Implications of unusual pitch-angle distributions observed by ISEE-1 and 2
<p><strong>Abstract.</strong> Unusual energetic particle pitch angle distributions (PADs) were observed by the ISEE-1 and 2 satellites at 3 h MLT and a radial distance of about 10&ndash;15 <i>R<sub>E</sub></i> during the time period of 07:00-14:00 UT on 3 March 1979. The ISEE-1 satellite obtained complete 3-D distributions of energetic proton and electron fluxes as a function of energy, while ISEE-2 was configured to provide higher time resolution but less angular resolution than ISEE-1.
... ion than ISEE-1. The ISEE-1 observed a butterfly PAD (a minimum in the 90&deg; PA particle flux) for a period of about 2 h (10:00&ndash;12:00 UT) for the electrons, and 3 h (09:00&ndash;12:00 UT) for the protons over an energy range of 22.5&ndash;189 keV (E1&ndash;E4) for the electrons and 24&ndash;142 keV (P1&ndash;P4) for the protons. The small pitch angle (15&deg;, 30&deg;) charged particles (electrons and protons) are seen to behave collectively in all four energy ranges. The relative differences in electron fluxes between 15&deg; PA and 90&deg; PA are more significant for higher energy channels during the butterfly PAD period. Three different types of electron PADs (butterfly, isotropic, and peaked-at-90&deg;) were observed at the same location and time as a function of energy for a short period of time before 10:00 UT. Electron butterfly distributions were also observed by the ISEE-2 for about 1.5 h over 28&ndash;62 keV (E2&ndash;E4), although less well resolved than ISEE-1. Unlike the ISEE-1, no butterfly distributions were resolved in the ISEE-2 proton PADs due to less angular resolution. The measured drift effects by ISEE-1 suggest that the detected protons were much closer to the particle source than the electrons along their trajectories, and thus ruled out a nightside source within 18:00 MLT to 03:00 MLT. Compared to 07:30 UT, the charged particle fluxes measured by ISEE-1 were enhanced by up to three orders of magnitude during the period 08:30&ndash;12:00 UT. From 09:10:00 UT to 11:50 UT, the geomagnetic conditions were quiet (AE&lt;100 nT), the LANL geosynchronous satellites observed no substorms, and the local magnetic field measured by ISEE-1 was almost constant, while the small PA charged particle (both electron and proton) fluxes measured by ISEE-1 increased gradually, which implies a particle source other than the substorm source. Based on detailed particle trajectory tracings in a realistic geomagnetic field model, the 50&ndash;200 keV protons with small PA at 10:00 UT ISEE-1 location on 3 March 1979 were passing through the northern high-altitude and high-latitude morningside region where the cusp should be located under a dawnward IMF component condition, while those protons with large PA may connect to the high-latitude morningside magnetopause. It is possible that the cusp source is responsible for the all particles observed during the event.</p>