Wind Observations of Upstream MeV Ion Bursts
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.