Low‐energy ion pitch angle distributions in the outer magnetosphere: Ion zipper distributions

Abstract

Ion pitch angle distributions, measured in a near‐synchronous orbit, are predominantly field aligned at low energies and predominantly peaked perpendicular to the magnetic field at higher energies. The transition from field‐aligned fluxes to fluxes peaked predominantly perpendicular to the magnetic field occurs over a very narrow energy range. These ion distributions have been observed at all local times between 5.3 and 7.8 RE. This transition energy correlates with the deep minimum observed in the ion spectra. There is no apparent correlation between the ion transition energy and magnetic local time, L, Kp, or Dst. However, the transition energy does respond to observed particle injections. The transition energy decreases prior to injection, increases abruptly at injection by as much as 10–20 keV, and then decreases slowly after injection. Fresh low‐energy ions are supplied at injection and decrease in intensity over several hours to instrument threshold level. Ion drift trajectory calculations indicate that the low‐energy component below the transition energy drifts in from the nightside plasma sheet via local morning to the dayside. The high‐energy component, above the transition energy, arrives on the dayside via local evening.