SM23C-01 INVITED
The Nonlinear Dynamics of Magnetic Reconnection and Applications to Solar Eruptions
It is widely accepted that magnetic reconnection plays a fundamental role in releasing magnetic energy in solar eruptions, but what initiates an eruption remains an unresolved problem. We present a model for the catastrophic onset of fast magnetic reconnection in weakly collisional plasmas, which potentially explains why the energy release begins abruptly. In particular, we show that magnetic reconnection is bistable: both the slow (Sweet-Parker) and fast (Hall) modes of reconnection independently exist for a wide range of parameters. However, the Sweet-Parker solution disappears catastrophically at a critical condition, leading to the onset of Hall reconnection and the accompanying increase in the rate of magnetic energy release by many orders of magnitude. We present theory and simulations confirming the model. We propose that the disappearance of reconnection solutions is caused by saddle-node bifurcations controlling the nonlinear dynamics of reconnection, and derive a simple nonlinear model that displays this dynamical behavior [1]. The model predicts the existence of an unstable reconnection solution, which we demonstrate numerically. The unstable solution reveals that the physics controlling the onset of Hall reconnection occurs locally near the X-line as opposed to remotely at the boundaries [2]. We discuss potential mechanisms for onset and show that the model is consistent with results from laboratory experiments and solar and stellar flare observations. [1] P. A. Cassak, Doctoral Thesis, 2006; [2] P. A. Cassak et al., Phys. Rev. Lett., 98, 215001, 2007.
SM23C-02 INVITED
Three Dimensional Dynamics of Collisionless Magnetic Reconnection in Large-Scale Pair Plasmas
Using the largest three dimensional particle-in-cell simulations to date, collisionless magnetic reconnection in large-scale electron-positron plasmas without a guide field is shown to involve complex interaction of tearing and kink modes. The reconnection onset is patchy and occurs at multiple sites which self-organize to form a single, large diffusion region. There is a basic tendency for the diffusion region to further elongate in the outflow direction and become unstable to secondary kinking and formation of "plasmoid-rope" structures with finite extent in the current direction. The secondary kink leads to folding of the reconnection current layer, while plasmoid ropes at times follow the folding of the current layer. The interplay among these secondary instabilities plays a key role in controlling the time dependent reconnection rate in large-scale systems.
SM23C-03
Experimental Study of the Two-Scale Structure of the Electron Diffusion Layer and MRX Scaling
The most recent results from the analysis of the electron diffusion layer in the Magnetic Reconnection
Experiment (MRX) are presented along with insights into the two-fluid physics of the reconnection layer.
Recent 2D numerical simulations for the magnetic reconnection layer [1], demonstrate a two-scale diffusion
layer in which an electron diffusion layer of the width of the electron skin depth (~1.5 c/ωpe)
resides inside of the ion diffusion layer. In the reconnecting current sheet in MRX, the electron diffusion
region is verified and demagnetized electrons are found to accelerate in the outflow direction [2]. The
measured width of the electron diffusion region scales with the electron skin depth (~ 6-
8c/ωpe), and is much wider than 2D simulation results on collisionless reconnection. The electron
outflow scales with the electron Alfv'en velocity (0.15VA). While the electron outflow seems to be slowed
down by enhanced dissipation in the electron diffusion region, the exact cause of the widening of the electron
diffusion region is being studied[3]. Finally, MRX scaling [4] will be re-examined in which the reconnection
rate increases rapidly when the ion skin depth becomes larger than the Sweet-Parker width demonstrating a
clear linear dependence on λmfp/L.
*Research Supported by DoE, NSF and NASA.
1. i.e. W. Daughton et al, Phys. Plasmas, 13, 072101 (2006)
2. Y. Ren et al, Phys. rev. Letts. 101, 085003, (2008)
3. S. Dorfman et al, to be published in Phys. Plasmas (2008)
4. M. Yamada et al, Phys. Plasmas, 13, 052119 (2006)
http://mrx.pppl.gov/
SM23C-04
Non-steady Reconnection in Global Simulations of Magnetosphere Dynamics.
To analyze the non-steady magnetic reconnection during quasi-steady solar wind driving we employed high resolution global MHD model BATSRUS with non-MHD corrections in diffusion regions around the reconnection sites. To clarify the role of small-scale non-MHD effects on the global magnetospheric dynamic we performed simulations with different models of dissipation. We found that magnetopause surface is not in steady state even during extended periods of steady solar wind conditions. The so-called tilted reconnection lines become unstable due to formation of pressure bubbles, strong core field flux tubes, vortices, and traveling magnetic field cavities. Non-steady dayside reconnection results in formation of flux tubes with bended axis magnetically connecting magnetic field cavities generated at flanks and strong core segments formed near the subsolar region. We found that the rate of magnetic flux loading to the tail lobes is not very sensitive to the dissipation mechanism and details of the dayside reconnection. On the other hand the magnetotail reconnection rate, the speed of the reconnection site retreat and the global magnetotail dynamics strongly depend on the model of dissipation. THEMIS and Cluster observations are consistent with signatures predicted by simulations.
