Polar Observations of Isotropic Angular Distributions of Energetic Ions During Geomagnetic Storms

Observations of energetic ion pitch angle distributions during several geomagnetic storms are presented as evidence for pitch angle diffusion processes. Measurements by the CAMMICE/MICS and CEPPAD/IPS instruments on the Polar satellite are used to obtain the angular distributions during several storms. MICS provides ion composition of 1-200 keV/q ions of all major species including H+, He+, He++, and O+. These data are supplemented with proton observations in the energy range 20-1500 keV from IPS. The 20 keV H+ fluxes at L>5 in the dusk-to-midnight local time sector during several storms show nearly isotropic distributions with filled loss cones. This implies that pitch angle diffusion or scattering processes were active during the main and early recovery phases of the storms. Such distributions may be caused by two possible phenomena: interactions with electromagnetic ion cyclotron (EMIC) waves, or nonadiabatic motion of the ions in regions near current sheets. The isotropic distributions tend to be observed over a wider range of L than predicted by models of EMIC waves so that at least part of the data is more consistent with current sheet scattering than waves.