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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. A9, PAGES 17,637–17,643, 1994

Acceleration of Interstellar Pickup Ions in the Disturbed Solar Wind Observed on Ulysses

G. Gloeckler

Also at Institute for Physical Science and Technology, University of Maryland, College Park.

J. Geiss

Physikalisches Institut, University of Bern, Switzerland.

E. C. Roelof

Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland.

L. A. Fisk

Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor.

F. M. Ipavich

Department of Physics, University of Maryland, College Park.

K. W. Ogilvie

NASA Goddard Space Flight Center, Greenbelt, Maryland.

L. J. Lanzerotti

Bell Laboratories, Murray Hill, New Jersey.

R. von Steiger

Physikalisches Institut, University of Bern, Switzerland.

B. Wilken

Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, Germany.

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) = F₀w ⁻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.

Received 12 May 1994; accepted 13 June 1994.

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Citation: Gloeckler, G., J. Geiss, E. C. Roelof, L. A. Fisk, F. M. Ipavich, K. W. Ogilvie, L. J. Lanzerotti, R. von Steiger, and B. Wilken (1994), Acceleration of Interstellar Pickup Ions in the Disturbed Solar Wind Observed on Ulysses, J. Geophys. Res., 99(A9), 17,637–17,643.

Copyright 1994 by the American Geophysical Union.