SM23C-05
On the properties and limitations of magnetic reconnection in Hall MHD
Magnetic reconnection is a key process in nature whereby magnetic energy is converted into kinetic and thermal energy. Magnetic reconnection fundamentally affects space, astrophysical, and laboratory plasmas, and usually happens on very fast time scales, possibly unrelated to underlying dissipation mechanisms. However, despite substantial theoretical progress in the understanding of fast reconnection (J. Birn et al., , J. Geophys. Res. 106, 3715, 2001). A fundamental analytical model capable of explaining these time scales has been lacking. Developing such a model is of the essence not only to further the basic understanding of reconnection, but also to provide resolution to controversies arising from numerical computations, which by necessity can only cover a limited region in parametric space. In this presentation, we will discuss a recently- developed analytical framework for describing the dynamics of a 2D diffusion region in Hall MHD. Equations controlling the diffusion region can be coupled to those modeling a macroscopic driver, thus providing a time- dependent description of the reconnection process (A. N. Simakov, L. Chacón, D. A. Knoll, Phys. Plasmas, 13, 082103, 2006). A steady-state analysis of the microscopic equations gives insight into the properties and limitations of the 2D reconnecting system. Despite the steady-state assumption, such insight has been shown to be applicable to predict maximum reconnection rates of highly dynamic systems.c,d,f Furthermore, we have found that the steady-state model adequately describes all regimes of interest of the ion inertial length di (A. N. Simakov and L. Chacón, Phys. Rev. Lett., accepted (2008)), recovering the resistive (Sweet-Parker) and electron MHD solutions in the appropriate limits (L. Chacón, A. N. Simakov, and A. Zocco, Phys. Rev. Lett. 99, 235001 (2007)). It also describes finite electron inertia effects (A. Zocco, L. Chacón, A. N. Simakov, "Electron inertia effects in 2D driven reconnection in electron MHD," Proc. of the Joint Varenna-Lausanne International Workshop on the Theory of Fusion Plasmas, Varenna, Italy, Aug. 25-29 (2008)). The model gives predictions for the dissipation region aspect ratio and the reconnection rate Ez in terms of dissipation and inertial parameters, and has been found to be in excellent agreement with non-linear simulations. It confirms a number of long-standing empirical results, and resolves several controversies. In particular, we find that both open X-point and elongated dissipation regions are possible, and that Ez depends strongly on di and on the length of the current sheet. Moreover, when applied to electron-positron plasmas, the model demonstrates that fast dispersive waves are not instrumental for fast reconnection (L. Chacón, A. N. Simakov, V. Lukin, and A. Zocco, Phys. Rev. Lett., 101, 025003, 2008). and that small-scale dissipation holds the key for the understanding of this phenomenon. The latter is a striking prediction, with important implications for the understanding of highly dynamic reconnection processes.
SM23C-06
Generation of Electric Field and Net Charge in Hall Reconnection
Generation of Hall electric field and net charge associated with magnetic reconnection is studied under different initial conditions of plasma density and magnetic field. With inclusion of the Hall effects, decoupling of the electron and ion motions leads to the formation of a narrow layer with strong electric field and large net charge density along the separatrix. The asymmetry of the plasma density or magnetic field or both across the current sheet will largely increase the magnitude of the electric field and net charge. The results indicate that the asymmetry of the magnetic field is more effective in producing larger electric field and charge density. The electric field and net charge are always much larger in the low density or/and high magnetic field side than in the high density or/and low magnetic field side. Both the electric field and net charge density are linearly dependent on the ratios of the plasma density or the square of the magnetic field across the current sheet. For the case with both initial asymmetries of the magnetic field and density, rather large Hall electric field and charge density are generated. The dependence of Hall electric field and charge density on the ion inertial lengths and its possible applications are also discussed.
SM23C-07
Theory of Superthermal, Wide, Electron Phase-Space Holes and Bipolar Fields*
Laboratory reconnection experiments [1] and recent magnetospheric spacecraft observations [2] are beginning to find bipolar fields with a spatial half-width equal to many Debye lengths (10 or more) traveling at high speeds (faster than the thermal velocity of the bulk of electrons). Electron phase-space hole solutions of the nonlinear Poisson-Vlasov equations (stationary in a frame co-moving with the hole) are constructed analytically with these properties by assuming there is secondary component of the electron distribution. This component can be a tail on the electron distribution or a beam. The hole velocity will be close to the velocity at the end of the tail or the velocity of the beam, provided the ions are moving with sufficient velocity in the frame of the hole. Vlasov simulations are used to study accessibility and stability of these solutions. * Work supported by DOE, NASA, and NSF [1]W. Fox, M. Porkolab, J. Egedal, N. Katz, A. Le, and A. Vrublevskis, "Observation of electron phase-space holes during magnetic reconnection in the Versatile Toroidal Facility," Abstract GP6.00029, 50th Annual Meeting of the Division of Plasma Physics, American Physical Society (Dallas, Nov.~2008). [2] R. E. Ergun and J. Tao, private communication.