An empirical profile of VLF triggered emissions

Abstract

The Siple Transmitter Experiment operated from 1973 to 1988 and generated a wealth of observations of nonlinear wave-particle interactions including extensive recordings of triggered emissions generated by VLF signals injected into the magnetosphere from the transmitter at Siple Station, Antarctica. Due to their complex appearance and immensely varied behavior, triggered emissions remain poorly described and understood. This work provides a comprehensive statistical description of observed triggered emissions and establishes statistical bounds on triggered emission type (fallers, risers, and positive and negative hooks) and behavior (frequency changes between 1kHz and 2.5kHz with initial sweep rates between -2.5kHz/s and 2.5kHz/s, with risers undergoing a median frequency change of 556Hz and fallers a median frequency change of -198Hz). The statistical study also reveals an apparent dependence of the triggered emission behavior on the transmitted signal itself. Long tones and rising ramps generate more risers and positive hooks, while short tones and falling ramps produce more fallers and negative hooks. Triggered emissions also appear to favorably initiate with sweep rates similar to that of the triggering element, with the 1kHz/s rising ramps triggering initial risers with a median sweep rate of 1.03kHz/s and -1kHz/s triggering initial fallers with a median sweep rate of -0.73kHz/s. These results improve observations of wave modification resulting from wave-particle interactions in the radiation belts and can be used to validate numerical simulations of triggered emissions.