SSP seminar

Fast Reconnection in High-Lundquist-Number Plasmas Due to the Plasmoid Instability

Yi-Min Huang (Space Science Center, University of New Hampshire)

Monday 14th June 2010, 12:00pm
Pratt conference room, 60 Garden Street

The problem of fast magnetic reconnection in high-Lundquist-number (S) plasmas has been an active research topic for several decades. The main challenge is to explain why reconnection in nature or laboratories, such as solar corona and fusion devices, can proceed so quickly when the plasma involved is highly conducting. The classic Sweet-Parker theory based on resistive MHD predicts a reconnection rate that scales as S^{-1/2}. For many systems of interest with the Lundquist number S ranging from 10^6 to 10^14, the resulting Sweet-Parker reconnection rates are much slower than observation. On the other hand, more complicated models such as Hall MHD or particle-in-cell simulations can have reconnection significantly faster than resistive MHD.

A key feature of the Sweet-Parker theory is the existence of a long, thin reconnection layer, which becomes narrower as the Lundquist number becomes higher. This narrow "nozzle" for the plasma outflow is the reason why Sweet-Parker reconnection is so slow. However, the Sweet-Parker layer in a system that exceeds a critical value of Lundquist number is unstable to the plasmoid instability. Moreover, the plasmoid instability becomes more unstable the higher the Lundquist number is. Therefore, there is a fundamental flaw in the Sweet-Parker argument, simply because the long, thin Sweet-Parker layer is physically unrealizable at the high-Lundquist-number regime. In this talk, I will present our recent progress in understanding the effects of plasmoid instability on reconnection, within the framework of resistive MHD. In particular, I will discuss the scalings of reconnection rate, numbers of plasmoids, current sheet thickness, etc., in this regime.


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