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

[摘要] New observations of energetic helium ionfluxes in the Earth's radiation belts have been obtained with the CAMMICE/HITinstrument on the ISTP/GGS POLAR spacecraft during the extended geomagneticallylow activity period April through October 1996. POLAR executes a highinclination trajectory that crosses over both polar cap regions and passes overthe geomagnetic equator in the heart of the radiation belts. The latterattribute makes possible direct observations of nearly the full equatorialhelium ion pitch angle distributions in the heart of the Earth's radiation beltregion. Additionally, the spacecraft often re-encounters the same geomagneticflux tube at a substantially off-equatorial location within a few tens ofminutes prior to or after the equatorial crossing. This makes both theequatorial pitch angle distribution and an expanded view of the localoff-equatorial pitch angle distribution observable. The orbit of POLAR alsopermitted observations to be made in conjugate magnetic local time sectors overthe course of the same day, and this afforded direct comparison of observationson diametrically opposite locations in the Earth's radiation belt region atclosely spaced times. Results from four helium ion data channels covering ionkinetic energies from 520 to 8200 KeV show that the distributions displaytrapped particle characteristics with angular flux peaks for equatoriallymirroring particles as one might reasonably expect. However, the helium ionpitch angle distributions generally flattened out for equatorial pitch anglesbelow about 45°. Significant and systematic helium ion anisotropy difference atconjugate magnetic local time were also observed, and we report quiet timeazimuthal variations of the anisotropy index.

Key words. Magnetospheric physics (energetic particles· trapped; magnetospheric configuration and dynamics; plasmasphere)

[学科分类] 地球科学(综合)