SM21B-01 INVITED
Magnetic Energy Conversion From Whistlers to Electrons
The most important aspect of magnetic reconnection is the fast conversion of magnetic energy into particle
energy. Fast reconnection is facilitated by two-fluid effects. In the Electron MHD regime (EMHD), magnetic
energy is converted into electrons on whistler time scales. In the absence of collisions and inertial effects the
heating mechanism is not obvious. Laboratory experiments observing efficient electron heating by nonlinear
whistler modes will be presented. Electron energization occurs in magnetic null lines by inductive electric
fields along the separator, irrespective of the null point topology (X or O type). A finite electron transit time
through the null line provides an inertial "resistivity" which allows dissipation and electron heating. Note that
simple electron acceleration (jetting) does not convert magnetic energy in EMHD. The heating is strongest
(from 3eV to 30eV) and fastest in field topologies called "whistler spheromaks" [Phys. Rev. Lett. 99, 265005
(2007)]. The energized electrons have anisotropic distributions since secondary whistler instabilities are
observed. Thus, intense whistlers with magnetic null lines efficiently heat electrons even on scales larger
than the electron inertial length. Such phenomena may arise in strong whistler turbulence near the center of
neutral sheets.
http://www.physics.ucla.edu/plasma-
exp/research/PropertiesNonlinearWhistlers/index.html
SM21B-02 INVITED
Intermittent Character of Fast Reconnection and its Role in Self-Regulation of a Marginally Collisionless Corona.
I will review the current status of our understanding of magnetic reconnection, with a specific emphasis on the problem of onset of fast reconnection, i.e., on the transition between the slow resistive-MHD regime in collisional plasmas to the fast collisionless regime. I will formulate the condition for this transition and explain its critical role in various magnetically self-organized systems. As my main example, I will focus on solar coronal heating and will demonstrate how the highly intermittent nature of reconnection results in a dynamic self-regulation of this process to maintain the corona at about marginal collisionality.
SM21B-03
Fast MHD Magnetic Reconnection on Macroscopic Scales: Turbulent Regime Based on Plasma Self-Feeding
We find that on the same given macroscopic system described with a
visco-resistive MHD (without any Hall or extended MHD terms) approach, magnetic reconnection can
progress in two
entirely different ways. The first is the well-known laminar
Sweet-Parker process. But a second, completely different and
chaotic reconnection process is possible. This regime presents
properties of immediate observational and experimental relevance [1]:
i) it is much faster, developing on scales of the order of the
Alfvén time rather than the usual diffusive time of resistive
processes, and ii) the areas of reconnection become distributed
chaotically over a macroscopic area of the system. The onset of this
fast chaotic reconnection process is the formation of closed
circulation patterns where the jet going out of the reconnection
region turns around and forces its way back in, carrying along
copious amounts of magnetic flux and allowing fast macroscopic
reconnection.
[1] G. Lapenta, Self-Feeding Turbulent Magnetic Reconnection on Macroscopic Scales, Physical Review
Letters, 100, 235001, 2008.
http://arxiv.org/abs/0805.0426
SM21B-04
Onset of Fast Reconnection and its Nonlinear Stabilization in Laboratory and Space Plasmas
The onset of fast reconnection is widely studied in laboratory experiments, in situ satellite measurements in the Earth's magnetosphere, and solar flares. These observations place strong constraints on theory, which must explain not only a fast reconnection rate but also a sudden increase in the time-derivative of the reconnection rate. We will show that important features of such dynamics can be accounted for by Hall MHD (or two-fluid) reconnection models in one unifying framework. The theory also elucidates the role of diamagnetic drifts that can quench nonlinearly the onset of fast reconnection. Thus, the theory explains not only when reconnection is near-explosive, but also when it is not. We will compare the predictions of theory with data from tokamaks, laboratory experiments, and magnetospheric and solar observations. The problem takes on additional complexity when it is applied to large sytsems, which have been the subject of considerable interest recently. We have carried out a sequence of simulations using the same initial conditions for large systems using resistive and Hall MHD models, with resistivity and electron inertia providing the mechanism for breaking field lines. It is shown that the dynamics of thin current sheets in large sytsems is sensitive to the mechanism that breaks field lines, and that velocity shear along the thin current sheets plays an important role in controlling their geometry and stability. We address the implications of these results for fully kinetic simulations and observations of reconnection in large space and astrophysical plasmas.
SM21B-05
Experimental onset threshold and magnetic pressure pileup for 3D Sweet-Parker reconnection
In events in many space, astrophysical and laboratory plasmas, magnetic reconnection converts magnetic energy into particle thermal and flow energy during unsteady, impulsive, explosive events. How the onset and cessation of these events abruptly comes to pass has long been an open question. We show a three dimensional laboratory example of the onset and stagnation of Sweet-Parker type magnetic reconnection that is self consistently driven by magnetohydrodynamic attraction and collision of magnetized, parallel current (flux) ropes. The mutually attracting flux rope plasmas advect oppositely directed magnetic fields towards each other by, accompanied by the annihilation of magnetic flux. When the inflow speed increases past the Sweet-Parker speed vSP = vA / S1/2, where vA is the Alfven speed and S is the Lundquist number for the reconnection layer, there is a threshold for magnetic flux and pressure pile up around the reconnection region, along with changes in the magnetic topology. The piled up fields generate forces that react back on the incoming flux ropes to stall the inflow and thus the reconnection process.
SM21B-06
Three-dimensional instability of spontaneous fast magnetic reconnection
MHD numerical study for spontaneous fast magnetic reconnection is presented. As well-known in two- dimensional numerical MHD studies, if a current-driven anomalous resistivity is assumed, one-dimensional current sheet is destabilized by a resistive perturbation, resulting in two-dimensional fast magnetic reconnection. In this paper, it is shown that such a two-dimensional fast magnetic reconnection process can be moreover destabilized by a three-dimensional resistive perturbation, resulting in three-dimensional fast magnetic reconnection which is strongly localized in sheet current direction. The three-dimensional fast magnetic reconnection process intermittently and randomly ejects three-dimensional plasmoids downstream. This numerical study is applicable for magnetic reconnection problems in geo-magnetotail and solar flares, in which three-dimensional plasmoids are intermittently ejected from one-dimensional current sheet. It fact, this numerical result is very similar to the intermittent downflow observed in solar flares by TRACEfs EUV instrument.
SM21B-07
Conditions for the Onset and Cessation of Reconnection in Solar Wind Current Sheets
The recent discovery of reconnection in solar wind current sheets has provided an unprecedented opportunity to study the structure and dynamics of reconnection without the complex boundary conditions often found in the Earth's magnetosphere. Studies of solar wind reconnection exhausts have revealed clear evidence for component reconnection with magnetic shear as low as 15 degrees, for reconnection X-lines extending tens of thousands of ion skin depths and remaining quasi-steady over thousands of ion gyroperiods. In this presentation we discuss the results of a survey of Wind, ACE, and THEMIS data which indicate that the occurrence of reconnection in solar wind current sheets depends on both the plasma beta and the magnetic shear. At low beta reconnection occurs for a large range of magnetic shear whereas at high beta, reconnection exhausts are detected only for large magnetic shear. The dependence on plasma beta is also evident from the finding that reconnection, present in the convecting solar wind current sheets, often ceases downstream of the bow shock where the plasma beta is enhanced.