Shock drift acceleration of energetic (E ≥ 50 keV) protons and (E ≥ 37 keV/n) alpha particles at the Earth's bow shock as a source of the magnetosheath energetic ion events

Abstract

We present simultaneous energetic proton (E ≥ 50 keV), alpha particle (E ≥ 37 keV/n) and electron (E ≥ 220 keV) observations obtained by the spacecraft IMP 8 and IMP 7 for an extended time period on May 2 and 3, 1976. During that time period, IMP 8 crossed successively the quasi‐perpendicular duskside of the bow shock in the presence of high fluxes of the ambient solar energetic particle population and under solar wind conditions which implied in a large value of the induced electric field E and a small ratio rG/Rc of the initial proton gyroradius to the local radius of curvature. The observations indicate that three different types of energetic ion populations can be distinguished inside the magnetosheath for the time period examined: (1) energetic ions of solar origin accelerated at (quasi‐)perpendicular regions of the bow shock via the shock drift acceleration (SDA) process, (2) energetic ions of solar origin not affected by the bow shock, and (3) energetic ions of magnetospheric origin. Protons and alpha particles accelerated and transmitted downstream from the bow shock conserve the same spectral index of the ambient solar population at low energies (50 ≤ E ≤ 360 keV) and present small values of the peak‐to‐background intensity ratio p/b ranging between 2.1 and 3.8 in the cases examined. A characteristic signature of the SDA, the double‐peaked anisotropy of the angular distributions of the particle intensities is not always observable in the low‐energy range (50 ≤ E ≤ 290 keV), because of the influence of the plasma flow in the magnetosheath, but is always observed in the high‐energy range (E ≥ 290 keV). Acceleration effects in the vicinity of the (quasi‐)perpendicular (θBn ≃ 79°‐90°) bow shock are observed in the intensities of the energetic protons with energies as high as E ≃ 4 MeV. Protons of magnetospheric origin present values of the ratio p/b as high as ∼6 × 10² and softer energy spectra (γ ≥ 4) than those of the protons accelerated at the bow shock.