Whistler Triggered Upper Band Chorus Observed in Alaska
Abstract
VLF radiation from lightning discharges is one of several sources of energy injection into the inner magnetosphere from the Earth. Lightning discharges initially produce a broadband impulse or `sferic' but after propagation in the dispersive magnetosphere this waveform soon becomes quasi narrow band with the characteristic spectrographic form of the whistler. Most of the lightning induced VLF wave energy injected into the magnetosphere will be unducted with a k-vector which becomes increasingly oblique. Although unducted radiation is ubiquitous throughout the inner magnetosphere, it is generally of a low amplitude due to Landau damping and is not expected to produce strong nonlinear phenomena such as triggered emissions and chorus waves. However, VLF wave energy ducted or trapped in field-aligned plasma density enhancements can have relatively large amplitudes due to focusing and also linear cyclotron resonance growth. Therefore high amplitude ducted whistler waves can trigger a number of complex nonlinear phenomena. These include the triggering of VLF emissions and triggering of VLF hiss or chorus. Such phenomena are generally considered to result from nonlinear electron cyclotron phase trapping. Observation of such VLF emissions triggered by natural whistlers have been reported since the 1970s in Antarctica. We present observations of whistlers triggered upper band chorus emission from Alaska. Dispersion analyze of whistlers determine the L-shell range to be 4.5 < L< 5.9 and equatorial plasma density range to be 50 - 200 el/cc. Moreover, there is a clear frequency band gap between upper and lower band of the observed chorus emissions. The observations point to ducted chorus generation in the vicinity of the plasmapause boundary.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMSM51B2457H
- Keywords:
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- 7829 Kinetic waves and instabilities;
- SPACE PLASMA PHYSICS;
- 7836 MHD waves and instabilities;
- SPACE PLASMA PHYSICS;
- 7867 Wave/particle interactions;
- SPACE PLASMA PHYSICS;
- 7954 Magnetic storms;
- SPACE WEATHER