Heavy Ion Composition in Upstream Events Near Earth:
New Challenges for Old Models

Mihir Desai
Department of Physics, University of Maryland, College Park, MD

The origin of ion populations observed in interplanetary space upstream of the Earth's bow shock has been controversial since they were first discovered in the 1960's. To date these observations have been explained in terms of either the leakage of ions accelerated inside the Earth's magnetosphere into the upstream region or the acceleration of solar wind ions via a first-order Fermi process occurring at the bow shock. One of the key measurements that has not been available until the launch of high-sensitivity mass spectrometers, such as the SupraThermal Energetic Particle (STEP) telescope on board the WIND spacecraft, is that of the ion composition of these events. We present here a detailed statistical survey of the energy spectra and composition of ions above ~70 keV/nucleon during 1225 upstream events observed by STEP from November 1994 until March 1999. We have examined the relationship between the occurrence rate of these events and the geomagnetic activity index, the solar wind speed, and the orientation of the interplanetary magnetic field. We have also investigated the spatial and temporal occurrence rates of the events. We compare our results with predictions of the two traditional models, namely, magnetospheric leakage and Fermi acceleration at the bow shock, that have been proposed to account for the origin and acceleration of upstream ions. We find that neither model in its current version can satisfactorily account for our results. We highlight the main constraints and new challenges posed by our results for both models.