On the bow shock θBn dependence of upstream 70 keV to 2 MeV ion fluxes

Abstract

We present a statistical study of 216 energetic ion events observed just upstream of Earth's bow shock by the Wind spacecraft. The ions that compose this database range in energy from 27 keV to 2 MeV. Of particular interest is the effect of shock geometry on the properties of the energetic ions. Our approach is different from previous studies in that we only include upstream ion events associated with a local bow shock crossing. In this way we determine the shock geometry locally, rather than from extrapolations of remote upstream observations to a model shock source location, providing a better quantitative determination of the properties of the energetic ions in relation to shock geometry. Under typical interplanetary conditions in the absence of a preexisting population of ambient energetic ions (E ≥ 50 keV), we find that the ion energy spectrum is characterized by an energy cutoff at ∼200–330 keV. Neither the energy cutoff nor the energetic ion spectrum show any dependence on shock geometry as measured by the angle between the shock normal and magnetic field θBn. However, when an ambient population of energetic particles is present in the interplanetary medium, ion flux levels measured just upstream of the terrestrial shock reach a minimum near θBn ∼ 45° irrespective of energy. The energy spectrum spans up to ∼2 MeV. Moreover, ions with energies E ≥ 550 keV are observed only at θBn ≥ 45°. The flux levels of these more energetic ions increase, in an average sense, with increasing values of θBn. Lee's [1982] self‐consistent theory of ion diffusive shock acceleration appears to explain the energetic ion flux levels observed at θBn ≤ 40°. Upstream energetic ions with energies E ≥ 550 keV appear to be the result of shock drift acceleration.