Ion composition of zipper events

Abstract

A class of ion distributions has recently been identified by Fennell et al. (this issue). The distributions are composed of two components, a low‐energy component with peak fluxes directed along the field line and a high‐energy component with peak fluxes in the perpendicular direction. The transition between the two components occurs over a very narrow range of energies but can occur anywhere between approximately several hundred electron volts and 20 keV. Because of the appearance of this distribution on an energy versus time spectrogram, the ion events have been called zippers. The purpose of this report is to examine the mass composition of the zipper events. We find that the low‐energy and parallel component is composed primarily of O+, with, to a lesser degree, H+ and a trace of He+. The high‐energy and perpendicular component is predominantly H+, with the relative abundances of O+ and He+ down from those of the low‐energy component by a factor of ∼10. These results suggest that whereas the low‐energy component is probably ionospheric in origin, the source of the high‐energy component is most probably the plasmasheet.