Observational test of shock drift and fermi acceleration on a seed particle population upstream of Earth's bow shock

Abstract

Simultaneous observations of energetic protons of solar origin by the IMP 7 and 8 spacecraft in the vicinity of the dawn and dusk bow shock, respectively, are examined to determine the conditions for acceleration of an ambient energetic particle population at the bow shock. The data show that acceleration effects are observed in the intensities of the energetic protons with energies as high as E ∼ 4 MeV only at the quasi‐perpendicular region of the bow shock. Furthermore, the ion acceleration is observed for high values of the solar wind velocity and the interplanetary magnetic field (IMF) and for an expanded bow shock. The energy spectra and angular distributions of the intensities of the accelerated ions both upstream and downstream from the bow shock are consistent with the predictions of the shock drift acceleration (SDA) mechanism operating efficiently at the quasi‐perpendicular (dusk) side of the bow shock while the IMF is in the nominal direction. At the same time no evidence for any particle acceleration at E ≳ 50 keV was detected by the spacecraft located at the quasi‐parallel (dawn) side of the bow shock, under conditions which favor the Fermi mechanism. These results, together with an observation of a magnetospheric burst during a period of radial IMF suggest the following: (1) acceleration of a seed particle population only occurs at the quasi‐perpendicular bow shock through SDA and (2) leakage of magnetospheric ions at E ≳ 50 keV rather than in situ acceleration is the principal source of observed upstream ion populations.