SH43A-01 

Observational evidence for an elongated (>50 ion skin depths) electron diffusion region during fast magnetic reconnection

* Phan, T D (phan@ssl.berkeley.edu), University of California, Berkeley, SSL, Berkeley, ca 94720, United States Drake, J F (drake@glue.umd.edu), University of California, Berkeley, SSL, Berkeley, ca 94720, United States Drake, J F (drake@glue.umd.edu), University of Maryland, U of Maryland, College Park, MD 20742, United States Shay, M A (shay@udel.edu), University of Delaware, Dept of Physics and Astronomy, Newark, DE 19716, United States Mozer, F S (fmozer@ssl.berkeley.edu), University of California, Berkeley, SSL, Berkeley, ca 94720, United States Eastwood, J P (eastwood@ssl.berkeley.edu), University of California, Berkeley, SSL, Berkeley, ca 94720, United States

We report observational evidence for an elongated electron reconnection diffusion region during fast reconnection. The Cluster in-situ observations in a magnetosheath reconnecting current sheet reveal a broad current layer (width around 12 ion skin depths) supporting the reversal of the reconnecting magnetic field with an embedded intense outflow current that implies a super-Alfvenic electron outflow jet with a transverse scale of ~8 electron skin depths. The oblique trajectory of the spacecraft through the reconnection layer allows the deduction of a fast reconnection rate of 9% of the upstream Alfven speed as well as the minimum extent of the super-Alfvenic electron outflow jet of 50 ion skin depths downstream from the X-line. The deduced reconnection rate is consistent with the directly measured rate of 8% based on the tangential electric field and the inflow plasma velocity. These observations confirm and extend the surprising finding from recent large-scale full particle simulations of the presence of an elongated (> 20 ion skin depths) electron diffusion region even during fast reconnection [Shay et al., 2007; Karimabadi et al., 2007].

SH43A-02 

The Multi-Scale Structure of the Electron Diffusion Region: Implications for Observations and Theory

* Daughton, W (daughton@lanl.gov), University of Iowa, 511 VAN, Iowa City, IA 52242, United States Karimabadi, H), UCSD, 777 S. Highway 101, Solana Beach, CA 92075, United States Scudder, J), University of Iowa, 511 VAN, Iowa City, IA 52242, United States

Recent 2D kinetic simulations with open boundary conditions¹ along with the largest periodic simulations ever considered have demonstrated that in large-scale systems the electron diffusion region (EDR) expands in time to form a highly elongated current layer with a width on the electron scale but a total length that can exceed tens of ion inertial lengths. This surprising result is nearly two orders of magnitude larger than previous predictions and challenges some of the central assumptions regarding the essential physics of collisionless reconnection. The formation of these layers involves a competition between the outward convection of flux with the non-ideal terms arising from the divergence of the electron pressure tensor. Although it is possible to achieve a balance over limited durations, over longer time scales these electron layers are unstable to secondary-island formation leading to a time dependent reconnection process. The formation of secondary islands is reminiscent of resistive MHD solutions in the presence of an imposed uniform resistivity. However, here the EDR exhibits multiple scales² in the outflow direction and the resulting structure has no fluid analogue. The elongation of the EDR and secondary island formation appear to be a generic feature of reconnection and remain in the presence of a finite guide field. The implications of these results are profound and bring into question several key expectations based on two-fluid theory including the size of the EDR, temporal behavior of reconnection, importance of the Hall term, role of electrons in the reconnection process, and even the structure of the quadrupole field. Finally, these results offer a wealth of new predictions that should be observationally testable: (1) highly elongated non-gyotropic electron layers extending large distance from the x-line, (2) a continuous but generally time-dependent reconnection rate in the range ~ 0.03-0.14, (3) repeated formation of secondary- islands and (4) strong modifications to the out-of-plane quadrupole field structure. ¹Daughton, Scudder and Karimabadi, Phys. Plasmas 13, 072101, 2006 ²Karimabadi, Daughton and Scudder, Geophys. Res. Lett. 34, L13104, 2007

SH43A-03 

Identification of a Two-scale Diffusion Layer during Magnetic Reconnection in a Laboratory Plasma*

