[1] Detailed comparisons are reported between laboratory observations of electron-scale dissipation layers near a reconnecting X-line and direct two-dimensional full-particle simulations. Many experimental features of the electron layers, such as insensitivity to the ion mass, are reproduced by the simulations; the layer thickness, however, is about 3 -5 times larger than the predictions. Consequently, the leading candidate 2D mechanism based on collisionless electron nongyrotropic pressure is insufficient to explain the observed reconnection rates. These results suggest that, in addition to the residual collisions, 3D effects play an important role in electron-scale dissipation during fast reconnection. Citation: Ji, H., Y. Ren, M. Yamada, S. Dorfman, W. Daughton, and S. P. Gerhardt (2008), New insights into dissipation in the electron layer during magnetic reconnection, Geophys. Res. Lett., 35, L13106, doi:10.1029 [2] Despite the disruptive influences of magnetic reconnection on large-scale structures in plasmas, the crucial topological changes and their associated dissipation take place only within thin current layers. The classical collisional models, where electrons and ions flow together through a single thin and long layer, fail to explain the observed fast reconnection rates. Modern collisionless models predict [Sonnerup, 1979;Mandt et al., 1994;Birn et al., 2001] that ions exhaust through a thick, ion-scale layer while mobile electrons flow through a thin, electron-scale layer, allowing for efficient release of magnetic energy. These ion layers have been frequently detected in space [e.g., Deng and Matsumoto, 2001;Øieroset et al., 2001;Mozer et al., 2002] and studied in detail in the laboratory [Ren et al., 2005;Yamada et al., 2006;Brown et al., 2006]. In contrast, the electron layers, where magnetic field dissipates, are rarely encountered in space and are often detected at places far from the reconnection X-line line [Scudder et al., 2002;Mozer, 2005;Wygant et al., 2005;Phan et al., 2007]. Therefore, whether the electron layers indeed exist near the X-line, and if yes, whether their associated dissipation results predominantly from laminar two-dimensional (2D) or three-dimensional (3D) dynamics as suggested by Xiao et al. [2006, 2007], is still an open question. Here we report detailed comparisons between recent laboratory observations of the electron layers near the X-line [Ren et al., 2008] and direct full-particle simulations in 2D. The measured electron layers display properties strikingly similar to predictions by 2D particle simulations, including their geometrical shape, insensitivity to ion mass, and sensitivity to the boundary conditions, but disagree on the electron layer thickness. As a consequence, the leading 2D mechanism based on collisionless electron nongyrotropic pressure is shown to be largely insufficient to explain the observed reconnection rates. These results suggest that, in addition to the residual Coulomb collisions, 3D effects play an important role in electron-scale dissipat...

Two-dimensional fully kinetic simulations are performed using global boundary conditions relevant to model the Magnetic Reconnection Experiment MRX M. Yamada et al., Phys Plasmas 4, 1936 1997. The geometry is scaled in terms of the ion kinetic scales in the experiment, and a reconnection layer is created by reducing the toroidal current in the flux cores in a manner similar to the actual experiment. The ion-scale features in these kinetic simulations are in remarkable agreement with those observed in MRX, including the reconnection inflow rate and quadrupole field structure. In contrast, there are significant discrepancies in the simulated structure of the electron layer that remain unexplained. In particular, the measured thickness of the electron layers is 3-5 times thicker in MRX than in the kinetic simulations. The layer length is highly sensitive to downstream boundary conditions as well as the time over which the simulation is driven. However, for a fixed set of chosen boundary conditions, an extrapolation of the scaling with the ion to electron mass ratio implies that at realistic mass ratio both the length and width will be too small compared to the experiment. This discrepancy implies that the basic electron layer physics may differ significantly between MRX and the two-dimensional, collisionless simulations. The two leading possibilities to explain the discrepancy are weak Coulomb collisions and three-dimensional effects that are present in the experiment but not included in the simulation model.

[1] Impulsive, local, 3-D reconnection is identified for the first time in a laboratory current sheet. The events observed in the Magnetic Reconnection Experiment are characterized by large local gradients in the third direction and cannot be explained by 2-D models. Detailed measurements show that the ejection of flux rope structures from the current sheet plays a key role in these events. By contrast, even though electromagnetic fluctuations in the lower hybrid frequency range are also observed concurrently with the impulsive behavior, they are not the key physics responsible. A qualitative, 3-D, two-fluid model is proposed to explain the observations. The experimental results may be particularly applicable to space and astrophysical plasmas where impulsive reconnection occurs.

The influence of current-aligned instabilities on magnetic reconnection in weakly collisional regimes is investigated using experimental observations from Magnetic Reconnection Experiment (MRX) [M. Yamada et al., Phys. Plasmas 4, 1936(1997] and large-scale fully kinetic simulations. In the simulations as well as in the experiment, the dominant instability is localized near the center of the reconnection layer, produces large perturbations of the magnetic field, and is characterized by the wavenumber that is a geometric mean between electron and ion gyroradii k $ ðq e q i Þ À1=2 . However, both the simulations and the experimental observations suggest the instability is not the dominant reconnection mechanism under parameters typical of MRX. V C 2013 AIP Publishing LLC.

[1] We report on measurements of ionospheric plasma dynamics conducted at the Arecibo Observatory between 20:00 and 24:00 local time (LT) on December 25 and 26, 2004 using the 430 MHz incoherent scatter radar (ISR). For interpretive purposes these measurements are supported by data from two nearby ionosondes and Global Positioning System (GPS) satellites. The ISR detected different ionospheric behaviors during the vertical-transmission periods on the consecutive, magnetically quiet nights. On the night of December 25 the ionosphere descended smoothly and spread F signatures faded. For about two hours on the following evening the bottomside ionosphere rose by $50 km, inducing plasma irregularities and intense spread F. Alternating cycles of bottom-side plasma rising and falling persisted through the remainder of the experiments. We postulate that this sinusoidal behavior is a response to gravity waves propagating above Puerto Rico. Nearly simultaneous data from two nearby stations show that GPS signals were modified by variations in total electron content (TEC) indicating the presence of traveling ionospheric disturbances (TIDs). The December 26 experiments were conducted about a day after an M W = 9.2 earthquake launched tsunami waves first across the Indian, then into the Atlantic and Pacific Oceans. We suggest that coupling at the tsunami sea-air interface launched gravity waves that propagated for great distances beneath the mesopause. GPS data recorded TEC variation in Asia, Europe, and the Caribbean, suggesting that TIDs were induced on a global scale at the wake of tsunami-launched gravity waves. Energy from imperfectly ducted gravity waves leaked into the ionosphere, partially over Puerto Rico. The wind-velocity field of these gravity waves caused local ionospheric plasma to rise, seeding bottomside irregularities via the generalized Rayleigh-Taylor instability.