TY - JOUR

T1 - Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field

AU - Riquelme, Mario

AU - Osorio, Alvaro

AU - Quataert, Eliot

N1 - Funding Information:
M.R. and E.Q. are grateful to the UC Berkeley-Chile Fund for support for collaborative trips that enabled this work. This work was also supported by NSF grants AST 13-33612 and 17-15054, a Simons Investigator Award to E.Q. from the Simons Foundation and the David and Lucile Packard Foundation. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. This research was also partially supported by the supercomputing infrastructure of the NLHPC (ECM-02) at the Center for Mathematical Modeling of University of Chile.
Funding Information:
M.R. and E.Q. are grateful to the UC Berkeley–Chile Fund for support for collaborative trips that enabled this work. This work was also supported by NSF grants AST 13-33612 and 17-15054, a Simons Investigator Award to E.Q. from the Simons Foundation and the David and Lucile Packard Foundation. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575. This research was also partially supported by the supercomputing infrastructure of the NLHPC (ECM-02) at the Center for Mathematical Modeling of University of Chile.
Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved.

PY - 2017/12/1

Y1 - 2017/12/1

N2 - 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.

AB - 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.

KW - acceleration of particles

KW - accretion, accretion disks

KW - instabilities

KW - plasmas

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U2 - 10.3847/1538-4357/aa95ba

DO - 10.3847/1538-4357/aa95ba

M3 - Article

AN - SCOPUS:85037694415

SN - 0004-637X

VL - 850

JO - Astrophysical Journal

JF - Astrophysical Journal

IS - 2

M1 - 113

ER -