Pitch Angle Phase Shift in Ring Current Ions Interacting With Ultra‐Low‐Frequency Waves: Van Allen Probes Observations

Abstract

AbstractDrift‐bounce resonance between ultra‐low‐frequency (ULF) waves and ring current ions has been widely studied, because of its important role in ring current acceleration and relevant geomagnetic activities. To identify drift‐bounce resonance in observations, 180° phase shifts across resonant pitch angle have been proposed as diagnostic signatures. This study, however, presents observations that suggest this criterion may be invalid when phase space density (PSD) distributions vary nonmonochromatically with energy. We identified 14 ULF wave‐ion interaction cases from 2‐years Van Allen Probes data. In these cases, 180° phase shifts across pitch angle are observed at particular energies. Near these energies, pitch angle‐dependent PSD bump‐on‐tail distributions were also observed. As a result, at fixed energies, the sign of ion PSD energy gradient changes across pitch angle, which then can result in the observed 180° phase shift. Based on the observations, we suggest 180° phase shifts across pitch angle can also result from pitch angle‐dependent bump‐on‐tail distributions, which should be taken into account in future ULF wave‐ion interaction studies.