Magnetic local time survey of radiation belt helium ion structure conducted with data from the polar cammice/hit instrument

A study of geomagnetically confined helium ions in the range 0.52-8.2 MeV i on kinetic energy was made with the CAMMICE Heavy Ion Telescope (HIT) durin g April-October 1996. Much of the year 1996 was remarkably geomagnetically quiescent with absence of major magnetic storms, large shock transits throu gh the magnetosphere, or penetration injection events. Indeed, between two minor magnetic "storms" on January 13 (D-ST minimum similar to -88 nT) and October 23 (D-ST minimum similar to -110 nT) 1996 there was a long geomagne tically relatively undisturbed time period with DST excursions in the few t ens of nT at most. Using the POLAR ephemeris data, it was found that the no minal L-parameter location of the radial peaks in helium ion fluxes varied considerably with azimuthal location around the Earth, i.e., with magnetic local time (MLT), to be observed at L = 2.7-2.6 respectively in the dawn se ctor (near MLT similar to 5 hr) and typically a nominal L-parameter farther out in the dusk sector (near MLT similar to 17 hr). The empirical helium i on anisotropy could be reasonably approximated by an unambiguous sin(n) alp ha(0) dependence (where alpha(0) is equatorial pitch angle) only fairly clo se to the geomagnetic equator, at equatorial pitch angles alpha(0) > 45 deg rees. For smaller equatorial pitch angles, the distribution was often seen to be flatter than describable by this relation alone. There may be several interacting causes of these observed features, including (1) effects of di fferences between the real geomagnetic field and the model field (IGRF 95) used in the POLAR ephemeris, (2) consequences of coupling between ion trans port dynamics, spectra and exospheric interactions, and (3) possibly also r eal physical effects of the azimuthally asymmetric geoelectric field in con junction with large gradients in the helium ion distribution function. Furt her work is needed to delineate the relative importance of these influences on the structure of radiation belt helium ions. (C) 1998 Elsevier Science Ltd. All rights reserved.