of similar measurements at Polar and
FAST.
Both Polar and FAST provide electric and magnetic field measurement at 8 samples s-1 and
particle measurements in the regions of interest. For each spacecraft the electric and magnetic
field data were detrended and smoothed using eight overlapping temporal scale bands covering
the range pertinent for Alfven waves. In all cases these bands corresponded to the frequency
ranges: 5-20 mHz, 10-40 mHz, 25-100 mHz, 50-200 mHz, 100-400 mHz, 0.25-1 Hz, 0.5-2 Hz,
and 1-4 Hz. For reference, the fci is ~3 (220) Hz at Polar (FAST) during the simultaneous
observations. The field aligned Poynting flux (S, E1B2 /μ0) in units of ergs cm-2s-1 (or mW
m-2) and phase velocity (E1/B2) in km/s were calculated on a point by point basis for each
band. Using these values, the mean total and net earthward parallel wave energy (|S̄| and S̄
respectively, where the bar denotes the mean over the region) and the median E1/B2 ratio for
times of |S| >0.04 ergs cm-2s-1 (mapped to 100 km) observed during the orange band in Figure 2
7
were calculated for each frequency band. The characteristics of the time-series smoothed and
detrended data in each band were also examined to evaluate whether the Alfven waves were
traveling, standing or mixed and to evaluate the degree of coherency of the waves in the band.
Finally, the mean Poynting flux over the entire frequency band, 5 mHz-4 Hz, was calculated and
compared with the mean net electron energy flux in the region at both spacecraft.
4. Observations and Discussion
The region of comparison, the orange region in Figure 2, contains significant, variable,
earthward Poynting flux (S) over the Alfven wave frequency range at both spacecraft. When
mapped to the ionosphere, the Poynting flux has peaks of ~45 (~10) ergs cm-2s-1 at Polar (FAST)
with a mean net value of ~3.8 (~1.7) ergs cm-2s-1 in the frequency range of 5 mHz-4 Hz. This is
an average loss of ~2.1 ergs cm-2s-1 in Alfvenic flux in the entire latitude interval over the
altitude range from Polar to FAST. Figure 3 summarizes the frequency analysis for Polar (left)
and FAST (right) with frequency along the abscissa and the frequency bands for the plotted
points represented as the error bars at the bottom of panel c. The data depicted in panel a are the
mean total and net earthward Poynting flux, |S̄| in blue and S̄ in pink respectively, calculated as
described in section 3. Also included in panel a is the peak mapped Poynting flux, SMax in green,
in each frequency band. This can also be directly compared between events, although the peak
measurement is not necessarily indicative of S during the entire event. Panel b depicts the
percentage of S directed earthward in each frequency band. Panel c plots the median E1/B2
ratio as a phase velocity. Since E1/B2 is a ratio, the median value is used rather than the
mean. The error bars represent where 75% of the ratios for points with sufficient |S| fall in each
8
band. The light (dark) blue line in panel c represent the Alfven velocity, VA, for 100% H+ (O+),
and the dark green bar represents the E1/B2 ratio expected for static coupling to the
ionosphere. This ratio, in km s-1, is generally 800/P, where P is the Pederson conductivity in
mhos (generally in the range of 1-10 mhos). During the event a large scale, DC current, not
shown, was observed concurrently with the Alfven waves at Polar giving a direct measurement
of the E1/B2 ratio for ionospheric closure. This ratio was ~250 km s-1 yielding a P of ~3.2
mhos.
The Polar data shows that the mean total Poynting flux, |S̄|, has a monotonic falloff with
increasing frequency. The earthward portion of this Poynting flux also smoothly decreases from
~80-90% of the total Poynting flux in the lowest frequency bands through the O+ (H+)
gyrofrequency at Polar of ~0.2 (~2.8) Hz where S becomes roughly equally divided between the
upward and downward directions.
The FAST data, by contrast, indicate a nearly straight power law falloff for |S̄| and S̄ at lower
frequencies with nearly 100% of S directed earthward. Both fall off at a steeper rate with
frequency than at Polar. There is a break from the falloff in |S̄| in the mid-frequency bands, ~0.11 Hz, which correspond to the frequencies of the ionospheric Alfven resonator [Lysak, 1991,
1993]. S in these frequency bands is roughly evenly divided between upward and downward
going. This is consistent with the wave power in these frequency ranges being dominated by
local resonance enhancement making meaningful direct comparison with the signals at Polar at
these frequencies impossible. Finally, also in contrast to Polar, there is a significant amount,
~80%, of earthward directed S in the highest frequency bands.
