Energetic particle observations by Ulysses during the declining phase of solar cycle 23

Abstract

We present recent energetic particle measurements from 1–20 MeV/n recorded by the Ulysses/COSPIN/LET instrument from mid‐October to the end of November 2003 and from May 2004 to the end of February 2005. Long‐lasting periods with enhanced solar activity occurred during the declining phase of the current solar cycle contributing to the high‐intensity particle events and heavy ion signatures observed by Ulysses near the ecliptic plane, at ∼5 AU from the Sun. The energetic particles injected from the Sun during the January 2005 intense solar activity were not observed at Ulysses until a stream interface that acted as a barrier for the particles reached the spacecraft. Our observations during this period thus show that the propagation of energetic particles is largely governed by the existence of large‐scale solar wind structures that impede the transport of the particles. We use the elemental composition of the particle fluxes recorded by COSPIN/LET to gain insight into the possible origin of the particle events. We study and provide a possible interpretation of the composition signatures observed at ∼5 AU during periods of CME/CIR combinations. Although predominantly SEP‐like, the observed composition showed evidence for enhancements in He, which we suggest is of interstellar origin, consistent with CIR shocks accelerating ions from multiple sources.