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Title: Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field

Abstract

Using 1D and 2D particle-in-cell simulations of a plasma with a growing magnetic field $${\boldsymbol{B}}$$, we show herein that ions can be stochastically accelerated by the ion-cyclotron (IC) instability. As $${\boldsymbol{B}}$$ grows, an ion pressure anisotropy $${p}_{\perp ,i}\gt {p}_{| | ,i}$$ arises due to the adiabatic invariance of the ion magnetic moment ($${p}_{| | ,i}$$ and $$p_{⊥,i}$$ are the ion pressures parallel and perpendicular to $${\boldsymbol{B}}$$). When initially $$β_ i$$ = 0.5 ($${\beta }_{i}\equiv 8\pi {p}_{i}/| {\boldsymbol{B}}{| }^{2}$$, where $$p_i$$ is the ion isotropic pressure), the pressure anisotropy is limited mainly by inelastic pitch-angle scattering provided by the IC instability, which in turn produces a nonthermal tail in the ion energy spectrum. After $${\boldsymbol{B}}$$ is amplified by a factor of ~2.7, this tail can be approximated as a power law of index ~3.4 plus two nonthermal bumps and accounts for 2%–3% of the ions and ~18% of their kinetic energy. On the contrary, when initially $$β_i$$ = 2, the ion scattering is dominated by the mirror instability, and the acceleration is suppressed. This implies that efficient ion acceleration requires that initially, $$β_{i} \lesssim$$ 1. Although we focus on cases where $${\boldsymbol{B}}$$ is amplified by plasma shear, we check that the acceleration occurs similarly if $${\boldsymbol{B}}$$ grows due to plasma compression. Our results are valid in a subrelativistic regime where the ion thermal energy is ~10% of the ion rest-mass energy. This acceleration process can thus be relevant in the inner region of low-luminosity accretion flows around black holes.

Authors:
 [1]; ORCiD logo [2];  [3]; ORCiD logo [4];  [5]
  1. Univ. of Wisconsin, Madison, WI (United States)
  2. Univ. de Chile (Chile)
  3. Columbia Univ., New York, NY (United States)
  4. Univ. College London, Dorking (United Kingdom); Univ. of New Hampshire, Durham, NH (United States)
  5. P. Univ. Católica de Chile (Chile)
Publication Date:
Research Org.:
Columbia Univ., New York, NY (United States); National Science Foundation (NSF), Washington, DC (United States); National Fund for Scientific and Technological Development (FONDECYT)
Sponsoring Org.:
USDOE Office of Science (SC); National Science Foundation (NSF); National Fund for Scientific and Technological Development (FONDECYT); National Aeronautics and Space Administration (NASA); Science and Technology Facilities Council (STFC)
OSTI Identifier:
1612630
Grant/Contract Number:  
SC0016542; NST AST-1616037; FONDECYT REGULAR 1191673
Resource Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 880; 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; astronomy & astrophysics; accretion; accretion disks; instabilities; plasmas

Citation Formats

Ley, Francisco, Riquelme, Mario, Sironi, Lorenzo, Verscharen, Daniel, and Sandoval, Astor. Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field. United States: N. p., 2019. Web. https://doi.org/10.3847/1538-4357/ab2592.
Ley, Francisco, Riquelme, Mario, Sironi, Lorenzo, Verscharen, Daniel, & Sandoval, Astor. Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field. United States. https://doi.org/10.3847/1538-4357/ab2592
Ley, Francisco, Riquelme, Mario, Sironi, Lorenzo, Verscharen, Daniel, and Sandoval, Astor. Tue . "Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field". United States. https://doi.org/10.3847/1538-4357/ab2592. https://www.osti.gov/servlets/purl/1612630.
@article{osti_1612630,
title = {Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field},
author = {Ley, Francisco and Riquelme, Mario and Sironi, Lorenzo and Verscharen, Daniel and Sandoval, Astor},
abstractNote = {Using 1D and 2D particle-in-cell simulations of a plasma with a growing magnetic field ${\boldsymbol{B}}$, we show herein that ions can be stochastically accelerated by the ion-cyclotron (IC) instability. As ${\boldsymbol{B}}$ grows, an ion pressure anisotropy ${p}_{\perp ,i}\gt {p}_{| | ,i}$ arises due to the adiabatic invariance of the ion magnetic moment (${p}_{| | ,i}$ and $p_{⊥,i}$ are the ion pressures parallel and perpendicular to ${\boldsymbol{B}}$). When initially $β_ i$ = 0.5 (${\beta }_{i}\equiv 8\pi {p}_{i}/| {\boldsymbol{B}}{| }^{2}$, where $p_i$ is the ion isotropic pressure), the pressure anisotropy is limited mainly by inelastic pitch-angle scattering provided by the IC instability, which in turn produces a nonthermal tail in the ion energy spectrum. After ${\boldsymbol{B}}$ is amplified by a factor of ~2.7, this tail can be approximated as a power law of index ~3.4 plus two nonthermal bumps and accounts for 2%–3% of the ions and ~18% of their kinetic energy. On the contrary, when initially $β_i$ = 2, the ion scattering is dominated by the mirror instability, and the acceleration is suppressed. This implies that efficient ion acceleration requires that initially, $β_{i} \lesssim$ 1. Although we focus on cases where ${\boldsymbol{B}}$ is amplified by plasma shear, we check that the acceleration occurs similarly if ${\boldsymbol{B}}$ grows due to plasma compression. Our results are valid in a subrelativistic regime where the ion thermal energy is ~10% of the ion rest-mass energy. This acceleration process can thus be relevant in the inner region of low-luminosity accretion flows around black holes.},
doi = {10.3847/1538-4357/ab2592},
journal = {The Astrophysical Journal (Online)},
number = 2,
volume = 880,
place = {United States},
year = {2019},
month = {7}
}

Journal Article:
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Figures / Tables:

Figure 1 Figure 1: Panels (a) and (b) show a sketch of the simulation domain in our 2D shearing simulations at t= 0 and t> 0, respectively. The 2D domain follows the shearing flow of the plasma (red arrows), acquiring a parallelogram shape. Magnetic flux conservation changes the magnitude and orientation ofmore » the background magnetic field B, which is always parallel to the nonhorizontal sides of the parallelogram. The blue lines show the domain of our 1D runs.« less

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