Statistical study of low‐energy heliosphere particle fluxes from 1.4 to 5 AU over a solar cycle

Abstract

Throughout the entire Ulysses mission, the Heliosphere Instrument for Spectra, Composition, and Anisotropy at Low Energies (HI‐SCALE) has collected measurements of low‐energy interplanetary ions and electrons. Time series of electron, proton, and ion fluxes have been obtained since 1990. We present statistical studies of high‐resolution ion and electron energy spectra (∼50 keV to ∼5 MeV) as measured by the HI‐SCALE instrument on the Ulysses spacecraft over a time interval longer than a solar cycle (1990 to 2004). Ulysses is the only spacecraft that continually measured the inner (∼1.4 to ∼5 AU) heliosphere particle population during these years. The data thus provide measures of the lower‐energy population of particles that a spacecraft traveling outward from Earth would have encountered and that also could have impacted the atmosphere and surface of Mars and of its satellites during this interval. Comparisons of Ulysses particle fluxes with those from the Electron, Proton, and Alpha Monitor (EPAM) instrument on the Advanced Composition Explorer (ACE) spacecraft (the HI‐SCALE backup instrument) have shown that azimuthal and heliolatitude dependencies of particle fluxes in the inner heliosphere following solar events are not as extreme as might be expected. Thus the Ulysses measurements, while taken over a range of heliolatitudes, can provide important statistical information that can be used to estimate the low‐energy radiation dosages and potential sputtering fluxes to planetary surfaces and to heliosphere spacecraft surfaces and solar arrays over a solar cycle.