ENERGETIC ION DISTRIBUTIONS ON BOTH SIDES OF THE EARTHS MAGNETOPAUSE

The AMPTE/CCE spacecraft, with an apogee of approximately 8.8R(E) and an inclination of approximately 4.3-degrees, sampled the outer dayside equatorial magnetosphere for extended time periods and often crossed into the magnetosheath whenever the solar wind pressure was sufficient ly high to compress the magnetopause to <8.8R(E). We have analyzed ion distributions on both sides of the magnetopause in order to investiga ted any local time and energy dependence, giving information about phy sical processes at the magnetopause and the bow shock. Particle measur ements 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 d escribes the magnetospheric distributions quite well, and for the purp ose of this study, adequately describes the high-energy part of the sh ocked solar wind. Thus both solar wind and magnetospheric components c an be recognized in a mixed particle distribution. Case studies of rep resentative 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 greater-than-or -equal-to 50 keV; spectra were harder at the duskside than at the dawn side 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 magneto sheath. 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. Over all, the results about the origin of the greater-than-or-equal-to 50 k eV magnetosheath ions are consistent with the continuous leakage of ma gnetospheric particles across a tangential discontinuity magnetopause, locally distributed according to magnetospheric drift paths. Magnetic reconnection, although present, should not be a dominant source on av erage, 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 grea ter-than-or-equal-to 50 keV because of upper-energy cutoff limitations . Our observations also indicate a significant magnetospheric contribu tion to energies as low as approximately 10 keV, where the magnetosphe re-magnetosheath intensity gradient reverses. However, in order to exa mine the relative strength and local time distribution of all possible sources at these energies, a detailed analysis is required.