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 B, we show that ions can be stochastically accelerated by the ion-cyclotron (IC) instability. As B grows, an ion pressure anisotropy p(perpendicular to,i) > p(parallel to,i) arises due to the adiabatic invariance of the ion magnetic moment (p(parallel to,i) and p(perpendicular to,i) are the ion pressures parallel and perpendicular to B). When initially beta(i) = 0.5 (beta(i) equivalent to 8 pi P-i/vertical bar B vertical bar(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 similar to 2.7, this tail can be approximated as a power law of index similar to 3.4 plus two nonthermal bumps and accounts for 2%-3% of the ions and similar to 18% of their kinetic energy. On the contrary, when initially beta(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, beta(i) less than or similar to 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 similar to 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.