NERSC Initiative for Scientific Exploration (NISE) 2011 Awards

Global Effects on the Dynamics of Plasmoids and Flux Ropes During Magnetic Reconnection

Amitava Bhattacharjee, University of New Hampshire

Associated NERSC Project: Center for Integrated Computation and Analysis of Reconnection and Turbulence (m148)

Magnetic reconnection drives some of the most dramatic, explosive energy releasing processes in the solar system, such as solar flares and coronal mass ejections. These violent events rapidly convert huge amounts of stored magnetic energy into heat and kinetic energy. The same type of reconnection process that governs these explosions is also widely believed to govern the onset of magnetospheric substorms, as well as sawtooth crashes in tokamaks. One of the main focus areas of reconnection research is to answer the following two questions: "How does reconnection happen so rapidly?" and "What triggers onset of fast reconnection?" Recent progress in this area suggests that a rapidly growing plasmoid instability may play an important role in triggering the onset of fast reconnection. This instability breaks up a long thin current sheet into a chain of plasmoids (or magnetic flux ropes in three dimensions) connected by even thinner current sheets. So far, studies of the plasmoid instability have largely focused on two-dimensional, highly idealized systems, often with periodic boundary conditions. Furthermore, reconnection itself is often considered as a localized event, and any feedback with boundary conditions are neglected. As such, the applicability of these results to real systems remains an open question. The principal objective of our proposed research is to bring the study a step closer to reality, by incorporating boundary effects and three-dimensional geometry into simulations.  In many physical systems of interest, magnetic field lines are anchored in much denser plasmas such as the solar photosphere or the astrophysical accretion disk, which may be treated as a boundary condition to the reconnecting system.  This so-called line-tied boundary condition plays a dual role in magnetic reconnection. On the one hand, it may facilitate the formation of thin current sheets via shuffling the footpoints of magnetic field lines, thereby enabling magnetic reconnection to occur. On the other hand,  the line-tying effect is known to be stabilizing for various plasma instabilities, including the tearing mode, which is the mechanism underlying the plasmoid instability.  Consequently, boundary conditions are expected to significantly alter previous results obtained in simple 2D configurations. We plan to study the interplay between the global geometry and reconnection through large scale 2D and 3D simulations.  In particular, we propose to address the following questions: How does the global configuration affect reconnection? And how do the effects of reconnection exercise feedback on the global configuration of the magnetic field?  Answers to these questions  will allow us to further assess the role of the plasmoid instability in triggering the onset on fast reconnection in the context of systems with real, physical boundaries.