* Yamada, M (myamada@pppl.gov), Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, Princeton Plasma Physics Laboratory, Princeton University, James Forrestal Rd, Princeton, NJ 08540, United States Ren, Y (yren2@wisc.edu), University of Wisconsin, 1150 University Ave., Madison, WI 53706, United States Ji, H (hji@pppl.gov), Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, Princeton Plasma Physics Laboratory, Princeton University, James Forrestal Rd, Princeton, NJ 08540, United States Gerhardt, S (sgerhard@pppl.gov), Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, Princeton Plasma Physics Laboratory, Princeton University, James Forrestal Rd, Princeton, NJ 08540, United States Dorfman, S (sdorfman@pppl.gov), Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, Princeton Plasma Physics Laboratory, Princeton University, James Forrestal Rd, Princeton, NJ 08540, United States McGeehan, B (bmcgeeha@pppl.gov), Center for Magnetic Self-organization in Laboratory and Astrophysical Plasmas, Princeton Plasma Physics Laboratory, Princeton University, James Forrestal Rd, Princeton, NJ 08540, United States

The recent 2D numerical simulations for magnetic reconnection layer, predict a two-scale diffusion layer in which an electron diffusion layer of the width of the ion skin depth resides inside of the ion diffusion layer of the ion skin depth width. In the reconnecting current sheet [1] in the Magnetic Reconnection Experiment (MRX), the electron diffusion region is verified and found that demagnetized electrons are accelerated in the outflow direction [2]. The measured width of the electron diffusion region scales with the electron skin depth (~8c/ωpe) and the electron outflow scales with the electron Alfv'en velocity (0.11VA). While the electron outflow seems to slow down by enhanced dissipation in the electron diffusion region, the total electron outflow flux remains independent of the width of the electron diffusion region. This result is in good agreement with the recent numerical simulation data. The ion outflow cannel is shown to be much broader than the electrons, which is also consistent with numerical simulations. The experimental results are compared with the recent 2-D numerical simulations [3] and magnetosphere data. *Research Supported by DoE, NSF and NASA. 1. M. Yamada, Y. Ren, H. Ji et al, Phys. Plasmas, v.13 052119 (2006) 2. Y. Ren et al, to be submitted for publication (2007) 3. S. Dorfman , W. Daughton et al, to be submitted (2007) http://mrx.pppl.gov/

SH43A-04 

Two-scale structure of the electron diffusion region during magnetic reconnection: Implications for reconnection rate and experimental observations

* Shay, M A (shay@udel.edu), University of Delaware, Dept. of Physics and Astron. 217 Sharp Lab University of Delaware, Newark, DE 19716, United States Drake, J F (drake@plasma.umd.edu), University of Maryland, Institute for Research in Electronics and Applied Phyiscs University of Maryland, College Park, MD 20742, United States Swisdak, M (swisdak@umd.edu), University of Maryland, Institute for Research in Electronics and Applied Phyiscs University of Maryland, College Park, MD 20742, United States

Particle in cell (PIC) simulations of collisionless magnetic reconnection are presented which demonstrate that the electron diffusion region develops a distinct two-scale structure along the outflow direction, extending large distances (10s of ion inertial lengths) downstream from the X-line[1]. This surprising finding that the electron diffusion region can be macroscopic has significant implications for satellite missions such as the Magnetospheric Multiscale Mission (MMS) whose goal is to observe the regions where electrons are not frozen-in. In addition, it is at odds with past two-fluid and hybrid simulation studies of reconnection because it requires a kinetic description of the electrons. In this two-scale structure, the inner electron diffusion region is the typical electron diffusion region with a strong out-of-plane current. The length of this inner region is found to decrease with decreasing electron mass, approaching the ion inertial length for a proton-electron plasma. Due to the microscopic nature of this inner region, the rate of reconnection remains fast in very large systems, independent of boundary conditions and the mass of the electrons. A surprise is the existence of an outer electron diffusion region downstream of the inner one. This outer region extends very large distances downstream from the x-line (40 ion inertial lengths in the largest simulations) and consists of a super-Alfvenic outflowing jet of electrons which are decoupled from the magnetic field. This non-frozen-in jet is supported by electron momentum transport, which manifests itself in Ohm's law as an off-diagnoal electron pressure force. The fast electron jet creates a quadrupolar Hall magnetic field which is not localized near the separatrices, different from previous hybrid and two-fluid simulations. Signatures of this two-scale electron diffusion region and its implications for satellite and laboratory observations will be discussed. [1]Shay, M. A., J. F. Drake, and M. Swisdak, "Two-scale structure of the electron dissipation region during collisionless magnetic reconnection," Physical Review Letters, In Press, also available at: arXiv:0704.0818v1 [physics.plasm-ph]

