Evidence for acceleration of ions to ∼ 1 Mev by adiabatic‐like reflection at the quasi‐perpendicular Earth's bow shock

Abstract

On December 6, 1994, during a CIR (Corotating Interaction Region) event, the WIND 3D‐Plasma and energetic particle experiment observed a burst (≤24 s) of 238–676 keV protons close to the electron foreshock boundary, followed by a 156–236 keV, 101–156 keV and 33–101 keV proton bursts about ∼40 s, 65 s and 85 s later, respectively. Similar dispersed bursts of helium with energies between ∼50 keV and ∼1 MeV followed the proton bursts. During this time, the IMF direction varied slowly with an almost monotonic decrease in θBn. The proton energy spectrum is initially peaked at ∼350 keV, and progresses to lower energies with time. The proton 3D angular distributions are peaked at ∼30° pitch‐angle, propagating away from the shock, but they are nongyrotropic. Finally, we show that the proton energy‐spectra as well as the pitch angle distribution agree quantitatively with a model of a single adiabatic reflection of the incident energetic interplanetary ions by the quasi‐perpendicular shock.