TY - JOUR
T1 - PIC Simulations of Velocity-space Instabilities in a Decreasing Magnetic Field
T2 - Viscosity and Thermal Conduction
AU - Riquelme, Mario
AU - Quataert, Eliot
AU - Verscharen, Daniel
N1 - Funding Information:
This work was supported by NSF grants AST 13-33612 and 1715054, a Simons Investigator Award to E.Q. from the Simons Foundation and the David and Lucile Packard Foundation. D.V. also acknowledges support from NSF/ SHINE grant AGS-1460190, NSF/SHINE grant AGS-1622498, NASA grant NNX16AG81G, and a UK STFC Ernest Rutherford Fellowship (ST/P003826/1). We are also grateful to the UC Berkeley–Chile Fund for support for collaborative trips that enabled this work. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575.
Funding Information:
This work was supported by NSF grants AST 13-33612 and 1715054, a Simons Investigator Award to E.Q. from the Simons Foundation and the David and Lucile Packard Foundation. D.V. also acknowledges support from NSF/SHINE grant AGS-1460190, NSF/SHINE grant AGS-1622498, NASA grant NNX16AG81G, and a UK STFC Ernest Rutherford Fellowship (ST/P003826/1). We are also grateful to the UC BerkeleyChile Fund for support for collaborative trips that enabled this work. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575.
Publisher Copyright:
© 2018. The American Astronomical Society.
PY - 2018/2/20
Y1 - 2018/2/20
N2 - We use particle-in-cell (PIC) simulations of a collisionless, electron-ion plasma with a decreasing background magnetic field, B, to study the effect of velocity-space instabilities on the viscous heating and thermal conduction of the plasma. If |B| decreases, the adiabatic invariance of the magnetic moment gives rise to pressure anisotropies with p∥j > p⊥,j (p∥j and p⊥,j represent the pressure of species j (electron or ion) parallel and perpendicular to B). Linear theory indicates that, for sufficiently large anisotropies, different velocity-space instabilities can be triggered. These instabilities in principle have the ability to pitch-angle scatter the particles, limiting the growth of the anisotropies. Our simulations focus on the nonlinear, saturated regime of the instabilities. This is done through the permanent decrease of |B| by an imposed plasma shear. We show that, in the regime 2 ≲ βj ≲ 20 (βj ≡ 8πpj |B|2), the saturated ion and electron pressure anisotropies are controlled by the combined effect of the oblique ion firehose and the fast magnetosonic/whistler instabilities. These instabilities grow preferentially on the scale of the ion Larmor radius, and make δpe/p∥,e ≈ δpi/p∥,i (where δpj = p⊥j - p∥j). We also quantify the thermal conduction of the plasma by directly calculating the mean free path of electrons, λe, along the mean magnetic field, finding that λe depends strongly on whether |B| decreases or increases. Our results can be applied in studies of low-collisionality plasmas such as the solar wind, the intracluster medium, and some accretion disks around black holes.
AB - We use particle-in-cell (PIC) simulations of a collisionless, electron-ion plasma with a decreasing background magnetic field, B, to study the effect of velocity-space instabilities on the viscous heating and thermal conduction of the plasma. If |B| decreases, the adiabatic invariance of the magnetic moment gives rise to pressure anisotropies with p∥j > p⊥,j (p∥j and p⊥,j represent the pressure of species j (electron or ion) parallel and perpendicular to B). Linear theory indicates that, for sufficiently large anisotropies, different velocity-space instabilities can be triggered. These instabilities in principle have the ability to pitch-angle scatter the particles, limiting the growth of the anisotropies. Our simulations focus on the nonlinear, saturated regime of the instabilities. This is done through the permanent decrease of |B| by an imposed plasma shear. We show that, in the regime 2 ≲ βj ≲ 20 (βj ≡ 8πpj |B|2), the saturated ion and electron pressure anisotropies are controlled by the combined effect of the oblique ion firehose and the fast magnetosonic/whistler instabilities. These instabilities grow preferentially on the scale of the ion Larmor radius, and make δpe/p∥,e ≈ δpi/p∥,i (where δpj = p⊥j - p∥j). We also quantify the thermal conduction of the plasma by directly calculating the mean free path of electrons, λe, along the mean magnetic field, finding that λe depends strongly on whether |B| decreases or increases. Our results can be applied in studies of low-collisionality plasmas such as the solar wind, the intracluster medium, and some accretion disks around black holes.
KW - accretion, accretion disks
KW - instabilities
KW - plasmas
KW - solar wind
UR - http://www.scopus.com/inward/record.url?scp=85042720990&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85042720990&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/aaa6d1
DO - 10.3847/1538-4357/aaa6d1
M3 - Article
AN - SCOPUS:85042720990
SN - 0004-637X
VL - 854
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2
M1 - 132
ER -