Nonthermal electron acceleration in low Mach number collisionless shocks. II. Firehosemediated Fermi acceleration and its dependence on preshock conditions
Abstract
Electron acceleration to nonthermal energies is known to occur in low Mach number (M{sub s} ≲ 5) shocks in galaxy clusters and solar flares, but the electron acceleration mechanism remains poorly understood. Using twodimensional (2D) particleincell (PIC) plasma simulations, we showed in Paper I that electrons are efficiently accelerated in low Mach number (M{sub s} = 3) quasiperpendicular shocks via a Fermilike process. The electrons bounce between the upstream region and the shock front, with each reflection at the shock resulting in energy gain via shock drift acceleration. The upstream scattering is provided by oblique magnetic waves that are selfgenerated by the electrons escaping ahead of the shock. In the present work, we employ additional 2D PIC simulations to address the nature of the upstream oblique waves. We find that the waves are generated by the shockreflected electrons via the firehose instability, which is driven by an anisotropy in the electron velocity distribution. We systematically explore how the efficiency of wave generation and of electron acceleration depend on the magnetic field obliquity, the flow magnetization (or equivalently, the plasma beta), and the upstream electron temperature. We find that the mechanism works for shocks with high plasma beta (≳ 20) atmore »
 Authors:
 HarvardSmithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
 NASA Einstein Postdoctoral Fellow. (United States)
 Publication Date:
 OSTI Identifier:
 22370051
 Resource Type:
 Journal Article
 Resource Relation:
 Journal Name: Astrophysical Journal; Journal Volume: 797; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
 Country of Publication:
 United States
 Language:
 English
 Subject:
 79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; ANISOTROPY; DISTRIBUTION; EFFICIENCY; ELECTRON TEMPERATURE; EMISSION; GALAXY CLUSTERS; HOSE INSTABILITY; MACH NUMBER; MAGNETIC FIELDS; MAGNETIZATION; PLASMA; PLASMA SIMULATION; SCATTERING; SHOCK WAVES; SOLAR FLARES; SYNCHROTRON RADIATION; TWODIMENSIONAL CALCULATIONS
Citation Formats
Guo, Xinyi, Narayan, Ramesh, and Sironi, Lorenzo. Nonthermal electron acceleration in low Mach number collisionless shocks. II. Firehosemediated Fermi acceleration and its dependence on preshock conditions. United States: N. p., 2014.
Web. doi:10.1088/0004637X/797/1/47.
Guo, Xinyi, Narayan, Ramesh, & Sironi, Lorenzo. Nonthermal electron acceleration in low Mach number collisionless shocks. II. Firehosemediated Fermi acceleration and its dependence on preshock conditions. United States. doi:10.1088/0004637X/797/1/47.
Guo, Xinyi, Narayan, Ramesh, and Sironi, Lorenzo. Wed .
"Nonthermal electron acceleration in low Mach number collisionless shocks. II. Firehosemediated Fermi acceleration and its dependence on preshock conditions". United States.
doi:10.1088/0004637X/797/1/47.
@article{osti_22370051,
title = {Nonthermal electron acceleration in low Mach number collisionless shocks. II. Firehosemediated Fermi acceleration and its dependence on preshock conditions},
author = {Guo, Xinyi and Narayan, Ramesh and Sironi, Lorenzo},
abstractNote = {Electron acceleration to nonthermal energies is known to occur in low Mach number (M{sub s} ≲ 5) shocks in galaxy clusters and solar flares, but the electron acceleration mechanism remains poorly understood. Using twodimensional (2D) particleincell (PIC) plasma simulations, we showed in Paper I that electrons are efficiently accelerated in low Mach number (M{sub s} = 3) quasiperpendicular shocks via a Fermilike process. The electrons bounce between the upstream region and the shock front, with each reflection at the shock resulting in energy gain via shock drift acceleration. The upstream scattering is provided by oblique magnetic waves that are selfgenerated by the electrons escaping ahead of the shock. In the present work, we employ additional 2D PIC simulations to address the nature of the upstream oblique waves. We find that the waves are generated by the shockreflected electrons via the firehose instability, which is driven by an anisotropy in the electron velocity distribution. We systematically explore how the efficiency of wave generation and of electron acceleration depend on the magnetic field obliquity, the flow magnetization (or equivalently, the plasma beta), and the upstream electron temperature. We find that the mechanism works for shocks with high plasma beta (≳ 20) at nearly all magnetic field obliquities, and for electron temperatures in the range relevant for galaxy clusters. Our findings offer a natural solution to the conflict between the bright radio synchrotron emission observed from the outskirts of galaxy clusters and the low electron acceleration efficiency usually expected in low Mach number shocks.},
doi = {10.1088/0004637X/797/1/47},
journal = {Astrophysical Journal},
number = 1,
volume = 797,
place = {United States},
year = {Wed Dec 10 00:00:00 EST 2014},
month = {Wed Dec 10 00:00:00 EST 2014}
}

Electron acceleration to nonthermal energies in low Mach number (M{sub s} ≲ 5) shocks is revealed by radio and Xray observations of galaxy clusters and solar flares, but the electron acceleration mechanism remains poorly understood. Diffusive shock acceleration, also known as firstorder Fermi acceleration, cannot be directly invoked to explain the acceleration of electrons. Rather, an additional mechanism is required to preaccelerate the electrons from thermal to suprathermal energies, so they can then participate in the Fermi process. In this work, we use two and threedimensional particleincell plasma simulations to study electron acceleration in low Mach number shocks. We focusmore »

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