Electromagnetic ion cyclotron rising tone emissions observed by THEMIS probes outside the plasmapause

Abstract

We report observations of electromagnetic ion cyclotron (EMIC) triggered emissions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes outside the plasmasphere. Although these phenomena have recently received much attention because of the possibility of strong interaction with particles, only a few events of EMIC triggered emissions have been reported near the equatorial plasmapause. We performed a survey of the THEMIS probe data and found various types of emissions mainly on the dayside at radial distances of 6–10 RE. We study three distinctive events in detail. The first is a typical event with an obvious rising tone emission in the afternoon sector. The emissions in the second event are simultaneously excited in different frequency bands separated by the cyclotron frequency of helium ions. In the third event, which occurred near local noon, rising tone emissions were excited in an extended region near the equator where the field‐aligned B gradient was much reduced because of compression of the magnetosphere by the solar wind. We compare these events with the nonlinear wave growth theory developed by Omura et al. (2010). In all events, it is found that the observed relationship between the amplitudes and frequencies of the emissions are in good agreement with the theory.