Abstract
Kinetic plasma turbulence cascade spans multiple scales ranging from macroscopic fluid flow to sub-electron scales. Mechanisms that dissipate large scale energy, terminate the inertial range cascade, and convert kinetic energy into heat are hotly debated. Here, we revisit these puzzles using fully kinetic simulation. By performing scale-dependent spatial filtering on the Vlasov equation, we extract information at prescribed scales and introduce several energy transfer functions. This approach allows highly inhomogeneous energy cascade to be quantified as it proceeds down to kinetic scales. The pressure work, −(P·∇)·u, can trigger a channel of the energy conversion between fluid flow and random motions, which contains a collision-free generalization of the viscous dissipation in collisional fluid. Both the energy transfer and the pressure work are strongly correlated with velocity gradients.
Highlights
The classical energy cascade scenario is of great importance in explaining the heating of corona and solar wind:1–5 In these applications, one envisions that significant amounts of energy reside in large-scale fluctuations
1⁄4 ðug aua À u~au~aÞ and ~sba 1⁄4 ðue aÂBÀu~a  B~ Þ is the flux of the fluid flow energy transfer across scales; UuaT 1⁄4 ÀðPa Á rÞ Á u~a is (a) the fluid flow energy converted into thermal energy due to pressure work; Kuab 1⁄4 ÀqanaE~ Á u~a is the rate of fluid flow energy conversion into electromagnetic energy, i.e., electromagnetic work done on the fluid
Global energy equations derived from the VlasovMaxwell system indicate the crucial role of the pressure tensor in transforming fluid flow energy into thermal energy
Summary
The classical energy cascade scenario is of great importance in explaining the heating of corona and solar wind: In these applications, one envisions that significant amounts of energy reside in large-scale fluctuations. By examining the filtered equations for energy transfer, we can assess the relative importance of different transfer terms at all scales ranging from MHD to electron scales This approach provides extensions of what fluid models tell us about the plasma cascade. We show that the global energy exchange between fluid flow and particles (i.e., kinetic and thermal energies), derived from the Vlasov equation, is bridged immediately by the collaboration of pressure tensor and strain stress (i.e., velocity gradient) This possibly provides a new perspective on the collisionless dissipation mechanism and on the collisionless plasma cascade, in general. Yang et al in Ref. 37 (hereafter, paper I) used some global diagnostics, such as volume averages and contour maps, to introduce some of the present concepts and to show the possible importance of pressure work in generating internal energy While following this basic idea, here we explore this novelty in a more comprehensive and detailed way. This suggests, but not conclusively, that the numerical solution at the scales studied here, separated from the Debye scale, is represented well as a Vlasov solution
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