FAST Observations of Electrostatic and Electromagnetic VLF Emissions in
the Auroral Zone
R J Strangeway, L Kepko
R C Elphic
(Los Alamos National Laboratory)
C W Carlson, R E Ergun, J P McFadden, W J Peria,
G T Delory, C C Chaston, M Temerin
C A Cattell
E Moebius, L M Kistler
(Univ. New Hampshire)
D M Klumpar, W K Peterson, E G Shelley
R F Pfaff
The Fast Auroral Snapshot Explorer (FAST) is well
instrumented to study the microphysics of the Earth's auroral
zones, with a full complement of high time resolution ion and
electron spectrometers, as well as AC and DC fields experiments.
The main topic of interest for auroral zone physics concerns
the energization of the ions and electrons within the auroral
zone. What role do quasi-stationary fields have in the particle
acceleration? Is the acceleration due to plasma waves?
Earlier data from rockets and spacecraft such as S3-3, Viking
and Freja have clearly shown that both are important. Auroral
electrons appear to be accelerated by parallel electric fields,
while ions are heated and accelerated by waves.
Auroral electrons, having been accelerated are in turn a
source of free energy for waves, such as Auroral Kilometric
Radiation, VLF Hiss, and VLF Saucers.
FAST, with apogee ~ 4000 km, flies through the heart of the
auroral acceleration region. This allows us to observe in fine
detail both the particles and fields. For example, with FAST we
will be able to determine if the electron free energy for AKR is
the upgoing loss-cone, or the down-going beam, or the electrons
trapped between the magnetic mirror and the electric field.
We will also verify that the source of VLF saucers is the
return current carried by upgoing electrons, and further verify
that the VLF saucers are primarily electrostatic in nature.
In this poster we will present some initial efforts in
relating VLF waves to the underlying plasma and fields that are
the source for the waves.
Figure 1. Schematic of the planned and actual boom deployment for
FAST. In this poster we will be using VLF wave data from the long
wire antenna (V5-V8, electric field), and the 21"-core search coil
which is aligned along the magnetometer boom (magnetic field).
Figure 2. Legend and overview data for Orbit 1761. The left-hand
data plot shows data from the near midnight auroral oval, on into
the polar cap. The right-hand data panel continues over the polar
cap into the morning auroral oval. The nightside is characterized
by intense AKR, as well as a variety of VLF signals. The AKR is
associated with a density cavity. There is a net upwards current
throughout the electron precipitation region. VLF emissions are
observed well within the polar cap, lying slightly above the
"nominal" ion plasma frequency - the ion mass is arbitrary, in
this case we chose 9 (!). It should be remembered that the ion
plasma frequency as inferred from the Langmuir probe current
depends on both the average ion mass, and the electron
temperature. The morning auroral oval is characterized by many VLF
saucer emissions, and the associated upwards electrons and
Figure 3. Expanded plot of the nightside auroral oval data. The
region of broad-band VLF emissions corresponds to a slight density
enhancement. The low frequency cut-off of the AKR emission
increases at this time, indicating that the dispersive properties
of the plasma are modified by the relative density of hot to cold
plasma. The structure of the VLF emissions is quite complicated.
Inspection of the spin modulation of both the electric and
magnetic field shows variability as a function of frequency.
Figure 4. Solutions of the cold-plasma dispersion relation. The
right-hand panel shows the wave dispersion when the electron
plasma frequency is less than the electron gyro-frequency, as is
the case for the data shown in
Figure 3. Given the overlap of the
wave modes near the electron plasma frequency, mode identification
in the VLF frequency range is not easy. Later on we will use the
phasing of the spin modulation to draw some inferences about the
wave modes observed.
Figure 5. Expanded plot of the morningside auroral oval data. The
minimum frequencies of the VLF saucers are coincident with bursts
of upgoing electrons - the return current.
