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Title: Electron and Proton Heating in Transrelativistic Magnetic Reconnection

Abstract

Hot collisionless accretion flows, such as the one in Sgr A* at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here protons are nonrelativistic, while electrons can be ultrarelativistic. By means of 2D particle-in-cell simulations, we investigate electron and proton heating in the outflows of transrelativistic reconnection (i.e., $${\sigma }_{w}\sim 0.1\mbox{--}1$$, where the magnetization $${\sigma }_{w}$$ is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high $${\beta }_{{\rm{i}}}$$ (here $${\beta }_{{\rm{i}}}$$ is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression ("adiabatic heating"), while at low $${\beta }_{{\rm{i}}}$$ it is accompanied by a genuine increase in entropy ("irreversible heating"). For our fiducial $${\sigma }_{w}=0.1$$, the irreversible heating efficiency at $${\beta }_{{\rm{i}}}\lesssim 1$$ is nearly independent of the electron-to-proton temperature ratio $${T}_{{\rm{e}}}/{T}_{{\rm{i}}}$$ (which we vary from 0.1 up to 1), and it asymptotes to $$\sim 2 \% $$ of the inflowing magnetic energy in the low-$${\beta }_{{\rm{i}}}$$ limit. Protons are heated more efficiently than electrons at low and moderate $${\beta }_{{\rm{i}}}$$ (by a factor of ~7), whereas the electron and proton heating efficiencies become comparable at $${\beta }_{{\rm{i}}}\sim 2$$ if $${T}_{{\rm{e}}}/{T}_{{\rm{i}}}=1$$, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection ($${\sigma }_{w}\gtrsim 1$$). Finally, our results have important implications for the two-temperature nature of collisionless accretion flows and may provide the subgrid physics needed in general relativistic MHD simulations.

Authors:
ORCiD logo [1];  [2];  [1]
  1. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  2. Columbia Univ., New York, NY (United States). Dept. of Astronomy
Publication Date:
Research Org.:
Columbia Univ., New York, NY (United States). The Trustees
Sponsoring Org.:
USDOE; National Aeronautics and Space Administration (NASA); National Science Foundation (NSF)
OSTI Identifier:
1510956
Grant/Contract Number:  
SC0016542; NNX14AB47G; TG-AST80026N; TG-AST12001
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Volume: 850; Journal Issue: 1; Journal ID: ISSN 1538-4357
Publisher:
Institute of Physics (IOP)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS

Citation Formats

Rowan, Michael E., Sironi, Lorenzo, and Narayan, Ramesh. Electron and Proton Heating in Transrelativistic Magnetic Reconnection. United States: N. p., 2017. Web. doi:10.3847/1538-4357/aa9380.
Rowan, Michael E., Sironi, Lorenzo, & Narayan, Ramesh. Electron and Proton Heating in Transrelativistic Magnetic Reconnection. United States. doi:10.3847/1538-4357/aa9380.
Rowan, Michael E., Sironi, Lorenzo, and Narayan, Ramesh. Tue . "Electron and Proton Heating in Transrelativistic Magnetic Reconnection". United States. doi:10.3847/1538-4357/aa9380. https://www.osti.gov/servlets/purl/1510956.
@article{osti_1510956,
title = {Electron and Proton Heating in Transrelativistic Magnetic Reconnection},
author = {Rowan, Michael E. and Sironi, Lorenzo and Narayan, Ramesh},
abstractNote = {Hot collisionless accretion flows, such as the one in Sgr A* at our Galactic center, provide a unique setting for the investigation of magnetic reconnection. Here protons are nonrelativistic, while electrons can be ultrarelativistic. By means of 2D particle-in-cell simulations, we investigate electron and proton heating in the outflows of transrelativistic reconnection (i.e., ${\sigma }_{w}\sim 0.1\mbox{--}1$, where the magnetization ${\sigma }_{w}$ is the ratio of magnetic energy density to enthalpy density). For both electrons and protons, we find that heating at high ${\beta }_{{\rm{i}}}$ (here ${\beta }_{{\rm{i}}}$ is the ratio of proton thermal pressure to magnetic pressure) is dominated by adiabatic compression ("adiabatic heating"), while at low ${\beta }_{{\rm{i}}}$ it is accompanied by a genuine increase in entropy ("irreversible heating"). For our fiducial ${\sigma }_{w}=0.1$, the irreversible heating efficiency at ${\beta }_{{\rm{i}}}\lesssim 1$ is nearly independent of the electron-to-proton temperature ratio ${T}_{{\rm{e}}}/{T}_{{\rm{i}}}$ (which we vary from 0.1 up to 1), and it asymptotes to $\sim 2 \% $ of the inflowing magnetic energy in the low-${\beta }_{{\rm{i}}}$ limit. Protons are heated more efficiently than electrons at low and moderate ${\beta }_{{\rm{i}}}$ (by a factor of ~7), whereas the electron and proton heating efficiencies become comparable at ${\beta }_{{\rm{i}}}\sim 2$ if ${T}_{{\rm{e}}}/{T}_{{\rm{i}}}=1$, when both species start already relativistically hot. We find comparable heating efficiencies between the two species also in the limit of relativistic reconnection (${\sigma }_{w}\gtrsim 1$). Finally, our results have important implications for the two-temperature nature of collisionless accretion flows and may provide the subgrid physics needed in general relativistic MHD simulations.},
doi = {10.3847/1538-4357/aa9380},
journal = {The Astrophysical Journal (Online)},
issn = {1538-4357},
number = 1,
volume = 850,
place = {United States},
year = {2017},
month = {11}
}

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