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

Abstract

© 2019 Institute of Physics Publishing. All rights reserved.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⊥, 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 βi = 0.5 (bi ≡ 8πpi |B|2, where pi 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 ac