[GTP] HOA-GTP Seminar January 18, 2012 Amitava Bhattacharjee

[Carolyn Mueller]

Joint HOA GTP Seminar
Onset of Fast Reconnection in High-Lundquist-Number Plasmas Mediated by 
the Plasmoid Instability
Herbert Amitava Bhattacharjee
Center for Integrated Computation and Analysis of Reconnection and
Turbulence, University of New Hampshire, Durham, NH

The problem of fast magnetic reconnection in high-Lundquist-number (S) 
plasmas has been an active area of research for several decades. The 
main challenge is to explain why reconnection in nature or laboratory 
devices (including fusion devices) can proceed rapidly from a relatively 
quiescent state in a weakly collisional plasma characterized by high 
values of the Lundquist number (S). The classical Sweet-Parker theory, 
based on resistive MHD, predicts a reconnection rate that scales as . 
For many systems of
interest, the Sweet-Parker reconnection rates are much slower than those 
observed. Recent work has demonstrated that there is a fundamental flaw 
in the Sweet-Parker argument, even within the framework of resistive 
MHD. When the Lundquist number exceeds a critical value, the 
Sweet-Parker layer is unstable to a super-Alfvenic tearing instability, 
hereafter referred to as the plasmoid instability, with a growth rate 
that increases with increasing S. Thus, the original Sweet-Parker 
current sheet breaks down into a chain of plasmoids and progressively 
thinner current sheets. Numerical simulations, supported by heuristic 
scaling arguments, strongly suggest that within the framework of 
resistive MHD, the nonlinear reconnection rate mediated by the plasmoid 
instability becomes insensitive to the value of S. Because the plasmoid 
instability can initiate a cascade to current sheets that are much 
thinner than the original Sweet-Parker sheet, the so-called Hall terms 
in the generalized Ohm’s law become important, triggering the onset of 
Hall reconnection, which lead to higher reconnection rates. We will 
present recent results from the largest 2D Hall MHD simulations to date 
that demonstrate the rich dynamics enabled by the interplay between the 
plasmoid instability and the Hall current. It is shown that the topology 
of Hall reconnection is not inevitably a single stable X-point. There 
exists an intermediate regime where the single X-point topology itself 
exhibits instability, causing the system to alternate between a single 
X-point and an extended current sheet with multiple X-points produced by 
the plasmoid instability. Examples of applications will be drawn from 
laboratory, magnetospheric, and solar coronal plasmas.

Wednesday, January 18, 2012
Center Green 1, South Auditorium
Lecture at 1:30pm

-- 
Carolyn Mueller
NCAR IMAGe
1850 Table Mesa Drive 
Boulder, CO 80305 
www.image.ucar.edu 
Tel: 303 497-2491
Fax: 303-497-2483