Fast Reconnection in HighLundquistNumber Plasmas Due to the Plasmoid Instability
YiMin 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 highLundquistnumber (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
SweetParker 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
SweetParker reconnection rates are much slower than observation. On
the other hand, more complicated models such as Hall MHD or
particleincell simulations can have reconnection significantly faster than
resistive MHD.
A key feature of the SweetParker 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 SweetParker reconnection is so slow. However, the
SweetParker 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 SweetParker
argument, simply because the long, thin SweetParker layer is
physically unrealizable at the highLundquistnumber 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.
