POLAR spacecraft observations of helium ion angular anisotropy in the Earth's radiation belts

New observations of energetic helium ion ̄uxes in the Earth's radiation belts have been obtained with the CAMMICE/HIT instrument on the ISTP/GGS POLAR spacecraft during the extended geomagnetically low activity period April through October 1996. POLAR executes a high inclination trajectory that crosses over both polar cap regions and passes over the geomagnetic equator in the heart of the radiation belts. The latter attribute makes possible direct observations of nearly the full equatorial helium ion pitch angle distributions in the heart of the Earth's radiation belt region. Additionally, the spacecraft often re-encounters the same geomagnetic ̄ux tube at a substantially o€equatorial location within a few tens of minutes prior to or after the equatorial crossing. This makes both the equatorial pitch angle distribution and an expanded view of the local o€-equatorial pitch angle distribution observable. The orbit of POLAR also permitted observations to be made in conjugate magnetic local time sectors over the course of the same day, and this a€orded direct comparison of observations on diametrically opposite locations in the Earth's radiation belt region at closely spaced times. Results from four helium ion data channels covering ion kinetic energies from 520 to 8200 KeV show that the distributions display trapped particle characteristics with angular ̄ux peaks for equatorially mirroring particles as one might reasonably expect. However, the helium ion pitch angle distributions generally ̄attened out for equatorial pitch angles below about 45°. Signi®cant and systematic helium ion anisotropy di€erence at conjugate magnetic local time were also observed, and we report quiet time azimuthal variations of the anisotropy index.