Anisotropy characteristics of geomagnetically trapped ions

Abstract

Radiation belt ions (protons plus heavier ions) are found to exhibit quiet time pitch angle anisotropies that vary significantly and nonmonotonically with L shell and energy in the range 20 keV to several MeV. Data from two spacecraft are reported: Explorer 45 measurements obtained in June 1972 and ISEE 1 measurements obtained from November 1977 through December 1978. Both data sets show that the radial distribution of pitch angle anisotropy contains one major minimum for most energies at some midrange L shell. The 1972 data collected over a 15‐day period in a localized region of the magnetosphere (between L = 2 and L = 5 in the afternoon local time sector) revealed an anisotropy profile for most energies that increased with lower L shells below L ∼ 2.5, minimized near L = 3, then attained a local maximum around L ∼ 4, and evolved to more isotropic distributions near the Explorer 45 orbit apogee at L = 5.2. The 1977–1978 pitch angle anisotropies observed by ISEE 1 were obtained over all local times from approximately L = 2.5 to the outer limit of trapping. These data in general tended to confirm the 1972 finding of nonmonotonic anisotropy distributions, but with some important modifications. The later quiet time distributions showed a broader region of low anisotropy and less regular L shell dependence than the 1972 data, and showed systematic trends in the anisotropy that were clearly dependent not only on L shell but also on energy. A single, major anisotropy minimum was observed at virtually every local time, the location of this minimum occurred at lower L shells with progressively increasing energy. Comparison of the two quiet time data sets separated by about 6 years suggests that there may also be an anisotropy dependence on the phase of the solar cycle, possible resulting from ion composition differences or from different wave‐particle interactions.