Triggering process of electromagnetic ion cyclotron rising tone emissions in the inner magnetosphere

Abstract

Spacecraft observations and simulations show generation of coherent electromagnetic ion cyclotron (EMIC) triggered emissions with rising tone frequencies. In the inner magnetosphere, the spontaneously triggered EMIC waves are generated by the energetic protons with large temperature anisotropy. We reproduced EMIC triggered emissions in the Earth's magnetosphere by real scale hybrid simulations with cylindrical magnetic geometry. We obtained spontaneously triggered nonlinear EMIC waves with rising frequencies in the H+band of the EMIC dispersion relation. The proton holes in the phase space are formed. We have also derived the theoretical optimum wave amplitude for triggering process of the EMIC nonlinear wave growth. The optimum wave amplitude and the nonlinear transition time show a good agreement with the present simulation result. The nonlinear wave growth over a limited time forms a subpacket structure of a rising tone emission. The formation process of a subpacket is repeated because of a new triggering wave generated by the phase‐organized protons, which are released from the previous subpacket. Then, the EMIC triggered emission is formed as a train of subpackets generated at different rising frequencies.