Stochastic electron acceleration by the whistler instability in a growing magnetic field

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, B. In this setup, an electron pressure anisotropy with p(perpendicular to),(e) > p(parallel to,e) enaturally arises due to the adiabatic invariance of the electron magnetic moment (p(parallel to),(e) and p(perpendicular to),(e) are the pressures parallel and perpendicular to B). If the anisotropy is large enough, then the whistler instability arises, efficiently scattering the electrons and limiting Delta p(e) (equivalent to p(perpendicular to,e) = p(parallel to,e)). In this context, Delta p(e) taps into the plasma velocity shear, producing electron heating by the so-called anisotropic viscosity. In our simulations, we permanently drive the growth of vertical bar B vertical bar 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 beta(e) similar to 1, gradually decreasing its efficiency for larger values of beta(e) (equivalent to 8 pi p(e)/vertical bar B vertical bar(2)). If initially beta(e) similar to 1, then the final electron energy distribution can be approximately described by a thermal component, plus a power-law tail with a spectral index of similar to 3.7. In these cases, the nonthermal tail accounts for similar to 5% of the electrons and for similar to 15% 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.