Field‐Aligned Structures of the Poloidal‐Mode ULF Wave Electric Field: Phase Relationship Implications

Spacecraft observations within recent decades have demonstrated that ultralow frequency (ULF) waves play an important role in the dynamics of Earth's magnetosphere through drift and/or drift‐bounce resonance with different particle species. The phase relationship between resonant particles and waves can help us understand the wave‐particle interacting processes. It has been revealed that the phase relationship between the drift resonant particles and the fundamental poloidal‐mode electric field is a signature to judge the energy transfer direction between particles and waves, and the local phase space density gradient. Here we explore whether there are other physical meanings of the phase relationship for drift‐bounce resonant particles in addition to the similar ones as drift resonant particles. In this study, we find that (1) under different field‐aligned structures, the poloidal‐mode electric fields will show different phase relationship with drift‐bounce resonant particles at the magnetic equator. It indicates that the phase relationship can be used to diagnose the parallel morphologies of ULF wave electric field. (2) If particle's phase space density variations caused by resonance effect become more dominant while compared with the nonresonant effect, the phase difference becomes much closer to inphase or antiphase. (3) When wave's parallel morphology is given, the phase relationship can be used to judge the energy transfer direction between waves and particles. These findings can provide new sights into exploring ULF waves' propagation and distribution along magnetic field lines and studying their interaction with charged particles in the magnetosphere. Key Points ULF waves with different field‐aligned structures will show different phase relationships between drift‐bounce resonant particles The phase relationship can be used to diagnose the parallel structures of ULF wave electric field and the energy transfer directions The ratio between resonant and nonresonant effects can affect the phase difference between waves and particles