Dominant acceleration processes of energetic protons at the earth's bow shock

Abstract

We examined magnetic field and energetic (E ⩾ 30 keV) proton and (E ⩾ 220 keV) electron observations obtained by the spacecraft IMP-6, 7, and 8 in the vicinity of both the (quasi-) parallel and (quasi-) perpendicular bow shock of the Earth. The observations analysed show that: (a) for a quasi-perpendicular (45 ⩽ θBn < 90°) region of the bow shock, the downstream proton fluxes are higher (lower) than the upstream fluxes, at low (high) energies, (b) for an oblique (30 ≲ θBn < 70°) bow shock there exists a strong positive correlation of the (≲ 30 keV) beam events published by Paschmann et al. [15] with the ambient solar ∼ 50 keV proton flux, and similar positive correlation with the value of the induced electric field E = -USW × B (c) for parallel or quasi-parallel shocks and strong upstream magnetic wave activity, the presence of upstream > 50 keV ion events is not a continuous phenomenon, but is strongly correlated with the substorm activity of the magnetosphere. The above observations along with other previously published observations are discussed in the context of the Shock Drift Acceleration (SDA) and the Diffusive (1st order Fermi) acceleration theories. We infer that: (a) a seed population of superthermal (energetic) protons are accelerated to higher energies at the oblique (quasi-perpendicular) bow shock via the SDA process, and (b) there is no convincing evidence for acceleration of either the solar wind or ambient energetic ions to energies ≳ 30–50 keV via the Fermi acceleration process.