Title:  
           Double Layers, Magnetic Reconnection, and Alfven Waves as Acceleration  
                              Mechanisms in Space and Astrophysical Plasmas

                                                          Nagendra Singh
                                        Electrical and Computer Engineering
                                        University of Alabama in Huntsville
                                                        Huntsville, Al 35899

Abstract:

          Electric currents are ubiquitous in space and astrophysical plasmas.  For example, in the near ŠEarth space, currents parallel to EarthÕs magnetic field play a crucial role in generating auroral light. In astrophysical plasmas currents are an inherent part of large-scale structures as well as a part of the prevailing MHD turbulence. As the magnetic loops associated with the turbulence convect and collide, they create current sheets. When Alfven waves are propagating in plasmas, the associated charged-particle drifts generate currents. Some types of Alfven waves, such as the Inertial Alfven waves, carry large currents parallel to the ambient magnetic fields. A fundamental question is about the mechanisms, which dissipate the currents in collisionless plasmas. The dissipation of the currents results in acceleration and heating of the plasma particles.   We will discuss the role of double layers in disrupting currents resulting into acceleration of electrons and ions, be it in the aurora or in solar flares. For the case of current sheets between current loops, we will discuss the role of current-driven Buneman instability in triggering magnetic reconnection and acceleration of charged particles. We will further discuss the generation of current by Alfven waves in a multi-ion plasma and its dissipation by cross-field instabilities.

       In space and astrophysical plasmas the ultimate source of energy lies in large-scale processes. But the absorption of the energy by the plasma particles involves microscopic physics. The study of the coupling between macro- and micro-scale plasma processes is a grand challenge. As in hydrodynamics and gas dynamics, the approach in such studies usually involves understanding the mico-physics and to include it in large-scale models heuristically. Our study of the current dissipation involves microphysics and is based on simulations using particle codes.