Butterfly pitch-angle distributions observed by ISEE-1

The ISEE-1 satellite has observed butterfly pitch-angle distributions (PAD) in protons and electrons from 20 keV to 2 MeV in the low latitude outer magnetosphere, which showed a sharp dropout in the flux of 90degrees pitch angle particles. These distributions were present on the nightside near the magnetic equatorial plane and were far more prevalent in electrons than in protons. These distributions have been known for some time and have been associated with drift shell splitting in combination with either shadowing by the magnetopause or with a radial intensity gradient close to the Earth as equatorially mirroring particles drift around the Earth at radial distances from 6 to 12R(E). In both a study of individual three-dimensional distributions and in a statistical survey, the occurrence of the butterfly PADs was found to have the following consistent behavior: (1) The anisotropy in the distributions increased with an increase in particle energy for both the protons and the electrons. (2) The anisotropy was less pronounced in the protons than in the electrons and usually disappeared altogether for the higher energy protons. (3) The radial location of the transition from and to the butterfly distribution was found to have a dependence on particle energy. (3a) The transition position occurred closer to the Earth with increasing particle energy for the butterfly PAD to normal PAD transition whereas (3b) the transition from the butterfly PAD to isotropic PAD occurred closer to the Earth with decreasing particle energy. In other words, the PADs were isotropic near the apogee of ISEE-1, peaked at 90degrees pitch angle close to the Earth, and butterfly over a range of radial distances, the width of which expanded in both directions with increasing energy. Modeling results are presented that are interpreted to demonstrate that the magnetopause and its motion are responsible for producing the butterfly PADs. The differences between ion and electron butterfly PADs are interpreted to indicate that the magnetopause has a layer of energetic particles that becomes a source of energetic particles to the inner magnetosphere and preferentially provides ions as close as the geostationary orbit routinely. These ions have a softer spectrum than the ions present at the same location on stably trapped drift trajectories (i.e., for those particles mirroring well away from the equator). (C) 2003 Elsevier Science Ltd. All rights reserved.