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:

Ley, Francisco ^[1]; Riquelme, Mario  ^[2];

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Sironi, Lorenzo ^[3]; Verscharen, Daniel  ^[4];

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Sandoval, Astor ^[5]

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+ Show Author Affiliations

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:

2019-07-30

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