Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field

Abstract

Abstract We use 2D particle-in-cell simulations to study the effect of the saturated whistler instability on the viscous heating and nonthermal acceleration of electrons in a shearing, collisionless plasma with a growing magnetic field, . In this setup, an electron pressure anisotropy with naturally arises due to the adiabatic invariance of the electron magnetic moment ( and are the pressures parallel and perpendicular to ). If the anisotropy is large enough, then the whistler instability arises, efficiently scattering the electrons and limiting ( ). In this context, taps into the plasma velocity shear, producing electron heating by the so-called anisotropic viscosity. In our simulations, we permanently drive the growth of by externally imposing a plasma shear, allowing us to self-consistently capture the long-term, saturated whistler instability evolution. We find that besides the viscous heating, the scattering by whistler modes can stochastically accelerate electrons to nonthermal energies. This acceleration is most prominent when initially , gradually decreasing its efficiency for larger values of ( ). If initially , then the final electron energy distribution can be approximately described by a thermal component, plus a power-law tail with a spectral index of ∼3.7. In these cases, the nonthermal tail accounts for of the electrons and for of their kinetic energy. We discuss the implications of our results for electron heating and acceleration in low-collisionality astrophysical environments, such as low-luminosity accretion flows.