Upstream energetic ions under radial IMF: A critical test of the Fermi Model

Abstract

We have surveyed eight years of interplanetary magnetic field (IMF) and energetic particle observations obtained by the IMP‐8 spacecraft upstream from the bow shock and accumulated 63 cases when the upstream IMF remained radial for extended periods of time (> 1 hour). Of these, two cases have been selected during which measurable fluxes of ambient solar or corotating energetic particle events were absent. These conditions provide an excellent test to the theories of the origin of upstream energetic ions. It is shown that there are extended periods with radial IMF when no upstream energetic ions (E ≥ 50 keV) were detected. It is further shown that energetic ions in the range 50 keV ≤ E ≤ 1 MeV are not continuously present but appear in bursts of intensities varying by more than an order of magnitude under persistently radial IMF. These measurements contradict a fundamental prediction of the Fermi mechanism for the origin of the upstream energetic ions, namely that such ions should always be present on radial IMF lines. The observations are consistent with the hypothesis that energetic (≳ 50 keV) ions leak out from, and appear in the upstream medium sporadically, following the onset of magnetic activity within the magnetosphere.