Comparing magnitudes of |S̄| and S̄ in the low-frequency bands indicate a loss of ~70% (~90%)
in |S̄| and ~40% (~80%) of S̄ in the 5-20 mHz (25-100mHz) band from Polar to FAST. The
9
resulting average reflection (R) and dissipation (D) coefficients in this low-frequency range are
plotted in Figure 4. The dissipation is split between the portion above and below FAST, AF and
BF respectively. These results are generally consistent with frequency response predicted by the
model of Strelsov and Lotko [2003, Figure 12]. The major discrepancy between our results and
those presented by Strelsov and Lotko [2003, Figure 12] are the larger value of DBF resulting in a
corresponding decrease in DAF, particularly in the lowest frequency band. This discrepancy may
be the result of the combination of FAST’s altitude being higher than the low altitude boundary
of the anomalous resistivity region used in Strelsov and Lotko’s model and the effects of a
possible discrepancy between the perpendicular scale width used in the model versus those of the
observed waves which, unfortunately, cannot be directly measured.
As mentioned, the wave power in the mid-frequency ranges at FAST is dominated by local
resonance enhancement, as theoretically predicted, and does not give us information that can be
directly compared with the signals at Polar altitudes. The higher frequencies, >1 Hz, however,
result in perhaps the most interesting comparison. Here |S̄| and particularly S̄ are larger at FAST
than at Polar. This and the fact that these frequencies are at or above the local ion gyrofrequency
at Polar indicate that these waves are likely generated between the two spacecraft by an, as yet,
unspecified process. These waves are also partially reflected, reflectivity ~0.1, and absorbed
below FAST. Absorption between FAST and the ionosphere and in the ionosphere cannot be
separated out in our data.
The E1/B2 ratios at the various frequencies, panel c of Figure 3, provide further insight into
the properties of the waves at both altitudes. At Polar the E1/B2 plot shows the
characteristics of kinetic Alfven waves [Lysak and Lotko, 1996], with low frequency waves
being near VA, and higher frequency waves becoming more electrostatic. Our results, however,
10
differ from kinetic Alfven theory in that the E1/B2 ratios at the lower frequencies are closer
to VA-O+ rather than VA-H+. Figure 5 shows similar plots for two other Alfven wave events at
Polar, 1 May 1997, ~20:30 UT and 9 May 1997, ~05:40 UT. These events show similar
characteristics although the 22 October 1999 event has generally a somewhat lower E1/B2
than either of these other events. In all three cases E1/B2 is significantly higher than
expected from ionospheric closure and shows the general characteristics of kinetic Alfven waves.
While the TIMAS data available during the 22 October 1999 event indicate that there is some O+
present at Polar, degradation of the instrument for O+ at low energies (<700 eV) by that date
makes an exact determination of composition ratios for the event impossible. Further the
characteristic ion energy during the FAST pass was outside the range of the TEAMS instrument
making it impossible to determine ion composition from the FAST data either. TIMAS was
operational for both of the May 1997 events and indicated that, while O+ was present in both
cases, ~34% (~9%) in number density for the May 1 (9) event, the composition cannot account
for the values of E1/B2 at low frequencies.
The FAST E1/B2 ratios indicate that the lowest frequencies at FAST are coupled to the
ionosphere, while the highest frequencies are more Alfvenic. The fact that the low frequency
Poynting flux is ~100% earthward and coupled to the ionosphere is consistent with such waves
being less than one wavelength from the ionosphere. Their existence, however, indicates that a
portion of the wave energy observed at Polar is also coupled to the ionosphere. This would have
the effect of lowering the E1/B2 ratio at Polar below those expected from kinetic Alfven
wave theory.
Although previous Polar studies did not specifically consider the frequency dependence of the
E1/B2 ratio of the Alfven waves in relation to those predicted by kinetic Alfven theory, the
11
observations presented herein are consistent with the conclusions of those studies that kinetic
Alfven waves were present [Wygant et al., 2000, 2002; Keiling et al., 2002] and that the
E1/B2 ratios in general are significantly higher than expected from ionospheric closure.
