Acceleration of interstellar pickup ions in the disturbed solar wind observed on Ulysses

Abstract

Acceleration of interstellar pickup H⁺ and He⁺ as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at ~4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H⁺ and He⁺) and the low thermal speed solar wind ions (H⁺ and He⁺⁺) are derived using velocity distribution functions of protons, pickup He⁺ and alpha particles from <1 to 60 keV/e and of ions (principally protons) above ~60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of the CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H⁺ and He⁺ are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He⁺, and intermediate for alpha particles, (4) both H⁺ and He⁺ have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w)=F_Ow^-4 for 1<w<~5, where w is the ion speed divided by the solar wind speed. Above w~5-10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.