Modulation of whistler mode chorus waves: 1. Role of compressional Pc4-5 pulsations

Abstract

[1] Using observations from the THEMIS spacecraft, we investigate the modulation of whistler mode chorus waves in the magnetosphere by compressional Pc4–5 pulsations (i.e., pulsations with periods from tens of seconds to a few minutes) with an anticorrelation between the total electron density and the background magnetic field intensity. We find that such compressional pulsations are associated with modulations of resonant electron fluxes and chorus intensity. Changes in the total electron density, background magnetic field, and the flux and anisotropy of resonant electrons could all be responsible for triggering the excitation of chorus waves. To quantitatively investigate which parameters dominantly contribute to chorus generation, we evaluate the changes in linear growth rates of whistler mode waves due to variations in either the ratio of resonant electrons to the total electrons R(VR) or the electron anisotropy A(VR). In the majority of cases, the modulation of whistler mode wave intensity is dominated by R(VR) variations caused by compressional Pc4–5 pulsations and tends to occur at large L shells of 8–12 in the dawn sector. Only a small fraction of events are associated with A(VR) modulations and these typically occur at lower L shells (<∼8).