Observation of Non‐Resonance Interactions Between Cold Protons and EMIC Waves of Different Polarizations in the Inner Magnetosphere

Using the high‐quality measurements of plasma by the Magnetospheric Multiscale (MMS) spacecraft, we investigate the wave‐particle interaction between cold protons and Electromagnetic ion cyclotron (EMIC) waves of different polarizations in the inner magnetosphere. Both the left‐hand and right‐hand polarized EMIC waves can interact with cold protons via non‐resonance interaction, and energize the cold protons (∼eV) up to suprathermal energies (∼tens of eV), resulting in a series of bridge‐like arcs in the ion energy spectrogram. Cold protons experience different types of phase bunching in left‐hand and right‐hand polarized EMIC waves. However, the relative angles between the velocities of bunched protons and wave magnetic or electric fields are independent of the polarizations of EMIC waves. Thus, the energy transfer process is not affected by the polarization of EMIC waves. The observations indicate that EMIC waves can affect the magnetospheric particle dynamics by energizing the cold protons. Plain Language Summary Electromagnetic ion cyclotron (EMIC) waves play an important role in the magnetospheric particle dynamics via wave‐particle interactions. Previous studies mainly focus on the resonance interaction between particles and EMIC waves. The resonance interaction is effective but it needs the energy of particles to be above the minimum resonant energy. In our observation, we find that the cold protons (∼eV) are energized up to suprathermal energies (∼tens of eV) by the EMIC waves, resulting in a series of bridge‐like arcs in the ion energy spectrogram. The energy of cold protons is much less than the minimum resonant energy, suggesting that the cold protons should be energized by the non‐resonance interaction. The polarization of EMIC waves could change the gyrophase distribution of bunched cold protons, but the energy transfer process is not affected by the polarization of EMIC waves. The observations suggest EMIC waves can affect the magnetospheric particle dynamics by energizing the cold protons. Key Points Both the left‐hand and right‐hand polarized electromagnetic ion cyclotron (EMIC) waves can interact with cold protons via non‐resonance interaction The cold protons experience different types of phase bunching with different polarizations of EMIC waves The energy transfer process is not affected by the polarization of EMIC waves