FAST Observations of Electrostatic and Electromagnetic VLF Emissions in the Auroral Zone

R J Strangeway, L Kepko
(IGPP/UCLA)

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
(UC Berkeley)

C A Cattell
(Univ. Minnesota)

E Moebius, L M Kistler
(Univ. New Hampshire)

D M Klumpar, W K Peterson, E G Shelley
(Lockheed Martin)

R F Pfaff
(NASA/GSFC)

Introduction

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 downwards current.

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 [1976] 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 source.

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 Figures 2, 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 intervals within 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 Figure 9. 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 Figure 4. 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 Figure 9. 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 columns.

Conclusions

VLF Waves observed by FAST include:

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 waves.

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Created by R. J. Strangeway

email: strange@igpp.ucla.edu

Last modified: July 7th, 1997.