Cold proton nonresonant response to EMIC waves: MMS observations and a hybrid simulation

Abstract Cold protons play an important role in the Earth’s magnetosphere by modifying the dispersion relation of plasma waves. Their energy flux can be enhanced through a nonresonant response to electromagnetic ion cyclotron (EMIC) waves. This study combines Magnetospheric Multiscale (MMS) observations and a hybrid simulation to investigate cold proton dynamics during this nonresonant process. It is found that the energy flux of cold protons with kinetic energies below ~ 200 eV increases due to the bulk flow induced by EMIC waves, and this enhancement becomes stronger and extends to higher energies at higher magnetic latitudes. Despite the flux enhancement, the temperature and number density of cold protons remain constant. Moreover, we identify the formation mechanism of proton phase-bunching distribution in stationary gyrophase. During the nonresonant process, particles bunch in anti-phase with the wave magnetic fields, without exchanging energy with waves. In contrast, during the resonant process, particles bunch in anti-phase with the wave electric fields, facilitating energy transfer from particles to waves. We demonstrate that particle phase-bunching is a necessary but not sufficient condition for identifying resonant interactions with waves, and that the occurrence of energy transfer determines whether the response is nonresonant or resonant. Graphical Abstract