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Ann. Geophys., 24, 3099-3113, 2006
www.ann-geophys.net/24/3099/2006/
© European Geosciences Union 2006
Implications of unusual pitch-angle distributions observed by ISEE-1 and 2
C. A. Zuluaga, E. S. Beiser, J. Chen, and T. A. Fritz
Center for Space Physics, Boston University, Boston MA, USA
Abstract. 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
10–15 RE 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. The ISEE-1 observed a butterfly PAD (a minimum in the
90° PA particle flux) for a period of about 2 h (10:00–12:00 UT)
for the electrons, and 3 h (09:00–12:00 UT) for the protons
over an energy range of 22.5–189 keV (E1–E4) for the electrons
and 24–142 keV (P1–P4) for the protons.
The small pitch angle (15°, 30°) charged particles (electrons and
protons) are seen to behave collectively in all four energy ranges.
The relative differences in electron fluxes between 15° PA and
90° 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°) 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 28–62 keV (E2–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 08:30–12:00 UT.
From 09:10:00 UT to 11:50 UT, the geomagnetic conditions were quiet
(AE<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 50–200 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.
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