An in situ Comparison of Electron Acceleration at Collisionless Shocks under Differing Upstream Magnetic Field Orientations

Journal Article · Mon Jul 10 04:00:00 EDT 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/AA76EA· OSTI ID:22663395

Masters, A.; Dougherty, M. K. ^[1]; Sulaiman, A. H. ^[2]; Stawarz, Ł. ^[3]; Reville, B. ^[4]; Sergis, N. ^[5]; Fujimoto, M. ^[6]; Burgess, D. ^[7]

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1. The Blackett Laboratory, Imperial College London, Prince Consort Road, London, SW7 2AZ (United Kingdom)
2. Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242 (United States)
3. Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Krakow (Poland)
4. School of Mathematics and Physics, Queens University Belfast, Belfast BT7 1NN (United Kingdom)
5. Office of Space Research and Technology, Academy of Athens, Soranou Efesiou 4, 11527 Athens (Greece)
6. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210 (Japan)
7. School of Physics and Astronomy, Queen Mary University of London, London E1 4NS (United Kingdom)

A leading explanation for the origin of Galactic cosmic rays is acceleration at high-Mach number shock waves in the collisionless plasma surrounding young supernova remnants. Evidence for this is provided by multi-wavelength non-thermal emission thought to be associated with ultrarelativistic electrons at these shocks. However, the dependence of the electron acceleration process on the orientation of the upstream magnetic field with respect to the local normal to the shock front (quasi-parallel/quasi-perpendicular) is debated. Cassini spacecraft observations at Saturn’s bow shock have revealed examples of electron acceleration under quasi-perpendicular conditions, and the first in situ evidence of electron acceleration at a quasi-parallel shock. Here we use Cassini data to make the first comparison between energy spectra of locally accelerated electrons under these differing upstream magnetic field regimes. We present data taken during a quasi-perpendicular shock crossing on 2008 March 8 and during a quasi-parallel shock crossing on 2007 February 3, highlighting that both were associated with electron acceleration to at least MeV energies. The magnetic signature of the quasi-perpendicular crossing has a relatively sharp upstream–downstream transition, and energetic electrons were detected close to the transition and immediately downstream. The magnetic transition at the quasi-parallel crossing is less clear, energetic electrons were encountered upstream and downstream, and the electron energy spectrum is harder above ∼100 keV. We discuss whether the acceleration is consistent with diffusive shock acceleration theory in each case, and suggest that the quasi-parallel spectral break is due to an energy-dependent interaction between the electrons and short, large-amplitude magnetic structures.

• OSTI ID: 22663395
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 843; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCELERATION
AMPLITUDES
COLLISIONLESS PLASMA
COMPARATIVE EVALUATIONS
COSMIC RADIATION
DATA ANALYSIS
EMISSION
ENERGY DEPENDENCE
ENERGY SPECTRA
KEV RANGE
MACH NUMBER
MAGNETIC FIELDS
MEV RANGE
RELATIVISTIC RANGE
SHOCK WAVES
SOLAR WIND
SUPERNOVA REMNANTS
TAIL ELECTRONS