Remote Sensing of Energetic Electron Anisotropy Using Lightning Triggered Upper Band Chorus
Abstract
Observations of magnetospheric ELF/VLF ( 300 Hz -30 kHz) chorus being triggered by lightning induced whistlers are rare but provide a unique opportunity to remotely sense the energetic electron anisotropy in the Earth's radiation belts. We present ground-based observations from Alaska of whistler triggered trains of banded chorus emissions lasting tens of seconds. The observations are unique in that whistlers originating from lightning in the southern hemisphere and propagating on L shells in the range 4.6 < L < 5.5 are seen to trigger upper band chorus repeatedly over the course of two hours, yielding 12 observation cases with very similar characteristics. Each whistler exhibits a distinct upper frequency cutoff and only the lowest L-shell whistler is observed to trigger upper band chorus. The observed cutoff frequencies along with L-shell and cold plasma density derived from dispersion analysis are used to estimate the anisotropy of the hot plasma distribution in the magnetosphere. Resulting anisotropy estimates are of 0.7 - 1.5 are in good agreement with previous in-situ measurements and past work. While the anisotropy of the hot plasma distribution determines wave growth in the linear regime, access to the nonlinear regime requires the in situ wave amplitude to exceed the threshold for phase trapping of energetic electrons. Calculation of the threshold for each whistler shows that the upper band triggering of chorus corresponds to a lower nonlinear threshold. The results suggest that while upper band chorus is less favorable to be spontaneously generated, the distribution in this band is ripe for triggering of the chorus instability with an external input wave.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMSM44A..05G
- Keywords:
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- 3369 Thermospheric dynamics;
- ATMOSPHERIC PROCESSESDE: 3389 Tides and planetary waves;
- ATMOSPHERIC PROCESSESDE: 7829 Kinetic waves and instabilities;
- SPACE PLASMA PHYSICSDE: 7836 MHD waves and instabilities;
- SPACE PLASMA PHYSICS