SUPRATHERMAL ELECTRONS AT SATURN'S BOW SHOCK

Journal Article · Wed Jul 20 04:00:00 EDT 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/826/1/48· OSTI ID:22665997

Masters, A.; Dougherty, M. K. ^[1]; Sulaiman, A. H. ^[2]; Sergis, N. ^[3]; Stawarz, L. ^[4]; Fujimoto, M. ^[5]

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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. Office of Space Research and Technology, Academy of Athens, Soranou Efesiou 4, 11527 Athens (Greece)
4. Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Krakow (Poland)
5. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210 (Japan)

The leading explanation for the origin of galactic cosmic rays is particle acceleration at the shocks surrounding young supernova remnants (SNRs), although crucial aspects of the acceleration process are unclear. The similar collisionless plasma shocks frequently encountered by spacecraft in the solar wind are generally far weaker (lower Mach number) than these SNR shocks. However, the Cassini spacecraft has shown that the shock standing in the solar wind sunward of Saturn (Saturn's bow shock) can occasionally reach this high-Mach number astrophysical regime. In this regime Cassini has provided the first in situ evidence for electron acceleration under quasi-parallel upstream magnetic conditions. Here we present the full picture of suprathermal electrons at Saturn's bow shock revealed by Cassini . The downstream thermal electron distribution is resolved in all data taken by the low-energy electron detector (CAPS-ELS, <28 keV) during shock crossings, but the higher energy channels were at (or close to) background. The high-energy electron detector (MIMI-LEMMS, >18 keV) measured a suprathermal electron signature at 31 of 508 crossings, where typically only the lowest energy channels (<100 keV) were above background. We show that these results are consistent with the theory in which the “injection” of thermal electrons into an acceleration process involves interaction with whistler waves at the shock front, and becomes possible for all upstream magnetic field orientations at high Mach numbers like those of the strong shocks around young SNRs. A future dedicated study will analyze the rare crossings with evidence for relativistic electrons (up to ∼1 MeV).

• OSTI ID: 22665997
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 826; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCELERATION
ASTROPHYSICS
COLLISIONLESS PLASMA
COSMIC RADIATION
DATA ANALYSIS
DISTRIBUTION
ELECTRONS
GALAXIES
INTERACTIONS
KEV RANGE
MACH NUMBER
MAGNETIC FIELDS
MEV RANGE
RELATIVISTIC RANGE
SATURN PLANET
SHOCK WAVES
SOLAR WIND
SPACE VEHICLES
SUPERNOVA REMNANTS
TAIL ELECTRONS