Evaluation of whistler‐mode chorus intensification on the nightside during an injection event observed on the THEMIS spacecraft

The intensification of the nightside whistler‐mode chorus emissions is observed in the low‐density region outside the plasmapause during the injection of anisotropic plasma sheet electrons into the inner magnetosphere. Time History of Events and Macroscale Interactions During Substorms data of the electron phase space density over the energy range between 0.1 keV and 30 keV are used to develop an analytical model for the distribution of injected suprathermal electrons. The path‐integrated gain of chorus waves is then evaluated with the HOTRAY code by tracing whistler‐mode chorus waves in a hot magnetized plasma. The simulated wave gain is compared to the observed wave electric field and magnetic field, respectively. The results indicate that lower‐energy (<1 keV) plasma sheet electrons can penetrate deeper toward the Earth but cause little chorus intensification, while higher‐energy (1 keV to tens of kiloelectron volts) electrons can be injected at relatively higher L ‐shells and are responsible for the intensification of lower‐band and upper‐band whistler‐mode chorus. Compared to the lower‐band chorus, a relatively higher electron anisotropy is required to generate upper‐band chorus. In addition, higher plasma density results in stronger wave intensity and a broader frequency band of chorus waves.