Suprathermal ions and MHD turbulence observed upstream of an interplanetary shock by Advanced Composition Explorer

Observations of a 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. The ions are observed by the Advanced Composition Explorer (ACE) Solar Wind Electron Proton Alpha Monitor (SWEPAM) and Energetic Proton Alpha Monitor (EPAM) instruments and associated MHD fluctuations are observed by the ACE Bartol Research Institute Magnetometer (MAG) instrument. The energy per charge of the ions observed by SWEPAM extends from ∼700 eV/q to ∼3 keV/q in the spacecraft frame, while the ions seen by EPAM extend to ∼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 are 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. Linear Vlasov theory predicts that the observed ions can drive fluctuations with the observed frequencies and polarization. In addition, fluctuating field amplitudes are in qualitative agreement with simulation predictions.