论文标题
在无碰撞等离子体湍流中形成的当前床单的自由能源
Free energy sources in current sheets formed in collisionless plasma turbulence
论文作者
论文摘要
宏观能量无碰撞耗散成热量是空间和天体物理等离子体的一个未解决的问题,例如太阳风和地球的磁石。正在考虑的最可行的过程是宏观能量对动力学尺度的湍流,无碰撞 - 质量处理可以消散能量。空间观测和数值模拟显示了湍流等离子体中动力学尺度电流板的形成。这些CS中的不稳定性可以提供无碰撞的耗散并影响湍流。物理量和非Maxwellian速度分布功能的空间梯度为CS血浆不稳定性提供了自由能源。为了确定由血浆密度和电子/离子散装速度的空间梯度在无碰撞的动荡等离子体中形成的,带有外部磁场$ \ mathbf {b} _0 $中的CS所提供的,我们进行了二维PIC-HYBRID模拟,并在二维pic-hybrid模拟中进行了模拟模型。 我们发现,无碰撞湍流等离子体中的离子尺度CS主要由电子剪切流,即CS内部的电子散装速度比离子散装速度大得多,而通过CS的密度变化相对较小($ <$ 10 \%)。电子式式玻璃速度,因此,纸内的当前密度主要与$ \ Mathbf {B} _0 $平行。垂直电子和离子 - 膨胀速度中的剪切产生平行电子和离子流涡度。在CS内部,平行电子流涡度超过平行离子 - 流动涡度,在CS中心和CS边缘附近的峰周围变化符号。离子温度各向异性在CS形成期间在CS附近发展。它与平行离子和电子流涡度具有正相关。理论估计支持模拟结果。
Collisionless dissipation of macroscopic energy into heat is an unsolved problem of space and astrophysical plasmas, e.g., solar wind and Earth's magnetosheath. The most viable process under consideration is the turbulent-cascade of macroscopic energy to kinetic-scales where collisionless-plasma-processes dissipate the energy. Space observations and numerical simulations show the formation of kinetic scale current sheets in turbulent plasmas. Instabilities in these CS can provide collisionless dissipation and influence the turbulence. Spatial gradients of physical quantities and non-Maxwellian velocity distribution functions provide the free-energy-sources for CS plasma instabilities. To determine the free-energy-sources provided by the spatial gradients of plasma density and electron/ion bulk velocities in CS formed in collisionless turbulent plasmas with an external magnetic field $\mathbf{B}_0$, we carried out two-dimensional PIC-hybrid simulations and interpret the results within the limitations of the simulation model. We found that ion-scale CS in a collisionless turbulent plasma are formed primarily by electron shear flows, i.e., electron bulk velocity inside CS is much larger than ion bulk velocity while the density variations through the CS are relatively small ($<$ 10\%). The electron-bulk-velocity and, thus, the current density inside the sheets are directed mainly parallel to $\mathbf{B}_0$. The shear in the perpendicular electron- and ion-bulk-velocities generates parallel electron- and ion-flow-vorticities. Inside CS, parallel electron-flow-vorticity exceeds the parallel ion-flow-vorticity, changes sign around the CS centers and peaks near the CS edges. An ion temperature anisotropy develops near CS during the CS formation. It has positive correlation with the parallel ion- and electron-flow-vorticities. Theoretical estimates support the simulation results.