The numerical simulation of VLF chorus and discrete emissions observed on the Geotail satellite using a Vlasov 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 200~1200Hz. 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 Geotail. These self-consistent simulations suggest that nonlinear trapping of cyclotron resonant electrons is the underlying mechanism behind VLF chorus and VLF emissions.