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Title: Electron Heating in Low-Mach-number Perpendicular Shocks. I. Heating Mechanism

Abstract

Recent X-ray observations of merger shocks in galaxy clusters have shown that the postshock plasma has two temperatures, with the protons hotter than the electrons. By means of two-dimensional particle-in-cell simulations, we study the physics of electron irreversible heating in low-Mach-number perpendicular shocks, for a representative case with sonic Mach number of 3 and plasma beta of 16. We find that two basic ingredients are needed for electron entropy production: (1) an electron temperature anisotropy, induced by field amplification coupled to adiabatic invariance; and (2) a mechanism to break the electron adiabatic invariance itself. In shocks, field amplification occurs at two major sites: at the shock ramp, where density compression leads to an increase of the frozen-in field; and farther downstream, where the shock-driven proton temperature anisotropy generates strong proton cyclotron and mirror modes. The electron temperature anisotropy induced by field amplification exceeds the threshold of the electron whistler instability. The growth of whistler waves breaks the electron adiabatic invariance and allows for efficient entropy production. For our reference run, the postshock electron temperature exceeds the adiabatic expectation by $$\simeq 15 \% $$, resulting in an electron-to-proton temperature ratio of $$\simeq 0.45$$. We find that the electron heating efficiency displays only a weak dependence on mass ratio (less than $$\simeq 30 \% $$ drop, as we increase the mass ratio from $${m}_{i}/{m}_{e}=49$$ up to $${m}_{i}/{m}_{e}=1600$$). We develop an analytical model of electron irreversible heating and show that it is in excellent agreement with our simulation results.

Authors:
ORCiD logo [1];  [2];  [1]
  1. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  2. Columbia Univ., New York, NY (United States)
Publication Date:
Research Org.:
Columbia Univ., New York, NY (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1511008
Grant/Contract Number:  
SC0016542
Resource Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 851; Journal Issue: 2; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS

Citation Formats

Guo, Xinyi, Sironi, Lorenzo, and Narayan, Ramesh. Electron Heating in Low-Mach-number Perpendicular Shocks. I. Heating Mechanism. United States: N. p., 2017. Web. doi:10.3847/1538-4357/aa9b82.
Guo, Xinyi, Sironi, Lorenzo, & Narayan, Ramesh. Electron Heating in Low-Mach-number Perpendicular Shocks. I. Heating Mechanism. United States. doi:10.3847/1538-4357/aa9b82.
Guo, Xinyi, Sironi, Lorenzo, and Narayan, Ramesh. Wed . "Electron Heating in Low-Mach-number Perpendicular Shocks. I. Heating Mechanism". United States. doi:10.3847/1538-4357/aa9b82. https://www.osti.gov/servlets/purl/1511008.
@article{osti_1511008,
title = {Electron Heating in Low-Mach-number Perpendicular Shocks. I. Heating Mechanism},
author = {Guo, Xinyi and Sironi, Lorenzo and Narayan, Ramesh},
abstractNote = {Recent X-ray observations of merger shocks in galaxy clusters have shown that the postshock plasma has two temperatures, with the protons hotter than the electrons. By means of two-dimensional particle-in-cell simulations, we study the physics of electron irreversible heating in low-Mach-number perpendicular shocks, for a representative case with sonic Mach number of 3 and plasma beta of 16. We find that two basic ingredients are needed for electron entropy production: (1) an electron temperature anisotropy, induced by field amplification coupled to adiabatic invariance; and (2) a mechanism to break the electron adiabatic invariance itself. In shocks, field amplification occurs at two major sites: at the shock ramp, where density compression leads to an increase of the frozen-in field; and farther downstream, where the shock-driven proton temperature anisotropy generates strong proton cyclotron and mirror modes. The electron temperature anisotropy induced by field amplification exceeds the threshold of the electron whistler instability. The growth of whistler waves breaks the electron adiabatic invariance and allows for efficient entropy production. For our reference run, the postshock electron temperature exceeds the adiabatic expectation by $\simeq 15 \% $, resulting in an electron-to-proton temperature ratio of $\simeq 0.45$. We find that the electron heating efficiency displays only a weak dependence on mass ratio (less than $\simeq 30 \% $ drop, as we increase the mass ratio from ${m}_{i}/{m}_{e}=49$ up to ${m}_{i}/{m}_{e}=1600$). We develop an analytical model of electron irreversible heating and show that it is in excellent agreement with our simulation results.},
doi = {10.3847/1538-4357/aa9b82},
journal = {The Astrophysical Journal (Online)},
number = 2,
volume = 851,
place = {United States},
year = {2017},
month = {12}
}

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