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Galactic Cosmic-Ray Anisotropies: Voyager 1 in the Local Interstellar Medium

Rankin, J. S. and Stone, E. C. and Cummings, A. C. and McComas, D. J. and Lal, N. and Heikkila, B. C. (2019) Galactic Cosmic-Ray Anisotropies: Voyager 1 in the Local Interstellar Medium. Astrophysical Journal, 873 (1). Art. No. 46. ISSN 1538-4357.

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Since crossing the heliopause on 2012 August 25, Voyager 1 observed reductions in galactic cosmic ray count rates caused by a time-varying depletion of particles with pitch angles near 90°, while intensities of particles with other pitch angles remain unchanged. Between late 2012 and mid-2017, three large-scale events occurred, lasting from ~100 to ~630 days. Omnidirectional and directional high-energy data from Voyager 1's Cosmic Ray Subsystem are used to report cosmic ray intensity variations. Omnidirectional (≳20 MeV) proton-dominated measurements show up to a 3.8% intensity reduction. Bidirectional (≳70 MeV) proton-dominated measurements taken from various spacecraft orientations provide insight about the depletion region's spatial properties. We characterize the anisotropy as a "notch" in an otherwise uniform pitch angle distribution of varying depth and width centered about 90° in pitch angle space. The notch averages 22° wide and 15% deep, signifying a depletion region that is broad and shallow. There are indications that the anisotropy is formed by a combination of magnetic trapping and cooling downstream of solar-induced transient disturbances in a region that is also likely influenced by the highly compressed fields near the heliopause.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Rankin, J. S.0000-0002-8111-1444
Stone, E. C.0000-0002-2010-5462
Cummings, A. C.0000-0002-3840-7696
McComas, D. J.0000-0001-6160-1158
Additional Information:© 2019 The American Astronomical Society. Received 2018 December 4; revised 2019 January 22; accepted 2019 January 31; published 2019 March 4. We thank E. Roelof for first suggesting particle trapping as a means for generating the anisotropy, R. Decker and the LECP team for providing background-corrected data, and E. Zirnstein for providing model-based information about the field enhancement toward the heliopause. This work was supported by NASA grant No. NNN12AA01C. J.S.R. and D.M. were also supported by the Interstellar Boundary Explorer (IBEX) mission, which is part of NASA's Explorer Program.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Subject Keywords:astroparticle physics – cosmic rays – ISM: magnetic fields – scattering – shock waves – turbulence
Issue or Number:1
Record Number:CaltechAUTHORS:20190321-114830889
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Official Citation:J. S. Rankin et al 2019 ApJ 873 46
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94021
Deposited By: Tony Diaz
Deposited On:21 Mar 2019 21:55
Last Modified:04 Feb 2020 00:29

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