Session UM1: Mini-conference on Interface between Fluid and Kinetic Processes in Laboratory, Space and Astrophysics I

Paradigms of Complexity in Modelling of Fluid and Kinetic Processes

The need to discuss and compare a wide variety of models of fluid and kinetic processes is motivated by the astonishing wide variety of complex physical phenomena which occur in plasmas in nature. Such phenomena include, but are not limited to: turbulence, turbulent transport and mixing, reconnection and structure formation. In this talk, I will review how various fluid and kinetic models come to grips with the essential physics of these phenomena. For example, I will discuss how the idea of a turbulent cascade and the concept of an ``eddy'' are realized quite differently in fluid and Vlasov models. Attention will be placed primarily on physical processes, the physics content of various models, and the consequences of choices in model construction, rather than on the intrinsic mathematical structure of the theories. Examples will be chosen from fusion, laboratory, space and astrophysical plasmas.   

Coronal heating by turbulence and origin of the fast solar wind

The central problem in explaining the existence of the fast solar wind is to transport sufficient nonthermal energy into the lower corona, and then to produce the wind. The process has fluid plasma features and distinctive kinetic signatures, e.g., enhanced perpendicular proton temperatures. We examine models in which a low frquency (reduced MHD) turbulent cascade might explain the heating and acceleration process. Energy input is due to low frequency Alfven waves injected at the coronal base. Inhomogeneities cause non-WKB reflection, permitting nonlinear interaction between upward and downward propagating Elsasser fields. Studies with prescribed large scale fields with an open top boundary, neglecting the wind speed itself, have been able to characterize the turbulence -- dependence on input wave frequency, sensitivity to density profile, conditions on several time scales involved, and the efficiency of conversion of input energy to heating. The efficiency and the heating profile seem quite compatible with observational requirements. More recently we have implemented a self consistent wind model in which the heating due to the turbulence takes place in large scale fields which themselves are influenced by the turbulence and by the heating, thus producing a wind. Results show that a fast wind can be produced in a coronal hole region starting with reasonable levels of low frequency wave injection at the coronal base.   

Beyond MHD: Hall-MHD turbulence and dynamo action

The relevance of two fluid effects has recently been pointed out in several studies of astrophysical and laboratory plasmas. The effect of adding the Hall current to the dynamics of the flow was studied in several scenarios, particularly dynamo action and reconnection. Some of the works in Hall-magnetohydrodynamics (Hall-MHD) present conflicting results, indicating in some cases that the Hall effect can help the growth of a large scale magnetic field or a large scale self-organization process, while in other cases the Hall currents were observed to generate small scales and filamentation. In this talk we will discuss both analytically and numerically three dimensional Hall-MHD turbulence as the result of a dynamo. The dynamo process, believed to be responsible for sustaining the magnetic fields observed in several astrophysical objects, is an example in which the small scale fluctuations can have an impact in the evolution of the fields in the largest scales in the system. The inclusion of the Hall effect in the small scales allows for novel interactions between separated scales dominated by MHD or by two fluid effects.   

Dissipation and Spectral Energy Transfer in the Relaxation of 3-D Force-Free Magnetic Sheet Pinch

Three-dimensional fully kinetic Particle-in-Cell plasma simulations are used to study the magnetic reconnection in an initially force-free plasma with a sheet pinch configuration. The configuration drives unstable collisionless tearing modes at multiple resonant surfaces. The nonlinear interactions among these modes eventually destroy the original flux surfaces and produce stochastic fields. This multi-layer reconnection process is accompanied by the production of sheets and filaments of intensified currents. In addition, the magnetic energy is transferred from the original shear length scale both to the large scales due to the global relaxation and to the smaller, kinetic scales for dissipation. The dissipation is dominated by thermal/pressure effects. The unique advantage of large-scale 3D PIC simulations (especially with increasing computing power) is that it offers the opportunity to capture both the fluid and kinetic effects simultaneously. We regard our present results as a step towards that goal.   

Boundary Conditions in Collisionless Magnetic Reconnection

Magnetic reconnection itself can be thought of as an interface between fluid and kinetic processes, for this multiscale energy release process is facilitated by kinetic physics at very small scales but drives effects at global scales. Because reconnection is inherently multiscale, kinetic simulations of reconnection cannot resolve the global scales, and therefore must simulate a smaller idealized geometry. As such, the boundary conditions of these idealized simulations are a key issue in determining what physics conclusions can be drawn from a particular simulation study. Boundary conditions and their role in reconnection scaling studies will be discussed. Recent simulation results which may shed light on boundary condition issues will be presented.   

Gyrokinetic simulations of nonlinear kinetic processes in magnetized plasmas

In fusion and space plasmas, kinetic processes play important roles in the excitation and saturation of compressional magnetic pulsations, in the plasma heating by Alfvenic turbulence, and in the cross field transport. The interplay between fluid and kinetic processes often regulates the nonlinear evolution of both microscopic and macroscopic phenomena. Nonlinear simulation needs to treat the disparate temporal-spatial scale dynamics on an equal footing. Physics progress and numerical methods for multiscale gyrokinetic particle simulations will be discussed for mirror modes in high-beta plasmas, spectral cascade and plasma heating in Alfven turbulence, and turbulent transport in fusion plasmas.