Energetic proton observations at 1 and 5 AU: 1. January–September 1997

Abstract

We present energetic particle observations by the Ulysses and Wind spacecraft during the years 1997–1998. During this period, Ulysses moved from +19° to −19° in heliographic latitude with heliocentric distances ranging from 4.71 to 5.41 AU. Wind was located near the Earth at 1 AU from the Sun in the ecliptic plane. The proximity of Ulysses to the ecliptic plane allows us to compare proton fluxes at different heliospheric distances and heliolongitudes but at similar latitudes. Enhancements in the energetic (approximately MeV) MeV proton fluxes were observed by both spacecraft in a close temporal association. We quantify this temporal coincidence and we distinguish two time intervals based on the energy range of the proton flux increases and on the solar wind streams observed by the Ulysses spacecraft. Each one of these two periods (period 1, January–September 1997 and period 2, October 1997 to December 1998) corresponds also to a different level of solar activity. We establish a direct correspondence between individual particle flux enhancements observed by both spacecraft throughout period 1. At 1 AU the proton flux enhancements are associated with the occurrence of coronal mass ejections (CMEs) at the Sun. At 5 AU the presence of high‐speed solar wind streams, the contribution of energetic particles from CMEs, and the evolution of the magnetic field structures along which energetic particles propagate determine the final shape of the proton flux enhancements and their time delays with respect to 1 AU. When a solar energetic particle (SEP) event is observed at 1 AU, particle flux enhancements associated with recurrent high‐speed solar wind streams at ∼5 AU show a higher particle flux intensity. We suggest that SEPs accelerated by CME‐driven shocks act as a background seed particle population available to be reaccelerated by shocks associated with corotating high‐speed streams.