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⊥,e > p∥,e naturally arises due to the adiabatic invariance of the electron magnetic moment (p∥,e and p⊥,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 Δpe(=p⊥,e - p∥,e). In this context, Δpe taps into the plasma velocity shear, producing electron heating by the so-called anisotropic viscosity. In our simulations, we permanently drive the growth of B Ö Ö 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 βe ~ 1, gradually decreasing its efficiency for larger values of βe (= 8πpe/|B|2). If initially βe ~ 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 ∼3.7. In these cases, the nonthermal tail accounts for ~5% of the electrons and for ~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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
- acceleration of particles
- accretion, accretion disks