Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session JI1: Reconnection |
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Chair: Masaaki Yamada, Princeton Plasma Physics Laboratory Room: Landmark A |
Tuesday, November 18, 2008 2:00PM - 2:30PM |
JI1.00001: Marshall N. Rosenbluth Dissertation Award Talk: Identification of the Electron Diffusion Region during Magnetic Reconnection in a Laboratory Plasma Invited Speaker: Magnetic reconnection is a process which converts magnetic energy to plasma kinetic and thermal energy. One of the important goals in magnetic reconnection research is to explain the fast reconnection rate observed in natural phenomena. Recent breakthroughs show that the Hall effect facilitates reconnection in the collisionless regime [1], by decoupling the ions and electrons on the ion skin depth scale, forming ion and electron diffusion regions. The width of the electron diffusion region is on the order of the electron skin depth, while the ion diffusion region is much wider, allowing the ions to flow out efficiently. The electron diffusion region is identified by observing an out-of-plane quadrupole magnetic field during the reconnection process in the Magnetic Reconnection Experiment (MRX) [2,3,4]. The width of the electron diffusion region scales with the electron skin depth ($\sim 5.5-7.5c/\omega_{pe} $), and the peak electron outflow velocity scales with the electron Alfven velocity ($\sim 0.12-0.16V_{eA}$), independent of the ion mass. The measured width of the electron diffusion region is much wider and the observed electron outflow is much slower than those obtained in 2D numerical simulations. Comparisons of these measurements with state-of-art, two- dimensional Particle-In-Cell simulations using boundary conditions similar to MRX will be also presented. In collaboration with M. Yamada, H. Ji, S. P. Gerhardt, R. Kulsrud, S. Dorfman and W. Daughton. This work is supported by DoE, NSF and NASA. \newline [1] J. Birn \textit{et al.}, J. Geophys. Res., 106, 3715, 2001\newline [2] Y. Ren \textit{et al.}, Phys. Rev. Lett., 95, 055003, 2005 \newline [3] M. Yamada \textit{et al.}, Phys. Plasmas, 13, 052119, 2006 \newline [4] Y. Ren \textit{et al.}, to appear in Phys. Rev. Lett. [Preview Abstract] |
Tuesday, November 18, 2008 2:30PM - 3:00PM |
JI1.00002: Experimental Investigation of Spontaneous Magnetic Reconnection in the Laboratory Invited Speaker: A new experimental configuration (the Versatile Toroidal Facility, or VTF) has been in operation at MIT for the study of collisionless magnetic reconnection under controllable conditions. In this experiment a plasma parameter regime of special interest can be formed where the reconnection process appears in rapid bursts [1]. This regime provides a unique opportunity to study the scientifically unresolved ``trigger problem'' of magnetic reconnection in current sheets related to the spontaneous and explosive onset of events observed on the sun, in the Earth's magneto-tail and in sawtooth oscillations in magnetic fusion devices. The most recent experiments document how the onset phase involves three-dimensional dynamics in the laboratory: The burst of reconnection starts at one toroidal location, and then propagates around the toroidal direction at the Alfven speed (calculated with the strength of the dominant guide field). The three dimensional measurements include the detailed time evolution of the plasma density, current density, the magnetic flux function, the electrostatic potential and the reconnection rate. In the talk Dr. Egedal will discuss the experimental methods and present detailed observations of the temporal evolution of the three dimensional dynamics associated with the fast and spontaneous onset of reconnection in the VTF current sheet. \\[1ex] [1] J Egedal, et al., (2007) Phys. Rev. Lett. {\bf 98}, 015003. [Preview Abstract] |
Tuesday, November 18, 2008 3:00PM - 3:30PM |
JI1.00003: Multi-satellite observations of the electron diffusion region, neighboring islands, and electron acceleration Invited Speaker: Based on data from the four-satellite mission Cluster and particle-in-cell simulations, we provide new insight into collisionless reconnection and particle energization. A spatially extended electron current sheet (ecs), its neighboring magnetic islands, and bursts of energetic electrons are identified during a magnetotail reconnection event with no appreciable guide field. One spacecraft crossed the ecs earthward of the reconnection null, and together with the other three spacecraft, registered the following properties: One, the ecs is co-located with a layer of electric fields normal to the ecs, and pointing toward the ecs. Two, within the ion diffusion region, the electron density varies by a factor of 3-4 along the ecs normal direction with a local maximum at the ecs center, and by about an order of magnitude along the exhaust direction. Three, in the inflow region up to the ecs and separatrices, electrons have a temperature anisotropy ($T_{e\parallel}/T_{e\perp}>1$) and the anisotropy increases toward the ecs. Four, multiple $d_i$-scale magnetic islands are attached to the two ends of the ecs, and within each island, there is a burst of suprathermal electrons (Nature Physics, 4, 19-23, 2008). To compare with observations, we have developed detailed maps of electron distribution functions and DC electric fields within the diffusion region using 2D PIC simulations. We find that the electric field normal to the ecs is originated from charge imbalance and is of the ecs scale, and that ions also exhibit ecs-scale structures in response to this electric field. The above results indicate that ions and electrons are electrostatically coupled at the ecs, electrons are highly compressible, and electron acceleration during reconnection is linked to the dynamics of magnetic islands. [Preview Abstract] |
