M. I. Desai, G. M. Mason, J. R. Dwyer
	University of Maryland, College Park, Maryland, USA
J. E. Mazur
	Aerospace Corp, Los Angeles, CA, USA
T. T. von Rosenvinge, R. P. Lepping
	NASA/GSFC, Greenbelt, Maryland, USA  

The origin of ion populations observed 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 above 70 keV/nucleon in these events. We present here a detailed statistical analysis of the energy spectra and composition of ions above ~30 keV/nucleon during 1225 such 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 KP, the solar wind speed, and the orientation of the interplanetary magnetic field. We have 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.