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
UR - http://www.scopus.com/inward/record.url?scp=85037694415&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85037694415&partnerID=8YFLogxK
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 -