NERSC Initiative for Scientific Exploration (NISE) 2010 Awards

Bridging the Gaps Between Fluid and Kinetic Magnetic Reconnection Simulations in Large Systems

Amitava Bhattacharjee, University of New Hampshire

Associated NERSC Project: Center for Integrated Computation and Analysis of Reconnection and Turbulence (m148), Principal Investigator: Amitava Bhattacharjee

Magnetic reconnection drives the most dramatic, explosive energy releasing processes in the solar system: solar flares and coronal mass ejections. These violent events rapidly convert huge amounts of stored magnetic energy into heat and kinetic energy. An X-class solar flare can release up to 6 x 10**25 Joules of energy in less than an hour. This energy release is comparable to tens of millions of atomic bombs exploding simultaneously. The same process occurs through current disruptions on smaller scales in laboratory devices seeking to magnetically confined plasma for nuclear fusion energy.

The main question driving reconnection research is, "How does reconnection happen so rapidly?" If magnetic energy were dissipated only by collisional plasma resistivity (the way a copper wire dissipates the energy stored in a battery) , a solar flare would take years to release its energy, rather than the sub-hour time scale that is actually observed. This is because collisions between the electrons and ions in astrophysical plasmas are exceedingly rare. Simulations of magnetic reconnection generally employ one of two basic strategies. In the kinetic (or Particle-in-Cell, PIC) approach, one follows the individual particles in the plasma subject to self-consistent electromagnetic fields , while the other approach is to model the plasma as a fluid, or multiple co-existing fluids. Particle based simulations include the most realistic dissipative (energy releasing) physics. However, particle models are too computationally expensive to use to model large systems relevant to space and astrophysical plasmas. Fluid models, however, excel in modeling the gross features of very large systems, although they show significant deviations from the predictions of kinetic models in the dissipation regions.

Particle simulations have shown that the dominant effect responsible for energy release in nearly collisionless plasmas is the electron pressure tensor in the generalized Ohm's law governing weakly collisional plasmas. In recent years, interesting analytic closure approximations that represent the electron pressure tensor in terms of multi-fluid variables have been developed, but they have not been tested sufficiently in global multi-fluid codes. The principal objective of our proposed research is to represent these closure approximations in our global multi-fluid codes, and to test the validity of these closure schemes by comparing the predictions of our global codes with results from smaller-scale kinetic simulations as well as with observations. One of the most challenging aspects of this work is that the simulations need to resolve the smallest physical scales (and associated short time scales) in large systems, necessitating the use of large computational grids, and small simulation time steps. The task of simulations is complicated further by the discovery by the PI and his collaborators (as well as a few others) of the tendency of large and thin current sheets embedded in reconnection layers to break up into a copious number of magnetic islands or plasmoids, which provide an additional mechanism for passage to fast reconnection.

The impact of the research would be to be able to simulate explosive eruptions driven by magnetic reconnection in weakly collisional astrophysical, space, or laboratory plasma systems.