Ion hole formation and nonlinear generation of electromagnetic ion cyclotron waves: THEMIS observations

Electromagnetic plasma waves are thought to be responsible for energy exchange between charged particles in space plasmas. Such an energy exchange process is evidenced by phase space holes identified in the ion distribution function and measurements of the dot product of the plasma wave electric field and the ion velocity. We develop a method to identify ion hole formation, taking into consideration the phase differences between the gyromotion of ions and the electromagnetic ion cyclotron (EMIC) waves. Using this method, we identify ion holes in the distribution function and the resulting nonlinear EMIC wave evolution from Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. These ion holes are key to wave growth and frequency drift by the ion currents through nonlinear wave‐particle interactions, which are identified by a computer simulation in this study. Key Points Wave‐particle interaction analysis method is applied to THEMIS data Energy exchange between EMIC waves and protons is directly detected Ion hole generated through nonlinear wave‐particle interaction is found