Inspection of the time series data in the various frequency bands indicates that, at all
frequencies at Polar, during the 22 October 1999 and both May 1997 events, and at frequencies
>~100 mHz at FAST, there are intervals with standing Alfven waves and others with traveling
Alfven waves. This, rather than a consistent phase relation between the electric and magnetic
field signals, is the cause of the varying percentages of net earthward Poynting flux. In the
higher frequency bands at Polar, the traveling waves are generally observed propagating in both
directions at different times during any given event. These results are consistent with results
from a recent study by Keiling et al. [2005] that indicates that in large-amplitude Polar PSBL
substorm expansion Alfven wave events, there is a mixture of standing and traveling waves at all
frequencies with their low-, mid-, high- frequency (<14 mHz, 14-25 mHz, 25-167 mHz) waves
being predominantly earthward traveling waves, standing, and bi-directional traveling waves
respectively. Our results are also consistent with their finding that the low-frequency (<14 mHz)
waves were carrying most of the earthward S, and that the E1/B2 ratio generally tended to
increase with frequency. It should be noted that, although the highest frequency bands in this
study are above the local ion gyrofrequency at Polar, both standing and traveling waves that are
definitively Alfvenic in nature are observed. However, at these frequencies |S̄| is ~1/100 that of
the lower frequency bands and the directionality results in negligible net earthward flux at these
frequencies compared to the lower frequencies.
Examination of the electron energy flux, Figure 2 panel e, indicates a net mean flux of ~2.2 ergs
cm-2s-1 over the orange region at FAST. The electron spectrogram at FAST, panel a, indicate a
12
significant electron population at energies between the normal peaks due to the plasma sheet and
ionosphere, in the ~30-700 eV range, consistently during the period of intense Alfvenic Poynting
flux. The Polar electron spectrogram does not show consistent energy flux enhancements in the
same energy range concurrent with its observed intense Alfvenic Poynting Flux. Examination of
the FAST electron energy flux distributions in this region (e.g. Figure 6 panel a) indicate that
these electrons consist of an isotropic population with a peak electron energy flux at ~300 eV
and a warm field aligned population with an earthward drift. This second population, except for
the upward thermal component, is contained within the loss cone, as are a portion of the isotropic
population, resulting in these electrons being lost to the ionosphere and contributing to the
aurora. Figure 6 panels b and c are electron energy flux distribution from the same passes just
equatorward of the orange region. In a region that does not have significant Alfven wave
Poynting flux. Panel b shows the typical central PS isotropic distribution with a peak energy
flux of ~1 keV. Panel c shows a secondary peak in isotropic electron energy flux at ~100 eV.
The PSBL is a region of transition from the central PS temperature and density to the lower
temperature and densities of the lobe. Therefore having isotropic distributions at lower than the
central PS temperature is not unexpected. However, the extremely intense lower temperature
isotropic population which also swamps the typical central PS energy levels and the warm field
aligned population are observed in this pass only coincident with the region of large Alfvenic
Poynting flux.
The electron energy flux at Polar is much more variable. The mean value over the orange
region is ~1.0 ergs cm-2s-1 (when mapped to the ionosphere), ~1.2 ergs cm-2s-1 less than observed
at FAST, although the variability of the flux calls into question the utility of the mean. The high
variability also suggests that, in order to properly compare the electron energy flux at the two
13
spacecraft, a structure by structure comparison is likely necessary. Such a study is underway;
however, such a comparison is incompatible with the averaging necessary for the frequency
analysis used in this study and therefore, the results will be presented elsewhere. The existence
of consistent significant earthward electron energy flux within the region of Alfven waves at
FAST but not at Polar and the consistent existence of significant intermediate energy electrons,
particularly the warm field aligned population at FAST and not at Polar indicate that there is
likely a relationship between the Alfvenic Poynting flux loss and the acceleration of these
particles.
A reasonable interpretation of the results of this study is that a source tailward of Polar
produces Alfven waves of various frequencies along the PSBL with more wave energy at lower
frequencies. These waves contain modes that are variously coupled to the ionosphere, absorbed
or reflected in route. In the ~5-100 mHz frequency range, ~50-60% of the wave energy is
reflected, ~10-30% is absorbed above FAST, and <~30% is coupled to the ionosphere. These
results are generally consistent with the model of Strelsov and Lotko [2003], although their
model infers a consistent partial reflection, while our event indicates modulated times/regions of
completely traveling waves, completely reflected waves and some partial reflection. The mean
absorbed energy flux in this event is ~2.1 ergs cm-2s-1 between Polar and FAST altitudes with
part, up to ~ 1.2 ergs cm-2s-1, accelerating electrons which generate aurora. There is evidence
that the higher frequency waves are kinetic Alfven waves, that they are damped when they are
near or above the local ion gyrofrequency, and that they are also generated between Polar and
FAST. This generation mechanism is open to speculation, but the energy to generate them may
come from the lower frequency Alfven waves [Chasten et al., 2003b; Wu, 2003; Fedun et al.,
2004; Watt et al., 2004; Voitenko and Goossens 2005]. This gives a possible electron
14
acceleration method of low frequency Alfven waves being converted into kinetic Alfven waves
which contain a parallel electric field which then accelerates the electrons.
