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

• Published in: Physics and chemistry of the earth. Part C, Solar-terrestrial and planetary science Solar-terrestrial and planetary science, 1999, Vol.24 (1-3), p.233-238
• Main Authors: Spjeldvik, W.N., Fritz, T.A., Sheldon, R.B., Chen, J.
• Format: Article
• Language: English
• ISSN: 1464-1917
• DOI: 10.1016/S1464-1917(98)00034-8

A study of geomagnetically confined helium ions in the range 0.52–8.2 MeV ion kinetic energy was made with the CAMMICE Heavy Ion Telescope (HIT) during April–October 1996. Much of the year 1996 was remarkably geomagnetically quiescent with absence of major magnetic storms, large shock transits through the magnetosphere, or penetration injection events. Indeed, between two minor magnetic “storms” on January 13 (DST minimum ∼ −88 nT) and October 23 (DST minimum ∼ −110 nT) 1996 there was a long geomagnetically relatively undisturbed time period with DST excursions in the few tens of nT at most. Using the POLAR ephemeris data, it was found that the nominal 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 sector (near MLT∼5 hr) and typically a nominal L-parameter farther out in the dusk sector (near MLT∼17 hr). The empirical helium ion anisotropy could be reasonably approximated by an unambiguous sinnα0 dependence (where α0 is equatorial pitch angle) only fairly close to the geomagnetic equator, at equatorial pitch angles α0 > 45°. 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 differences between the real geomagnetic field and the model field (IGRF 95) used in the POLAR ephemeris, (2) consequences of coupling between ion transport dynamics, spectra and exospheric interactions, and (3) possibly also real physical effects of the azimuthally asymmetric geoelectric field in conjunction with large gradients in the helium ion distribution function. Further work is needed to delineate the relative importance of these influences on the structure of radiation belt helium ions.