Pitch angle distribution analysis of radiation belt electrons based on combined release and radiation effects satellite medium electrons A data

[1] Using data from the Medium Electrons A instrument on the Combined Release and Radiation Effects Satellite (CRRES), a survey of pitch angle distributions (PADs) of energetic electrons is performed. The distributions are classified into three categories: 90 degrees-peaked, flattop, and butterfly. The categorizations are examined as a function of L-shell and orbit number and at electron energies of 153, 510, and 976 keV. The 90 degrees-peaked distributions dominate at the lowest energy channel, and butterfly distributions are more prevalent at higher L values. The PADs on the dayside are predominately 90 degrees-peaked distributions, while butterfly distributions become more common on the nightside at higher L-shells. Fitting the PADs to a sin(n) alpha form, where alpha is the local pitch angle, a profile of the parameter n versus L-shell is produced for local times corresponding to postnoon and midnight sectors for the 510-keV channel. We then compare the 510-keV data during times of moderate disturbance to the less disturbed case and the average case, and show an increase in butterfly distributions, which occurs at L > 6 for the nightside case and 3.5 < L < 5.5 for the dayside case. Comparing the profiles for n > 1 before and after the great storm on 24 March 1991, we find that there are significant differences before and after this event, the latter orbits being during a time of higher observed geomagnetic activity. Considering only those PADs with a calculated n > 1, the variation of the 90 degrees-peaked distributions versus L-shell and orbit shows increased steepness at lower L-shell. For the lowest energy channel, the low L-shell variation of the steepness of the distributions visually correlates with the average 2-day minimum plasmapause location calculated from a model based on the D-st index over the same time period. For the 510-keV electrons, a correlation can be seen with the development of flattop distributions inside of the calculated minimum plasmapause location.