Figure 6. Plot of the refractive index surface for the whistler-
mode. The theory of James  argues that the waves are
generated on the resonance cone through Landau resonance with
relatively low energy upward going electrons. Because the wave
group velocity is almost perpendicular to the wave vector, the ray
path for low frequencies is nearly field-aligned, while high
frequencies propagate across the field. A spacecraft flying over a
source will hence see emissions with a saucer-like structure, the
lowest frequency occurring when the spacecraft passes over the
Figure 7. Comparison of ion and electron data for the nightside
oval. The broad burst of VLF waves occurs when the electron energy
increases. However, there is also an associated decrease in the
ion energy. There is an energetic ion beam on either side of the
VLF burst. This indicates that the burst is generated within a
region where the accelerating potential has moved above the
spacecraft. Perhaps the increase in cold electrons is quenching
local AKR generation, favoring lower frequency VLF waves.
Figure 8. Comparison of ion and electron data for the morningside
oval. In addition to the upward electrons, there are regions of
enhanced ion "conics". Such conics are indicative of transverse
acceleration. The VLF electric field data show evidence of ion
Bernstein modes when such conics are present. Whether the harmonic
structure is actually an emission feature, or due to absorption by
the hot ions is an open question.
Figure 9. Power spectra for an interval of "burst mode" data
acquired around the time of the broad VLF emissions shown in
3 and 7.
The burst mode, or wave-form, data allow for
detailed comparison between the different components of the
electric and magnetic field data. For this poster we have analyzed
the phase and amplitude of the spin modulation as a function of
frequency for both the electric and magnetic field data.
Figure 10. Wave energy density (top panel), inferred phase
velocity (middle panel), and phase angle of the spin modulation
with respect to the DC magnetic field (bottom panel) for three
Figure 9. Casting the wave power into energy
density allows for more easy comparison. The spectra will be
identical for a wave traveling at the speed of light. The middle
panel in each row shows the inferred phase velocity, assuming a
purely electromagnetic wave (no electrostatic component). Because
of this assumption, both a superliminous wave and a strongly
electrostatic wave (with only a weak transverse component) will
have an apparent phase velocity greater than the speed of light.
The left-hand column shows data from the first third of
The electric field (blue) shows a weak harmonic structure below
0.4 kHz, with an emission up to about 5 kHz. There is an
additional emission peaking near 8 kHz. The proton gyro-frequency
is about 0.2 kHz, supporting the suggestion that the harmonic
structure is due to absorption. Below 5 kHz, the magnetic field
(red) is dominated by signals due to the search-coil spinning in a
large DC field. Only above 5 kHz is there any detectable "natural"
signal. Hence the inferred phase velocity has no meaning below 5
kHz for this interval. The spin phase for the electrostatic signal
below 5 kHz varies from perpendicular to the field at low
frequency to parallel at the upper frequency cut-off. This is to
be expected for a whistler-mode wave on the resonance cone.
Furthermore, the cut-off implies that the plasma frequency is
about 4 kHz.
Above 7 kHz, there appears to be a mixture of modes. One mode has
both E and B perpendicularly polarized and is subluminous, which
could correspond to the parallel propagating Z-mode in
The other mode, at higher frequency is polarized in a manner
consistent with a perpendicularly propagating O-mode wave, but
this wave is also apparently subluminous.
The center column corresponds to the intense band of emission in
At low frequencies, below 0.2 kHz, there is a wave with
both E and B perpendicularly polarized, and phase velocity near
the speed of light. This could be an Alfvén wave, since the Alfvén
velocity is close to the speed of light. Above 0.3 kHz there is an
intense emission. Both E and B are perpendicularly polarized. This
could be the superluminous L-X branch, converting to the L-O
branch at about 7 kHz.
The right-hand column appears to be a mixture of the other two
VLF Waves observed by FAST include:
Ion Bernstein modes. Usually observed in association with hot
transverse ions, or conics. Are the harmonics due to emission or
VLF saucers. Mainly electrostatic, probably on the whistler-
mode resonance cone, generated by upgoing electrons [James, 1976].
L-X waves? Possibly generated at altitudes where the density
is not low enough for AKR. Alternative interpretation is that
these waves are intense nearly electrostatic whistler-mode waves.
The magnetic signature could be due to weak wave fields, or
possibly electrostatic pick-up.
Z-mode and L-O mode waves.
Future work: More detailed comparisons of wave fields using burst
mode data (wave-form analysis). We need to be able to distinguish
between DC field spin-tone, possible electrostatic pick-up, and
true deviations from exactly electrostatic fields, as well as
subluminous electrostatic versus superluminous electromagnetic
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Created by R. J. Strangeway
Last modified: July 7th, 1997.