Multiscale issues in modeling magnetic reconnection.

James Drake
University of Maryland

Magnetic reconnection plays a fundamental role in the dynamics of plasma systems as the driver of explosive events such as disruptions in aboratory fusion experiments and solar and stellar flares. The fast release of magnetic energy in plasma systems requires the magnetic field to change topology or "reconnect", which can only occur
in narrow boundary layers. Thus, the large scale dynamics of plasma systems is controlled by the dynamics of a localized boundary layer,
the dissipation region, creating a challenging multiscale physics problem. Understanding the mechanisms that enable the magnetic field
to reconnect, the associated rate of reconnection and the deposition of magnetic energy into high speed flows and energetic particles have been major issues. The resistive magnetohydrodynamic (MHD) model with classical resistivity fails to explain the rates of reconnection observed in nature has led to the widespread use of enhanced or "anomalous" resistivity to boost reconnection rates. Recent kinetic models of reconnection have demonstrated that the coupling to dispersive waves at small spatial scales where the MHD model breaks
down facilitates fast reconnection even in astrophysical scale
systems. Moreover, observations and recent models suggest that the
intense currents generated in the dissipation region self-generate
high-frequency, small-scale turbulence that may also generate
electron-ion drag that has been so widely invoked in the past. Thus, the
dissipation region is emerging as kinetically-dominated and
turbulent. Present and future computational platforms will not be able
to model the disparate scales associated with reconnection without the
development of a new class of computational algorithms that can handle the complex dynamics of localized boundary layers embedded in much
larger scale MHD plasmas.

Presentation (PowerPoint File)

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