Energetic ion distributions on both sides of the Earth's magnetopause

Abstract

The AMPTE/CCE spacecraft, with an apogee of ∼8.8RE and an inclination of ∼4.3°, sampled the outer dayside equatorial magnetosphere for extended time periods and often crossed into the magnetosheath whenever the solar wind pressure was sufficiently high to compress the magnetopause to <8.8RE. We have analyzed ion distributions on both sides of the magnetopause in order to investigate any local time and energy dependence, giving information about physical processes at the magnetopause and the bow shock. Particle measurements are from the CHEM (1.5 to 300 keV/e) and MEPA (10 keV to 2 MeV) instruments. The wide total energy range (1.5 keV to 2 MeV) covered describes the magnetospheric distributions quite well, and for the purpose of this study, adequately describes the high‐energy part of the shocked solar wind. Thus both solar wind and magnetospheric components can be recognized in a mixed particle distribution. Case studies of representative magnetopause crossings at dawn, noon, and dusk, as well as a survey of several other crossings, indicated: (1) a local time and energy dependence of magnetosheath spectra at energies ≥50 keV; spectra were harder at the duskside than at the dawnside and also correlated with magnetospheric activity, (2) constantly much higher intensities in the magnetosphere than in the magnetosheath at energies >10 keV and an earthward gradient in the subsolar magnetosheath. In addition to the steady state magnetosheath population there exists a burst‐type component indicative of a magnetospheric source, and most of the time this is recognized as a flux transfer event. Overall, the results about the origin of the ≥50 keV magnetosheath ions are consistent with the continuous leakage of magnetospheric particles across a tangential discontinuity magnetopause, locally distributed according to magnetospheric drift paths. Magnetic reconnection, although present, should not be a dominant source on average, because it is not continuous in time. Fermi acceleration should not be dominant because it predicts the opposite local time asymmetry, and shock drift acceleration should be a minor contributor at E≥50 keV because of upper‐energy cutoff limitations. Our observations also indicate a significant magnetospheric contribution to energies as low as ∼10 keV, where the magnetosphere‐magnetosheath intensity gradient reverses. However, in order to examine the relative strength and local time distribution of all possible sources at these energies, a detailed analysis is required.