168 research outputs found
Particle-in-cell simulation study of the scaling of asymmetric magnetic reconnection with in-plane flow shear
We investigate magnetic reconnection in systems simultaneously containing
asymmetric (anti-parallel) magnetic fields, asymmetric plasma densities and
temperatures, and arbitrary in-plane bulk flow of plasma in the upstream
regions. Such configurations are common in the high-latitudes of Earth's
magnetopause and in tokamaks. We investigate the convection speed of the
X-line, the scaling of the reconnection rate, and the condition for which the
flow suppresses reconnection as a function of upstream flow speeds. We use
two-dimensional particle-in-cell simulations to capture the mixing of plasma in
the outflow regions better than is possible in fluid modeling. We perform
simulations with asymmetric magnetic fields, simulations with asymmetric
densities, and simulations with magnetopause-like parameters where both are
asymmetric. For flow speeds below the predicted cutoff velocity, we find good
scaling agreement with the theory presented in Doss et al., J.~Geophys.~Res.,
120, 7748 (2015). Applications to planetary magnetospheres, tokamaks, and the
solar wind are discussed.Comment: 17 pages, 4 figures, submitted to Physics of Plasma
Guide Field Dependence of 3D X-Line Spreading During Collisionless Magnetic Reconnection
Theoretical arguments and large-scale two-fluid simulations are used to study
the spreading of reconnection X-lines localized in the direction of the current
as a func- tion of the strength of the out-of-plane (guide) magnetic field. It
is found that the mech- anism causing the spreading is different for weak and
strong guide fields. In the weak guide field limit, spreading is due to the
motion of the current carriers, as has been pre- viously established. However,
spreading for strong guide fields is bi-directional and is due to the
excitation of Alfv\'en waves along the guide field. In general, we suggest that
the X-line spreads bi-directionally with a speed governed by the faster of the
two mecha- nisms for each direction. A prediction on the strength of the guide
field at which the spread- ing mechanism changes is formulated and verified
with three-dimensional simulations. Solar, magnetospheric, and laboratory
applications are discussed.Comment: 9 pages, 6 figures, Submitted to JG
Scaling of the magnetic reconnection rate with symmetric shear flow
The scaling of the reconnection rate during (fast) Hall magnetic reconnection in the presence of an oppositely directed bulk shear flow parallel to the reconnecting magnetic field is studied using two-dimensional numerical simulations of Hall reconnection with two different codes. Previous studies noted that the reconnection rate falls with increasing flow speed and shuts off entirely for super-Alfvenic flow, but no quantitative expression for the reconnection rate in sub-Alfvenic shear flows is known. An expression for the scaling of the reconnection rate is presented
Three-dimensional simulations of the orientation and structure of reconnection X-lines
This work employs Hall magnetohydrodynamic (MHD) simulations to study the
X-lines formed during the reconnection of magnetic fields with differing
strengths and orientations embedded in plasmas of differing densities. Although
random initial perturbations trigger the growth of X-lines with many
orientations, at late time a few robust X-lines sharing an orientation
reasonably consistent with the direction that maximizes the outflow speed, as
predicted by Swisdak and Drake [Geophys. Res. Lett., 34, L11106, (2007)],
dominate the system. The existence of reconnection in the geometry examined
here contradicts the suggestion of Sonnerup [J. Geophys. Res., 79, 1546 (1974)]
that reconnection occurs in a plane normal to the equilibrium current. At late
time the growth of the X-lines stagnates, leaving them shorter than the
simulation domain.Comment: Accepted by Physics of Plasma
Kinetic dissipation and anisotropic heating in a turbulent collisionless plasma
The kinetic evolution of the Orszag-Tang vortex is studied using
collisionless hybrid simulations. In the magnetohydrodynamic regime this vortex
leads rapidly to broadband turbulence. Significant differences from MHD arise
at small scales, where the fluid scale energy dissipates into heat almost
exclusively through the magnetic field because the protons are decoupled from
the magnetic field. Although cyclotron resonance is absent, the protons heat
preferentially in the plane perpendicular to the mean field, as in the corona
and solar wind. Effective transport coefficients are calculated.Comment: 4 pages, 4 figures. Submitted to PR
On the 3-D structure and dissipation of reconnection-driven flow-bursts
The structure of magnetic reconnection-driven outflows and their dissipation
are explored with large-scale, 3-D particle-in-cell (PIC) simulations. Outflow
jets resulting from 3-D reconnection with a finite length x-line form fronts as
they propagate into the downstream medium. A large pressure increase ahead of
this ``reconnection jet front'' (RJF), due to reflected and transmitted ions,
slows the front so that its velocity is well below the velocity of the ambient
ions in the core of the jet. As a result, the RJF slows and diverts the
high-speed flow into the direction perpendicular to the reconnection plane. The
consequence is that the RJF acts as a thermalization site for the ion bulk flow
and contributes significantly to the dissipation of magnetic energy during
reconnection even though the outflow jet is subsonic. This behavior has no
counterpart in 2-D reconnection. A simple analytic model predicts the front
velocity and the fraction of the ion bulk flow energy that is dissipated
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