Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field
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.
- Research Organization:
- Columbia Univ., New York, NY (United States); National Science Foundation (NSF), Washington, DC (United States); National Fund for Scientific and Technological Development (FONDECYT)
- Sponsoring Organization:
- 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)
- Grant/Contract Number:
- SC0016542; NST AST-1616037; FONDECYT REGULAR 1191673
- OSTI ID:
- 1612630
- Journal Information:
- The Astrophysical Journal (Online), Vol. 880, Issue 2; ISSN 1538-4357
- Publisher:
- Institute of Physics (IOP)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
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