Latitudinal anisotropy in ring current energetic neutral atoms

Stereo energetic neutral atom (ENA) images obtained by Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) are analyzed to quantify the latitudinal anisotropy of ring current ENAs, and by implication, the system-level pitch-angle response during a moderate geomagnetic storm on 6 April 2010. During a 15-min interval just after midday, TWINS 1 and 2 were at similar local times but different latitudes, and thus were able to simultaneously sample two different pitch angles from selected locations. The observations show a global dawn-dusk asymmetry in the ring current ENA latitudinal anisotropy, the dusk side being ∼1-2 orders of magnitude more anisotropic than the dawn side. The latitudinal dependence of the ENAs implies a duskside ion distribution that is more equatorial than isotropic; this anisotropy becomes less pronounced with increasing energy. The observed dawn-dusk ENA anisotropy may be caused by ring current ions drifting adiabatically in self-consistently varying electric and (nondipolar) magnetic fields.