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

New observations of energetic helium ion fluxes in the Earth's radiation be lts have been obtained with the CAMMICE/HIT instrument on the ISTP/GGS POLA R spacecraft during the extended geomagnetically low activity period April through October 1996. POLAR executes a high inclination trajectory that cro sses over both polar cap regions and passes over the geomagnetic equator in the heart of the radiation belts. The latter attribute makes possible dire ct observations of nearly the full equatorial helium ion pitch angle distri butions in the heart of the Earth's radiation belt region. Additionally, th e spacecraft often re-encounters the same geomagnetic flux tube at a substa ntially off-equatorial location within a few tens of minutes prior to or af ter the equatorial crossing. This makes both the equatorial Ditch angle dis tribution and an expanded view of the local off-equatorial pitch angle dist ribution observable. The orbit of POLAR also permitted observations to be m ade in conjugate magnetic local time sectors over the course of the same da y, and this afforded direct comparison of observations on diametrically opp osite locations in the Earth's radiation belt region at closely spaced time s. Results from four helium ion data channels covering ion kinetic energies from 520 to 8200 KcV show that the distributions display trapped particle characteristics with angular flute peaks for equatorially mirroring particl es as one might reasonably expect. However, the helium ion pitch angle dist ributions generally flattened out for equatorial pitch angles below about 4 5 degrees. Significant and systematic helium ion anisotropy difference at c onjugate magnetic local time were also observed, and we report quiet time a zimuthal variations of the anisotropy index.