Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field
Stochastic Electron Acceleration by the Whistler Instability in a Growing Magnetic Field
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不断增长的磁场中惠斯勒不稳定性导致的随机电子加速
DOI：
10.3847/1538-4357/aa95ba
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
E. Quataert
中科院分区：
文献类型：
--
作者：
M. Riquelme;A. Osorio;E. Quataert
我们用二维粒子模拟方法研究了饱和哨声不稳定性对剪切、无碰撞等离子体中电子的粘性加热和非热加速的影响。在这种情况下，由于电子磁矩的绝热不变性(压力是平行和垂直的)，自然会产生电子压力各向异性。如果各向异性足够大，那么哨声不稳定性就会出现，从而有效地散射电子并限制()。在这种情况下，利用等离子体的速度切变，产生电子加热所谓的各向异性粘度。在我们的模拟中，我们通过在外部施加等离子体切变来永久地驱动的增长，使我们能够自我一致地捕捉到长期的、饱和的哨声不稳定演化。我们发现，除了粘性加热外，哨声模的散射还可以随机地将电子加速到非热能。当()值较大时，这种加速在初始时最为显著，逐渐降低其效率。如果是初始的，那么最终的电子能量分布可以用一个热分量加上一个谱指数为∼3.7时的幂函数尾部来近似描述。在这些情况下，非热尾解释了电子和它们的动能。我们讨论了我们的结果对低碰撞性天体物理环境中的电子加热和加速的影响，例如低光度吸积流。
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, . In this setup, an electron pressure anisotropy with naturally arises due to the adiabatic invariance of the electron magnetic moment ( and are the pressures parallel and perpendicular to ). If the anisotropy is large enough, then the whistler instability arises, efficiently scattering the electrons and limiting ( ). In this context, taps into the plasma velocity shear, producing electron heating by the so-called anisotropic viscosity. In our simulations, we permanently drive the growth of 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 , gradually decreasing its efficiency for larger values of ( ). If initially , 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 of the electrons and for 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.