Numerical simulation of VLF risers, fallers, and hooks observed in Antarctica

The VLF database from Halley station, Antarctica, has been searched, and prominent examples of discrete emissions are presented. Risers, fallers, quasiconstant tones, and upward and downward hooks are all common. Also observed are risers and fallers triggered from the tops of hiss bands. A one-dimensional Vlasov VHS code has been used to simulate the main types of events observed. Using realistic plasma parameters, the code successfully reproduces the main kinds of events observed, in good qualitative agreement with observations. Distinct generation region structures are found to be associated with risers and fallers. Hook formation is interpreted as being a spontaneous transition between these two quasi-stable generation region structures.