Direct Measurement of Ion Cyclotron Resonance Between Wave Fields and Protons in Space Plasmas

Ion cyclotron resonance is one of the fundamental energy conversion processes through wave field-particle interaction in collisionless plasma. However, the key evidence for cyclotron resonance (i.e., the coherence between wave field and ion phase space density pertaining to the ion cyclotron resonance and responsible for the dissipation of ion cyclotron waves (ICWs)) has yet to be directly observed. Based on the high-quality measurements of space plasma by the Magnetospheric Multiscale (MMS) satellites, we observe that both the wave electromagnetic field vectors and the disturbed ion velocity distribution rotate around the background magnetic field. Moreover, we find that the gyrophase angle difference between the fluctuations in the ion velocity distribution functions and the wave electric field vectors are always in the range of (0, 90) degrees, clearly suggesting the ongoing energy conversion from wave fields to particles. By invoking plasma kinetic theory, we find that the field-particle correlation for the dissipative ion cyclotron waves in the theoretical model matches well with our observations. Furthermore, all the wave electric field vectors (Ewave), the ion current (Ji) and the energy transfer rate (Ji ·Ewave) exhibit quasi-periodic oscillations, and the frequency of Ji ·Ewave is about twice the frequency of Ewave and Ji, consistent with plasma kinetic theory. Therefore, our combined analysis of MMS observations and kinetic theory provides direct, thorough, and comprehensive evidence for ICW dissipation in space plasmas.