5. Conclusions
We have presented the first comparison of Alfven waves observed simultaneously at both Polar
at ~7 RE and FAST at ~2 RE geocentric. These waves were observed during a period of
decreasing AE during the main phase of a major geomagnetic storm on 22 October 1999 in the
PSBL at ~23 MLT. We presented an overview of the spacecraft passes and mapped the
corresponding regions at Polar and FAST, comparing the mean net Poynting flux (S) and
electron energy flux observed at the two spacecraft. For the Alfven waves, we also investigated
S and its directionality, and E1/B2 ratios in eight overlapping temporal scale (frequency)
bands ranging from 0.25 to 200 s (5 mHz - 4 Hz). The results were compared at the two
spacecraft and to the predictions from a model presented by Strelsov and Lotko [2003] and
kinetic Alfven wave theory [Lysak and Lotko, 1996].
We found:
1) There is a mean net loss in earthward Poynting flux of ~2.1 ergs cm-2s-1 between Polar and
FAST in the region of conjunction.
2) The frequency characteristics of the waves at Polar are consistent with kinetic Alfven waves
with some coupling to the ionosphere at the lower frequencies.
3) High frequency (>~1 Hz) kinetic Alfven waves are generated between Polar and FAST.
15
4) The waves in the low-frequency bands, <~0.1 Hz, account for most of the earthward Poynting
flux at Polar, and the reflectivity and dispersion characteristics of the waves at these frequencies
are generally consistent with the model of Strelsov and Lotko [2003].
5) The reflectivity is temporally and/or spatially modulated rather than being continuously
partially reflective.
6) There is evidence for acceleration of electrons between Polar and FAST with a mean net
electron energy flux gain of up to ~1.2 ergs cm-2s-1.
Taken together these conclusions indicate that current models for auroral acceleration from
kinetic Alfven waves, e.g. Chasten et al. [2002, 2003a] is consistent with the overall Poynting
and electron energy flux data. The kinetic Alfven wave model of Strelsov and Lotko [2003],
which includes the effects of anomalous resistivity, is also consistent with the flux data.
However, current models are not yet sufficiently well formulated to fully account for the
observations. Comparing Alfven waves at Polar and FAST simultaneously provides a tool for
investigating the details of the process, including the reflectivity modulation and the generation
of the higher frequency earthward directed Alfven waves which were observed at FAST.
Investigating these phenomena can lead to a more complete understanding of the auroral
acceleration process.
16
Acknowledgments: The authors would like to thank W. L. Peterson At LASP/Univ. of
Colorado and the TIMAS data team for access to the TIMAS data along with C. C. Chaston and
R. L. Lysak for fruitful discussions. This research was supported through NASA grants NAG512761, NAG5-12590 and SA3554 AM#3.
17
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20
Figure 1. The general situation during the simultaneous observations of Alfven waves at Polar
and FAST on 22 October 1999 depicting roughly the x-z GSE plane. Both spacecraft traversed
the northern PSBL region at ~23 MLT. The black lines represent the magnetic field lines of the
boundary of interest. (The width of this region is exaggerated in the diagram). Both spacecraft
cartwheel in their orbits, drawn in green (red) for Polar (FAST), with their spin planes nearly in
the x-z GSE plane during the event. The spin plane electric field probes for each spacecraft are
also depicted (not to scale). The thicker, dark blue portion of the drawn orbits, appearing as a dot
on the Polar orbit, represents ~30 seconds of orbit relative to the region width.
Figure 2. Summary plots of the FAST and Polar passes through the region of interest. Panels a,
b and c are the electron and ion spectra and electron density respectively, Panel d displays the
field aligned Poynting flux in the range of 5 mHz-4 Hz, with positive being earthward. Panel e
shows the net earthward electron energy flux with the red line being the measurements at the
sampling rate and the darker line being a smoothed average over 5 (69) seconds for FAST
(Polar). The orange, gray and brown bands are regions that topographically map between the
spacecraft, with the orange band being the region of comparison for the given study. The darker
orange band specifies the time of the FAST orange region on the Polar data. See text for a
complete description.
