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SPRG Seminars
October 14, 2008:
"Simulations of three-dimensional reconnection in low and high-beta
current sheets: Implications for energy release in solar flares"
Joachim Birn
Using three-dimensional magnetohydrodynamic (MHD) simulations, the energy release and transfer is investigated in a three-dimensional extension of the standard two-ribbon flare picture. In this scenario reconnection is initiated in a thin current sheet (suggested to form below a departing coronal mass ejection) above a bipolar magnetic field. Two cases are contrasted: an initially force-free current sheet (low beta) and a finite-pressure current sheet (high beta). The energy conversion process from reconnection consists of incoming Poynting flux (from the release of magnetic energy) turned into up- and downgoing Poynting flux, enthalpy flux and bulk kinetic energy flux. In the low-beta case, the outgoing Poynting flux is the dominant contribution, whereas the outgoing enthalpy flux dominates in the high-beta case. The bulk kinetic energy flux is only a minor contribution, particularly in the downward direction. The dominance of the downgoing Poynting flux in the low-beta case, which may be seen as an Alfvenic pulse, is consistent with an alternative to the thick target electron beam model for solar flare energy transport, suggested recently by Fletcher and Hudson. For plausible characteristic parameters of the reconnecting field configuration, we obtain energy release time scales and and energy output rates that compare favorably with those inferred from observations for the impulsive phase of flares.