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The Distance to the Solar Wind Termination Shock in 1993 and 1994 from Observations of Anomalous Cosmic Rays

Stone, E. C. and Cummings, A. C. and Webber, W. R. (1996) The Distance to the Solar Wind Termination Shock in 1993 and 1994 from Observations of Anomalous Cosmic Rays. Geophysical Research Letters, 101 (A5). pp. 11017-11025. ISSN 0094-8276. doi:10.1029/96JA00413.

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Using data from the Voyager and Pioneer spacecraft, we examine the energy spectra of anomalous cosmic rays (ACRs) during 1992–1994, a period of rapidly increasing intensities of these particles in the outer heliosphere. The 1992 period appears to be in nonequilibrium, marked by a rapid decrease in the tilt of the current sheet from ∼63° to ∼35° and a complex, evolving magnetic topology that affects interplanetary drifts and drift acceleration along the solar wind termination shock. In addition, large propagating interplanetary disturbances in 1991 may have weakened the shock temporarily, reducing the intensity of accelerated ions. The 1993–1994 period appears to be in quasi-equilibrium, marked by a stable tilt of the current sheet and a steady, large-scale magnetic topology that provides stable conditions for propagation and acceleration. The ACR intensities of higher-rigidity particles rise slowly during this period, suggesting that the shock intensity has stabilized. At lower rigidities the ACR intensities are increasing rapidly, which we attribute to decreasing modulation. During 1994 days 157–209, we estimate that the shock was at 85 ± 5 AU. The shock strength s is estimated to be 2.63 ± 0.14, significantly weaker than a strong shock (s = 4) and consistent with a termination shock modified by the pressure of galactic cosmic rays. By comparing these 1994 observations with those made in 1987, we estimate that the latitudinal gradient of ACR He with energies 31–57 MeV nucleon^(−1) is ∼1.9%/deg along the shock from the equator to the pole. We estimate that the ratio of the efficiency of injection of He^+ to H^+ pickup ions into the acceleration process is 7.0 ± 1.6.

Item Type:Article
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URLURL TypeDescription
Stone, E. C.0000-0002-2010-5462
Cummings, A. C.0000-0002-3840-7696
Additional Information:Copyright 1996 by the American Geophysical Union. (Received August 17, 1995; revised January 25, 1996; accepted January 29, 1996.) We are grateful to J. T. Hoeksema for providing the tilt observations prior to publication. We thank J. Richardson and J. Belcher for providing the Voyager 2 solar wind speed data. This work was supported by NASA under contract NAS7-918. The Editor thanks D. T. Hall and two other referees for their assistance in evaluating this paper.
Group:Space Radiation Laboratory
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Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory1995-04
Issue or Number:A5
Record Number:CaltechAUTHORS:20140612-105003970
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:46237
Deposited By: Deborah Miles
Deposited On:12 Jun 2014 23:18
Last Modified:10 Nov 2021 17:22

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