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Spectral Evolution of Anomalous Cosmic Rays at Voyager 1 beyond the Termination Shock

Senanayake, U. K. and Florinski, V. and Cummings, A. C. and Stone, E. C. (2015) Spectral Evolution of Anomalous Cosmic Rays at Voyager 1 beyond the Termination Shock. Astrophysical Journal, 804 (1). Art. No. 12. ISSN 0004-637X. doi:10.1088/0004-637X/804/1/12.

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When the Voyager 1 spacecraft crossed the termination shock (TS) on 2004 December 16, the energy spectra of anomalous cosmic rays (ACRs) could not have been produced by steady-state diffusive shock acceleration. However, over the next few years, in the declining phase of the solar cycle, the spectra began to evolve into the expected power-law profile. Observations at the shock led to a broad range of alternative theories for ACR acceleration. In spite of that, in this work we show that the observations could be explained by assuming ACRs are accelerated at the TS. In this paper, we propose that the solar cycle had an important effect on the unrolling of the spectra in the heliosheath. To investigate the spectral evolution of ACRs, a magnetohydrodynamic background model with stationary solar-wind inner boundary conditions was used to model the transport of helium and oxygen ions. We used a backward-in-time stochastic integration technique where phase-space trajectories are integrated until the so-called "injection energy" is reached. Our simulation results were compared with Voyager 1 observations using three different diffusion models. It is shown that the spectral evolution of ACRs in the heliosheath at Voyager 1 could be explained by an increase in the source strength and an enhancement in diffusion as a result of a decrease of the turbulent correlation length in the declining phase of the solar cycle. At the same time, drift effects seem to have had a smaller effect on the evolution of the spectra.

Item Type:Article
Related URLs:
URLURL TypeDescription DOIArticle
Florinski, V.0000-0001-5485-2872
Cummings, A. C.0000-0002-3840-7696
Stone, E. C.0000-0002-2010-5462
Additional Information:© 2015 The American Astronomical Society. Received 2014 December 6; accepted 2015 February 24; published 2015 April 23. This work was supported in part by NASA grants NNX10AE46G, NNX11AO64G, NNX12AH44G, and NNX13AF99G and by NSF grant AGS-0955700.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Subject Keywords:acceleration of particles; diffusion; shock waves; solar wind; Sun: heliosphere
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2015-04
Issue or Number:1
Record Number:CaltechAUTHORS:20150604-140432982
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Official Citation:Spectral Evolution of Anomalous Cosmic Rays at Voyager 1 beyond the Termination Shock U. K. Senanayake et al. 2015 ApJ 804 12
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58010
Deposited By: Tony Diaz
Deposited On:04 Jun 2015 21:12
Last Modified:10 Nov 2021 21:57

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