Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field
Abstract
We use 2D particleincell 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, {\boldsymbol{B}}. In this setup, an electron pressure anisotropy with {p}_{\perp ,e}> {p}_{  ,e} naturally arises due to the adiabatic invariance of the electron magnetic moment ({p}_{  ,e} and {p}_{\perp ,e} are the pressures parallel and perpendicular to {\boldsymbol{B}}). If the anisotropy is large enough, then the whistler instability arises, efficiently scattering the electrons and limiting {{Δ }}{p}_{e} (\equiv {p}_{\perp ,e}{p}_{  ,e}). In this context, {{Δ }}{p}_{e} taps into the plasma velocity shear, producing electron heating by the socalled anisotropic viscosity. In our simulations, we permanently drive the growth of  {\boldsymbol{B}} by externally imposing a plasma shear, allowing us to selfconsistently capture the longterm, 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 {β }_{e}∼ 1, gradually decreasing its efficiency for larger values of {β }_{e} (\equiv 8π {p}_{e}/ {\boldsymbol{B}}{ }^{2}). If initially {β }_{e}∼ 1, then the final electron energy distribution can be approximately described by a thermal component, plus a powerlaw 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 lowcollisionality astrophysical environments, such as lowluminosity accretion flows.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2017
 DOI:
 10.3847/15384357/aa95ba
 arXiv:
 arXiv:1708.07254
 Bibcode:
 2017ApJ...850..113R
 Keywords:

 acceleration of particles;
 accretion;
 accretion disks;
 instabilities;
 plasmas;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 7 pages, 6 figures, accepted in The Astrophysical Journal