51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009;
Atlanta, Georgia
Session BI2: Reconnection in Laboratory and Space
9:30 AM–12:30 PM,
Monday, November 2, 2009
Room: Centennial I
Chair: Hantao Ji, Princeton Plasma Physics Laboratory
Abstract ID: BAPS.2009.DPP.BI2.3
Abstract: BI2.00003 : Influence of Coulomb Collisions on the Dynamics of Magnetic Reconnection in Space and Laboratory Plasmas
10:30 AM–11:00 AM
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Abstract
Author:
Vadim Roytershteyn
(Los Alamos National Laboratory)
Magnetic reconnection is the process of a rapid change in the
magnetic field topology, frequently associated with a conversion
of magnetic energy into various forms of plasma kinetic energy.
Many systems of interest, such as the solar corona and laboratory
experiments, operate in the parameter regimes inaccessible to
both collisionless and fluid models, where the collisional mean
free path is comparable to the characteristic scale lengths of
interest and/or the reconnection electric field is of the order
of the runaway field. In this work, fully kinetic simulations
with a Monte-Carlo treatment of Landau collision integral are
used to analyze two problems in reconnection under such
conditions. Made practical by the recent progress in computing
capabilities, this powerful simulation technique allows a
seamless transition from collisionless to fully collisional
regimes. First, we present simulations with boundary
conditions~[1] mimicking the Magnetic Reconnection eXperiment
(MRX). A thorough comparison of the structure of the electron
reconnection layer between the experiment and the simulations
allows the relative roles of the collisional dissipation and that
of the collisionless effects in MRX to be quantified. Ultimately,
this provides important insights into a possible role of 3D
current-aligned instabilities and helps bridge the gap between a
small laboratory experiment and much larger systems in Nature. As
a second example, we discuss the transition between the
collisional and the kinetic reconnection regimes. In relatively
short systems with Lundquist number below $S \sim 10^3$ the
transition, signified by a rapid increase in the reconnection
rate, occurs at the temperature that corresponds to the width of
the Sweet-Parker current sheet $\delta_{\mathrm{SP}}\sim d_i$,
where $d_i$ is the ion inertial length. In larger systems $S >
10^3$, the transition is observed at significantly lower
temperatures than are expected from a simple criterion
$\delta_{\mathrm{SP}} \sim d_i$. In particular, the Sweet-Parker
current sheet is found to be unstable against a tearing-like
instability that leads to a significant modulation of the current
sheet thickness~[2]. The transition is achieved when the {\em
minimum} thickness of the current sheet falls below $d_i$. These
results may have strong implications for reconnection in the
solar corona, since many of the existing models are based on the
premise that the Sweet-Parker scaling is relevant at practically
important $S \sim 10^{12}$ and neglect the influence of secondary
islands in estimating the transition to kinetic scales.\\[4pt]
[1] S. Dorfman et al. Phys. Plasmas {\bf 15}, 102107 (2008)\\[0pt]
[2] W. Daughton et al. {\em ``Transition from Collisional to
Kinetic Regimes in Large-Scale Reconnection Layers''}, to appear
in Phys. Rev. Letters
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.DPP.BI2.3