[PAST EVENT] Physics Colloquium - Yang Yang
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Yang Yang, University of Delaware, Title of talk: Multiscale Nature of Turbulence in Space Plasmas
Abstract: 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. Based on what plasma properties we are interested in studying, be they dominant at small or large scales, a plasma can be treated as tractable models in various limits, such as the kinetic theory and magnetohydrodynamic (MHD) theory. Turbulence flows are characterized by the nonlinear transfer of energy and other quantities across a huge range of scales. Observed turbulence in space is expected to involve cross-scale energy transfer and subsequent dissipation and heating. 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. In this talk I will first give an overview of some basic properties for turbulence. Then I will briefly review turbulence theory application in space plasmas. I will discuss in detail 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 and estimate energy dissipation rate in weakly collisional plasmas. These also motivate several unresolved issues 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.