Princeton Center for Heliophysics Seminar

Space plasmas are frequently taken to be weakly collisional or collisionless. Therefore, an explicit form of viscous dissipation as in collisional (e.g., MHD) cases cannot be easily defined. A variety of approaches have attempted to characterize specific mechanisms (e.g., magnetic reconnection, wave-particle interaction and turbulent-driven intermittency) and to quantify the dissipation. However, the community has not come to a consensus solution applicable to all systems. Turbulence enters into space plasmas in many guises. The complexity and variability of the behavior of plasma turbulence is in large part due to the involvement of dynamics at many scales, ranging from macroscopic fluid to sub-electron scales. Observed turbulence in space is expected to involve cross-scale energy transfer and subsequent dissipation and heating. Instead of identifying specific mechanisms, we discuss how to disentangle multiscale properties, how plasma dynamics bridges multiple scales, what new ingredients are introduced in cross-scale transfer as models progress from fluid to kinetic, and how to identify key steps in energy transfer. This motivates several surrogates, arising from the energy transfer process, to estimate energy dissipation rate in weakly collisional plasmas. We investigate in detail the cross-scale energy transfer process and discuss unresolved issues (e.g., entropy and reversibility) that may be addressed by future studies. Where feasible, examples are given from MHD, Particle in Cell, and hybrid Vlasov-Maxwell simulations, and from Magnetospheric Multiscale (MMS) observations.