T. J. Freeman, D. E. Larson, R. P. Lin, K. Meziane University of California, Berkeley, CA, USA G. K. Parks University of Washington, Seattle, WA, USA
The 3d Plasma Analyzer instrument on the Wind spacecraft has measured
more than 100 upstream ion events which contain ions of ~2 MeV. The
detailed in situ observations made by Wind can shed light on a process
that not only produces high energy ions upstream of the Earth's bow
shock, but which scales geometrically to larger astrophysical systems,
and is thought to produce cosmic rays.
A survey of these events, using data from both the ion and electron detectors, is shown to demonstrate that these events do not contain protons or alpha particles, and therefore are probably due to oxygen ions.
Three-dimensional distribution functions of these bursts show these events to be highly non-gyrotropic, from ~100 keV to ~2 MeV. In a typical event, nearly all of the flux is gyrophase-bunched, with an angular width of ~60 degrees.
These observations are explained by a model of Fermi acceleration in which ions are energized through repeated reflections between upstream large-scale IMF rotations and the bow shock. Using measured IMF data to track particle trajectories backwards from the spacecraft location to the bow shock, it is shown that these particles come (most recently) from localized regions of the bow shock where the geometry is quasi-perpendicular. Tracing trajectories further back in time, to earlier bow shock reflections, we find that these particles originate in the suprathermal ion distribution at shock locations in which the geometry is quasi-parallel. These starting points are often quite far from the location of the final shock interaction before the particles were detected at the spacecraft.
If these events are due to the leakage of magnetospheric ions, and not Fermi acceleration, the ions would have to leak from specific regions of the bow shock in such a way as to produce the non-gyrotropic distributions observed upstream. The implications for each model will be discussed.