Figure 3. Summary of frequency band analysis of Polar and FAST Poynting flux (S) and
E1/B2 ratios. The error bars at the bottom of panel c represent the frequency band ranges for
the plotted data points. Panel a depicts the mean total and net earthward Poynting flux, |S̄| in
blue and S̄ in pink respectively, calculated as described in section 3, mapped to the ionosphere in
21
each band. The peak Poynting flux, SMax, in each band is also shown in green. Panel b shows the
percentage of S directed earthward for each band, and panel c displays the E1/B2 ratio in
each band with the Alfven velocity, VA, for 100% H+ (O+) shown as a light (dark) blue line. The
dark green line shows the E1/B2 ratio expected from ionospheric closure.
Figure 4. The coefficients of reflection (R), dissipation above FAST (DAF) and below FAST
(DBF) for the observed Polar waves in the lowest four frequency bands in black, blue and red
respectively.
Figure 5. Frequency analysis of the E1/B2 ratios for the Alfven wave events observed at
Polar on 1 May 1997 and 9 May 1997 in the same format as in Figure 3. In these cases the
E1/B2 ratio for ionospheric closure is not as well defined since there is no large scale DC
current observed concurrent with the Alfven waves. The displayed range is for ΣP of 1-10 mhos.
Figure 6. Electron energy flux distribution plots from the FAST pass during the 22 October
1999, ~02:09 UT Alfven wave event. The earthward field aligned direction is to the right, and
the loss cone is evident to the left in each plot. a) A typical electron energy flux distribution plot
in the region of intense Alfven wave Poynting flux (S) showing an isotropic population with an
energy flux peak of ~300 eV and an earthward drifting, warm field aligned population. b) An
electron energy flux distribution plot from a region without intense S showing the typical
isotropic central plasma sheet population with an energy flux peak at ~1 keV. c) A second
example from the region without intense S showing a secondary isotropic energy flux peak at
~100 eV.
22
Figure 1
Figure 1. The general situation during the simultaneous observations of Alfven waves at Polar
and FAST on 22 October 1999 depicting roughly the x-z GSE plane. Both spacecraft traversed
the northern PSBL region at ~23 MLT. The black lines represent the magnetic field lines of the
boundary of interest. (The width of this region is exaggerated in the diagram). Both spacecraft
cartwheel in their orbits, drawn in green (red) for Polar (FAST), with their spin planes nearly in
the x-z GSE plane during the event. The spin plane electric field probes for each spacecraft are
also depicted (not to scale). The thicker, dark blue portion of the drawn orbits, appearing as a dot
on the Polar orbit, represents ~30 seconds of orbit relative to the region width.
23
Figure 2
See separate file, due to landscape format
24
Figure 3
Figure 3. Summary of frequency band analysis of Polar and FAST Poynting flux (S) and
E1/B2 ratios. The error bars at the bottom of panel c represent the frequency band ranges for
the plotted data points. Panel a depicts the mean total and net earthward Poynting flux, |S̄| in
blue and S̄ in pink respectively, calculated as described in section 3, mapped to the ionosphere in
each band. The peak Poynting flux, SMax, in each band is also shown in green. Panel b shows the
percentage of S directed earthward for each band, and panel c displays the E1/B2 ratio in
each band with the Alfven velocity, VA, for 100% H+ (O+) shown as a light (dark) blue line. The
dark green line shows the E1/B2 ratio expected from ionospheric closure.
25
Figure 4
Figure 4. The coefficients of reflection (R), dissipation above FAST (DAF) and below FAST
(DBF) for the observed Polar waves in the lowest four frequency bands in black, blue and red
respectively.
26
Figure 5
Figure 5. Frequency analysis of the E1/B2 ratios for the Alfven wave events observed at
Polar on 1 May 1997 and 9 May 1997 in the same format as in Figure 3. In these cases the
E1/B2 ratio for ionospheric closure is not as well defined since there is no large scale DC
current observed concurrent with the Alfven waves. The displayed range is for ΣP of 1-10 mhos.
27
Figure 6
Figure 6. Electron energy flux distribution plots from the FAST pass during the 22 October
1999, ~02:09 UT Alfven wave event. The earthward field aligned direction is to the right, and
the loss cone is evident to the left in each plot. a) A typical electron energy flux distribution plot
in the region of intense Alfven wave Poynting flux (S) showing an isotropic population with an
energy flux peak of ~300 eV and an earthward drifting, warm field aligned population. b) An
electron energy flux distribution plot from a region without intense S showing the typical
isotropic central plasma sheet population with an energy flux peak at ~1 keV. c) A second
example from the region without intense S showing a secondary isotropic energy flux peak at
~100 eV.
28