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The Relative Recovery of Galactic and Anomalous Cosmic Rays in the Distant Heliosphere: Evidence for modulation in the heliosheath

McDonald, F. B. and Heikkila, B. and Lal, N. and Stone, E. C. (2000) The Relative Recovery of Galactic and Anomalous Cosmic Rays in the Distant Heliosphere: Evidence for modulation in the heliosheath. Journal of Geophysical Research A, 105 (A1). pp. 1-8. ISSN 0148-0227. doi:10.1029/1999JA900372.

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At Voyager 1 (46 AU, 33°N) the recovery of anomalous cosmic rays (ACR) is found to be very different from that of galactic cosmic rays (GCR) following the passage of the large interplanetary disturbances produced by the intensive solar activity of March/June 1991. If the modulation boundary for the GCR were at the termination shock, where anomalous cosmic rays are believed to originate, it would be expected that the intensity of the higher-energy galactic cosmic rays would recover more rapidly than the relatively low energy anomalous component. On the contrary, we find that the time constant for the recovery of 265 MeV/nucleon GCR He is approximately twice as large as that of 43 MeV/nucleon ACR He^+ and 13 MeV/nucleon O^+. A regression plot of the ACR versus GCR intensity indicates a broad plateau in the ACR intensity over a period of several years while the GCR continues to increase. These differences in the relative recovery of the ACR and GCR strongly suggest that the combined interplanetary disturbances in the form of a global merged interaction region (GMIR) produced by the March/June 1991 solar activity remain an effective modulation agent for GCR after passing beyond the termination shock and into the region of the heliosheath. Some 0.37 years after the passage of the leading portion of the GMIR by Voyager 1, there is a large anisotropy in the ACR He^+. One possible interpretation of this anisotropy is that it is produced by the initial flow of the ACR back into the heliosphere at the time that the leading portion of the interplanetary disturbance moves beyond the termination shock. If this interpretation is correct, then the inferred transit time between Voyager 1 and the termination shock of the GMIR along with an estimate of its velocity at 40 AU based on similar features in the Voyager 1 and Pioneer 11 energetic particle data give a value of the heliocentric distance to the termination shock of 88.5 ± 7 AU at ∼33°N in early 1992.

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Stone, E. C.0000-0002-2010-5462
Additional Information:© 2000 American Geophysical Union. Paper number 1999JA900372. (Received July 7, 1999; revised August 16, 1999; accepted August 18, 1999.) Janet G. Luhmann thanks James A. Van Allen and John Quenby for their assistance in evaluating this paper.
Group:Space Radiation Laboratory
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Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2000-15
Issue or Number:A1
Record Number:CaltechAUTHORS:20150219-103841210
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Official Citation:McDonald, F. B., B. Heikkila, N. Lal, and E. C. Stone (2000), The relative recovery of galactic and anomalous cosmic rays in the distant heliosphere: Evidence for modulation in the heliosheath, J. Geophys. Res., 105(A1), 1–8, doi:10.1029/1999JA900372.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:55004
Deposited By: Deborah Miles
Deposited On:23 Feb 2015 20:14
Last Modified:10 Nov 2021 20:40

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