Fermi acceleration of suprathermal solar wind oxygen ions

The Wind spacecraft has observed numerous sunward bursts of approximate to 2 MeV ions upstream of the Earth's bow shock. These energetic particles are not protons or alpha particles and are probably oxygen ions. The bursts ty pically last several minutes at the highest energies, but they can last for tens of minutes at intermediate energies (tens to hundreds of keV). In thi s paper, Wind observations of the November 30, 1994, bursts and numerical p article simulations are presented that demonstrate that suprathermal solar wind O6+ ions, which have undergone Fermi acceleration between the bow shoc k and interplanetary magnetic field (IMF) rotations, are the most likely so urce of these bursts. Each burst either coincides with or is closely follow ed by a large IMF rotation. By using measured magnetic field data the timin g of the bursts detected by Wind is reproduced in the simulation. Simulated spectra of H+, He2+, and O6+ fluxes fit the observed spectra, assuming an increase of similar to 2 orders of magnitude in the high-energy tail of the solar wind oxygen distribution, relative to the average solar wind abundan ce of oxygen. Such enhancements in CNO group ions above 40 keV/nucleon were measured by the Suprathermal and Energetic Particle (STEP) instrument duri ng this interval. This study concludes by predicting that ion composition a nd charge state measurements will show these bursts to be solar wind oxygen ions in high charge states.