A statistical survey of ions observed upstream of the Earth's bow shock: Energy spectra, composition, and spatial variation

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Abstract
The characteristics of 33 diffuse particle events in the energy range from ∼30 to ∼130 keV/Q observed upstream of the earth's bow shock have been determined. The measurements were made with the Ultra Low Energy Charge Analyzer (ULECA) sensor of the Max‐Planck‐Institut/University of Maryland instrument on ISEE‐1. The energy spectra of these events are clearly steeper than power law and are generally well described by either an exponential or Maxwellian in energy per charge, with average e folding values of ∼20 keV/Q and ∼15 keV/Q, respectively. The composition in these events is remarkably constant as a function of energy per charge and is similar (within a factor of ∼2) to that of the solar wind. The particle flux is found to decay exponentially with distance from the bow shock, with an e folding distance of ∼7 RE for H and He at 30 keV/Q. Inverse velocity dispersion was observed in each event, with an average delay time of ∼40 min between the 30 keV and 130 keV proton equilibrium intensity levels. Our measurements are consistent with a Fermi acceleration mechanism and the presence of a free escape boundary upstream of the bow shock. Although existing Fermi‐escape boundary models differ somewhat in detail, our present analysis is unable to differentiate among them.