1998A&A...335.1101C

During its southbound journey the Ulysses spacecraft crossed a series of corotating interaction regions (CIRs) building up due to the interaction of fast and slow solar wind streams. We analyse the forward and the reverse shocks marking off the 18 CIR encounters between July 1992 and December 1993. Our investigations look for a correlation between the particle acceleration efficiency expressed by the particle flux measured at the time of shock crossing and the MHD characteristics of the shocks; i.e., Alfven-Mach number (M_A1), density and magnetic field compression ratios (r_N, r_B), and the angle between shock normal and upstream magnetic field (θ_Bn). The results of this analysis show that the highest fluxes of 300 keV electrons and 1MeV protons are observed when the conditions M_A1>2.5 and 50°≤θ_Bn≤75° are simultaneously fulfilled by the shocks. These investigations are supplemented by a computation of the first critical Alfven-Mach number for typical parameters of CIR-related shocks. Furthermore, we discuss possible acceleration mechanisms by an analysis of the spectral indices of protons and Helium.