Composition and energy spectra of low energy ions observed upstream of the earth's bow shock on ISEE-1

The characteristics of eleven locally accelerated particle events in the energy range from 30 to 125 keV/Q observed upstream of the earth's bow shock have been determined, including composition, energy spectra, and intensity versus time profiles. The measurements were made with the Ultra Low Energy Charge Analyzer sensor on ISEE-1. The composition in these events is similar to that of the solar wind, with a He to proton ratio of 8% and a CNO to He ratio of 6%. The composition is reasonably constant only when evaluated at equal energy per charge. The energy spectra cannot be adequately fit by a single power law in energy; an exponential or Maxwellian in energy per charge gives a satisfactory representation of the spectra. The time-intensity profiles of these upstream events show an inverse velocity dispersion, which may provide clues to the responsible acceleration mechanism.