The Numerical Simulation of VLF Chorus and Discrete Emissions Observed on the Geotail Satellite using a Vlasov Code

Abstract

The work considers VLF chorus elements observed from the Geotail satellite, which passes the equator at 10 earth radii. We have used a VHS Vlasov simulation code to model these emissions, employing all the ambient plasma data observed on Geotail. Excellent agreement with observation results, with steep risers and slow fallers being reproduced. The results confirm the overall validity of the non linear trapping theory of VLF emissions, and also confirm the efficacy of the Vlasov VHS code.

The Geotail satellite skims the dayside magnetosphere in the equatorial region at about L = 10. During such passes the WFC (Waveform Capture Receiver) observes VLF chorus and discrete emissions in the band 200"' 1200 Hi. The most common waveforms observed are rising tones and rising chorus, with hooks and fallers being seen occasionally. Analysis shows that the k vectors are closely parallel to the ambient magnetic field. The Geotail satellite has available comprehensive wave and particle data observed in the vicinity of the generation regions of such VLF emissions. A well-established Vlasov simulation code has been used to simulate the observed emissions, using detailed data from Geotail. The code readily simulates rising frequency VLF emissions with a steep frequency gradient. With appropriate parameter values the code will produce fallers and hooks in good agreement with those observed on Gcotail. These self-consistent simulations suggest that nonlinear trapping of cyclotron resonant electrons is the underlying mechanism behind VLF chorus and VLF emissions.

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