Ley, F; Riquelme, M; Sironi, L; Verscharen, D; Sandoval, A; (2019) Stochastic Ion Acceleration by the Ion-cyclotron Instability in a Growing Magnetic Field. Astrophysical Journal , 880 (2) 10.3847/1538-4357/ab2592.

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Abstract

Using 1D and 2D particle-in-cell simulations of a plasma with a growing magnetic field B, we show that ions can be stochastically accelerated by the ion-cyclotron (IC) instability. As B grows, an ion pressure anisotropy p1_i > 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 B. When initially β i = 0.5 (Bi = 8π pi/|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 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 < 1. Although we focus on cases where B is amplified by plasma shear, we check that the acceleration occurs similarly if 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.

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