Heavy Ion Composition of Upstream Events: New Challenges for Old Models

Enhancements in the intensities of energetic ions above a few keV nucleon^-1 have been routinely observed upstream of the Earth's bow shock since the 1960's. Such upstream events were typically attributed either to the leakage of ionospheric ions accelerated inside the Earth' magnetosphere into the upstream region or to the acceleration of solar wind ions via a first-order Fermi process occurring at the bow shock. Although studied extensively until the early 1990's, the origin of upstream ions remained controversial primarily due to the lack of composition measurements in the suprathermal energy range i.e., from the thermal solar wind energy through the tens of keV nucleon^-1. However, the launch of the Wind spacecraft in 1994 with mass spectrometers such as the SupraThermal through Energetic Particle (STEP) instrument has offered new opportunities to investigate the origin of upstream ions. In this talk, we will focus on results of a comprehensive survey of the heavy ion composition and energy spectra above ~30 keV nucleon^-1 measured by the STEP instrument during more than a thousand upstream events. Our results show that energetic ions during such events originate primarily from the solar wind, and occasionally from ³He-rich impulsive solar flares and corotating interaction regions. Recently, measurements from the SupraThermal Ion Composition Spectrometer (STICS) have shown that ionospheric species such as O⁺ and N⁺ can also be present simultaneously during some of these events, indicating that the ionosphere can also be an important source of ions in upstream events. We will also present a detailed analysis of some events when both ionospheric and solar wind ions are present simultaneosuly and compare our new results with predictions of the above two models, namely, magnetospheric leakage and Fermi acceleration at the bow shock.