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

Abstract

Using 1D and 2D particle-in-cell (PIC) simulations of a plasma with a growing magnetic field $\vec{B}$, we show that ions can be stochastically accelerated by the ion-cyclotron (IC) instability. As $\vec{B}$ grows, an ion pressure anisotropy $p_{\perp ,i}>p_{||,i}$ arises, due to the adiabatic invariance of the ion magnetic moment ($p_{||,i}$ and $p_{\perp ,i}$ are the ion pressures parallel and perpendicular to $\vec{B}$). When initially ${\beta }_i=0.5$ (${\beta }_i\equiv 8\pi p_i/|\vec{B}{|}^2$, 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 non-thermal tail in the ion energy spectrum. After $\vec{B}$ is amplified by a factor $\sim 2.7$, this tail can be approximated as a power-law of index $\sim 3.4$ plus two non-thermal bumps, and accounts for $2-3\%$ of the ions and $\sim 18\%$ 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 ${\beta }_i\lesssim 1$. Although we focus on cases where $\vec{B}$ is amplified by plasma shear, we check that the acceleration occurs similarly if $\vec{B}$ grows due to plasma compression. Our results are valid in a sub-relativistic regime where the ion thermal energy is $\sim 10\%$ 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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Cited by in corpus (6)

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• Stochastic Electron Acceleration by Temperature Anisotropy Instabilities Under Solar Flare Plasma Conditions
• Fully kinetic shearing-box simulations of magnetorotational turbulence in 2D and 3D. I. Pair plasmas
• Nonthermal particle acceleration from maximum entropy in collisionless plasmas