Phase Relationship between ULF Waves and Drift-bounce Resonant Ions: A Statistical Study

Abstract:

We explore the phase relationship between the poloidal mode ULF wave electric field oscillations and drift-bounce resonant oxygen ions at the magnetic equator. Using Cluster data from 2001 to 2004, we identify 55 fundamental-mode poloidal wave events, among which 42 show "negative slope" pitch angle dispersion signatures in the southern hemisphere, 11 show "positive slope" dispersion in the northern hemisphere, and 2 near-equatorial events are associated with in-phase field-aligned signatures. For each event, the off-equatorial resonant ions are traced along their bouncing trajectories to determine the last time they moved across the equator. The resulting time-series of the resonant oxygen ion fluxes at the equator are found to be statistically in anti-phase with the wave electric fields. This anti-phase relationship suggests that the fundamental-mode poloidal wave electric fields are generally characterized by electric field intensity peaking near the magnetic equator.