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 0 w −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.