Suprathermal Ions and MHD Turbulence Observed Upstream of an Interplanetary Shock by Advanced Composition Explorer

R. L. Tokar, W. C. Feldman, S. P. Gary, J. T. Gosling, D. J. McComas, R. M. Skoug, C. W. Smith, N. F. Ness and D. Haggerty

Journal of Geophysical Research, submitted (1999).


Observations of a relatively cool, beam-like ion distribution upstream of an interplanetary shock near 1 AU are presented in this paper. Such observations upstream of interplanetary shocks are rare, with only one known example in the literature, a shock studied by Voyager 1 at about 2.25 AU. ISEE 1 and 2 studies reported only hotter, relatively isotropic ions in the upstream region of interplanetary shocks. The ions are observed by the Advanced Composition Explorer SWEPAM and EPAM instruments and associated MHD fluctuations are observed by the ACE MAG instrument. The energy per charge of the ions observed by SWEPAM extends from about 700 eV/q to about 3 keV/q in the spacecraft frame, while the ions seen by EPAM extend to about 100 keV in the spacecraft frame. The fluctuating fields observed by MAG have spacecraft frame frequencies from about 0.03 to 0.3 Hz. The particle and field data is combined to determine the ion phase space density parallel and perpendicular to the ambient magnetic field, and the results suggest that a field-aligned beam moving away from the shock is observed. The observations are compared with predictions from both linear Vlasov theory and plasma simulation. The instability theory predicts that the observed ions can drive fluctuations with the observed frequencies and polarizations. In addition, fluctuating field amplitudes are in qualitative agreement with simulation predictions.

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