ELECTRON INJECTION BY WHISTLER WAVES IN NON-RELATIVISTIC SHOCKS

Journal Article · Fri May 20 04:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/733/1/63· OSTI ID:21576765

Riquelme, Mario A ^[1]; Spitkovsky, Anatoly ^[2]

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1. Astronomy Department, University of California, Berkeley, CA 94720 (United States)
2. Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)

Electron acceleration to non-thermal, ultra-relativistic energies ({approx}10-100 TeV) is revealed by radio and X-ray observations of shocks in young supernova remnants (SNRs). The diffusive shock acceleration (DSA) mechanism is usually invoked to explain this acceleration, but the way in which electrons are initially energized or 'injected' into this acceleration process starting from thermal energies is an unresolved problem. In this paper we study the initial acceleration of electrons in non-relativistic shocks from first principles, using two- and three-dimensional particle-in-cell (PIC) plasma simulations. We systematically explore the space of shock parameters (the Alfvenic Mach number, M{sub A} , the shock velocity, v{sub sh}, the angle between the upstream magnetic field and the shock normal, {theta}{sub Bn}, and the ion to electron mass ratio, m{sub i} /m{sub e} ). We find that significant non-thermal acceleration occurs due to the growth of oblique whistler waves in the foot of quasi-perpendicular shocks. This acceleration strongly depends on using fairly large numerical mass ratios, m{sub i} /m{sub e} , which may explain why it had not been observed in previous PIC simulations of this problem. The obtained electron energy distributions show power-law tails with spectral indices up to {alpha} {approx} 3-4. The maximum energies of the accelerated particles are consistent with the electron Larmor radii being comparable to that of the ions, indicating potential injection into the subsequent DSA process. This injection mechanism, however, requires the shock waves to have fairly low Alfenic Mach numbers, M{sub A} {approx}< 20, which is consistent with the theoretical conditions for the growth of whistler waves in the shock foot (M{sub A} {approx}< (m{sub i} /m{sub e}){sup 1/2}). Thus, if the whistler mechanism is the only robust electron injection process at work in SNR shocks, then SNRs that display non-thermal emission must have significantly amplified upstream magnetic fields. Such field amplification is likely achieved by the escaping cosmic rays, so electron and proton acceleration in SNR shocks must be interconnected.

• OSTI ID: 21576765
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 733; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCELERATION
BEAM INJECTION
COSMIC RADIATION
COSMIC RADIO SOURCES
DIMENSIONLESS NUMBERS
ELECTROMAGNETIC RADIATION
ELECTRON BEAM INJECTION
ELECTRONS
ELEMENTARY PARTICLES
ENERGY SPECTRA
FERMIONS
IONIZING RADIATIONS
LEPTONS
MACH NUMBER
MAGNETIC FIELDS
NOISE
PLASMA SIMULATION
RADIATIONS
RADIO NOISE
RADIOWAVE RADIATION
SHOCK WAVES
SIMULATION
SPECTRA
SUPERNOVA REMNANTS
THREE-DIMENSIONAL CALCULATIONS
TWO-DIMENSIONAL CALCULATIONS
VELOCITY
WHISTLERS