Tuesday, November 18, 2008 3:30PM - 4:00PM |
JI1.00004: Scaling of Asymmetric Magnetic Reconnection Invited Speaker: Theories of magnetic reconnection traditionally assume that the plasmas on either side of the dissipation region have identical densities and magnetic field strengths. While this canonical description is ostensibly appropriate for reconnection in the magnetotail, it is not appropriate in many settings, notably at the dayside magnetopause where the magnetosphere and magnetosheath plasmas have considerably different properties. There has been wide interest in the shock structure of fast asymmetric reconnection, but a general theory of the scaling of the rate of reconnection and the structure of the dissipation region during asymmetric reconnection has not been addressed until recently. In this talk, we will present a first principles analysis of the scaling of the reconnection rate and speed of the outflow jet in terms of upstream densities and magnetic field strengths for two-dimensional anti-parallel asymmetric magnetic reconnection. This analysis generalizes the classical Sweet-Parker scaling analysis to allow for asymmetric conditions. However, most of the scaling results are independent of the dissipation mechanism and, therefore, apply to asymmetric reconnection in general. In addition, we show that, unlike in symmetric reconnection, the X-line and stagnation point need not be located in the same place for asymmetric reconnection, and in fact usually are not. As such, there is a bulk flow across the X-line. Results from numerical simulations of asymmetric reconnection using resistive magnetohydrodynamics (MHD) and Hall-MHD will be presented, finding good agreement with the predicted scaling laws and properties of the dissipation region. Potential applications to reconnection at the dayside magnetopause and its impact on solar wind-magnetospheric coupling will be discussed. Collaborator: Michael A. Shay, University of Delaware [Preview Abstract] |
Tuesday, November 18, 2008 4:00PM - 4:30PM |
JI1.00005: A quantitative, comprehensive analytical model for ``fast'' magnetic reconnection in Hall MHD Invited Speaker: Magnetic reconnection in nature usually happens on fast (e.g. dissipation independent) time scales. While such scales have been observed computationally [1], a fundamental analytical model capable of explaining them has been lacking. Here, we propose such a quantitative model for 2D Hall MHD reconnection without a guide field. The model recovers the Sweet-Parker and the electron MHD [2] results in the appropriate limits of the ion inertial length, $d_i$, and is valid everywhere in between [3]. The model predicts the dissipation region aspect ratio and the reconnection rate $E_z$ 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 allow ``fast'' reconnection and that $E_z$ depends on $d_i$. Moreover, when applied to electron-positron plasmas, the model demonstrates that fast dispersive waves are not instrumental for ``fast'' reconnection [4]. [1] J. Birn {\it et al.}, {\it J. Geophys. Res.} {\bf 106}, 3715 (2001). [2] L. Chac\'{o}n, A. N. Simakov, and A. Zocco, {\it Phys. Rev. Lett.} {\bf 99}, 235001 (2007). [3] A. N. Simakov and L. Chac\'on, submitted to {\it Phys. Rev. Lett.} [4] L. Chac\'on, A. N. Simakov, V. Lukin, and A. Zocco, {\it Phys. Rev. Lett.} {\bf 101}, 025003 (2008). [Preview Abstract] |
Tuesday, November 18, 2008 4:30PM - 5:00PM |
JI1.00006: Momentum transport during reconnection events in the MST reversed field pinch Invited Speaker: During reconnection events in the MST reversed field pinch momentum parallel to the magnetic field is observed to be suddenly transported from the core to the edge. This occurs simultaneous with a surge in multiple resistive tearing instabilities. From measurements of the plasma flow and the forces arising from tearing instability (Maxwell and Reynolds stresses) we have established that tearing instabilities induce strong momentum transport. Comparison with nonlinear MHD computation of tearing fluctuations supports this conclusion, although it also indicates that effects beyond single-fluid MHD are likely to be important. The radial profile of the parallel velocity is reconstructed from a combination of diagnostics: Rutherford scattering of injected neutral atoms (for majority ions), charge exchange recombination spectroscopy (for minority ions), and Mach probes (for edge majority ion flow). Maxwell stress has been measured previously in the core by laser Faraday rotation, and both stresses are measured in the edge with probes. A surprising observation is that both the Maxwell and Reynolds stresses are each ten times larger than needed to account for the observed momentum transport (i.e., larger than the inertial and viscous terms in the momentum balance equation). However, they are oppositely directed such that their difference is approximately equal to the rate of change of plasma momentum. The large magnitude of the individual stresses is not predicted by MHD theory; the Maxwell stress also produces a Hall dynamo effect, implying that a two-fluid theory might be necessary for a complete description of momentum transport. To test further the relation between momentum transport and tearing fluctuations, momentum transport was measured perturbatively, by altering plasma rotation with inserted biased electrodes. Biasing is applied in plasmas with large tearing activity and improved confinement plasmas in which tearing activity is reduced by inductive current profile control. We find that momentum transport in improved confinement (of energy and particles) plasmas is also reduced about five-fold. Work supported by U.S. DOE and NSF. [Preview Abstract] |
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