Frequency doubling and field‐aligned ion streaming in a long‐period poloidal pulsation

Abstract

We reexamine a long‐period (10–18 min) poloidal pulsation observed by THEMIS‐A in the outer dawn magnetosphere from 10:00 to 12:30 UT on 7 November 2007. The interval was originally reported by Korotova et al. (2009). Although the nonlinear compressional plasma and magnetic field perturbations observed by THEMIS‐A during this interval agree well with model predictions for the linear perturbations associated with antisymmetric waves generated by the ballooning‐mirror mode instability, the phase relationships between these perturbations indicates a complex frequency rather than the purely imaginary frequency that theory predicts for the outer dawnside magnetosphere. Variations in the radial plasma velocity confirm that a phase‐locked north/south oscillation in the equatorial line of nodes associated with the ballooning‐mirror mode waves doubles the frequency of the compressional component of the magnetic field during these pulsations. The same velocity and magnetic field perturbations exclude explanations for the frequency doubling in terms of spatial gradients sweeping back and forth across the spacecraft or drift‐bounce resonances. Azimuthal electric fields associated with the pulsations generate field‐aligned anisotropies in the pitch angle distributions that become more prominent with increasing ion energy due to the presence of drift‐shell splitting and radial flux gradients that steepen with increasing energy. Although there was no evidence for drift‐bounce interactions during this event, the role of such events in ion energization in other events and at other locations remains to be evaluated.