Pitch angle distribution analysis of radiation belt electrons based on Combined Release and Radiation Effects Satellite Medium Electrons A data

Abstract

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°‐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°‐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°‐peaked distributions, while butterfly distributions become more common on the nightside at higher L‐shells. Fitting the PADs to a sinnα form, where α 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°‐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 Dst 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.