SH43A-05 

Direct Detection of Electron Demagnetization and Agyrotropy at Electron Diffusion Regions

* Scudder, J D (jack-scudder@uiowa.edu), Department of Physics and Astronomy, University of Iowa, Jefferson at Dubuque Streets, Iowa City, IA 52242, United States Daughton, W (william-daughton@uiowa.edu), Los Alamos National Laboratory, Code X-1, Los Alamos, NM 87545, United States Li, S (shanshan-li@uiowa.edu), Department of Physics and Astronomy, University of Iowa, Jefferson at Dubuque Streets, Iowa City, IA 52242, United States Holdaway, R (robert-holdaway@uiowa.edu), Department of Physics and Astronomy, University of Iowa, Jefferson at Dubuque Streets, Iowa City, IA 52242, United States Glassberg, R (robert-glassberg@uiowa.edu), Department of Physics and Astronomy, University of Iowa, Jefferson at Dubuque Streets, Iowa City, IA 52242, United States

The central focus of the non-ideal layers of collisionless reconnection is the electron diffusion region where field lines lose their traceability. This paper focuses on a new scalar observable, agyrotropy, for in situ and 3-D PIC certification of thermal electrons as being demagnetized, the essential property of the electron diffusion region. Hithertofore inferences of penetrations of the "electron diffusion region" have either centered on a magnetometer null reading in anti-parallel geometries, or a collection of tests involving only the electromagnetic field for guide geometries. These latter tests are not sufficient for the task of identifying the electron diffusion region. Examples of the incidence and onset of electron agyrotropy while traversing and near to current sheets will be presented from the Polar Hydra data at anti-parallel and guide field crossings of the magnetopause. Examples of the incidence and spatial distribution of electron agyrotropy expected from full PIC coded in open geometry will also be presented to motivate the labeling role that agyrotropy has for finding the electron diffusion regions, that is especially relevant in the guide geometry and 3d PIC simulations.

SH43A-06 

Scaling of Reconnection and Stability of Current Sheets in Large Systems

Yang, H (hongang.yang@unh.edu), Space Science Center, University of New Hampshire, Durham, NH 03824, * Bhattacharjee, A (amitava.bhattacharjee@unh.edu), Space Science Center, University of New Hampshire, Durham, NH 03824, Bessho, N (naoki.bessho@unh.edu), Space Science Center, University of New Hampshire, Durham, NH 03824, Xin, Q (qian.xin@unh.edu), Space Science Center, University of New Hampshire, Durham, NH 03824,

The scaling of collisionless reconnection in large systems has been a subject of considerable interest recently. We have carried out a sequence of 2D simulations using the same initial conditions for large systems using resistive MHD, Hall MHD and fully kinetic particle-in-cell models. It is shown that the dynamics of thin current sheets is sensitive to the mechanism that breaks field lines (spatially uniform resistivity, electron inertia, and/or electron pressure tensor), and that velocity shear along the thin current sheets plays an important role in controlling their geometry and stability. In the resistive MHD model, the long thin current sheet spanning Y-points becomes near-explosively unstable to secondary tearing, producing plasmoids copiously. In Hall MHD, the nonlinear dynamics changes qualitatively, as the Y-points contract spontaneously to form X-points thwarting the secondary tearing instabilities seen in the resistive MHD study. A steady state is eventually realized due to a balance between the spatial gradients of the current density and the velocity shear. Collisionless PIC simulations show a very different dynamics, exhibiting the tendency to form extended thin current sheets and secondary tearing instabilities. We address the issue of scaling in all three models, especially the dependence on system size and the dissipation mechanism, and discuss the relevance of these results to solar flares and magnetospheric substorms.

SH43A-07 

Collisionless Magnetic Reconnection in an Asymmetric Current Sheet: Implications for Magnetopause Reconnection

* Pritchett, P L (pritchet@physics.ucla.edu), Department of Physics and Astronomy, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1547, United States

Reconnection at the magnetopause probes several aspects of basic reconnection physics that are not duplicated elsewhere in the magnetosphere. At the magnetopause, reconnection occurs between two topologically distinct regions, the shocked solar wind and the magnetosphere, which have quite different properties (plasma density, plasma temperature, and magnetic field strength). Thus the standard symmetric Harris current sheet equilibrium is not well-suited for investigating magnetopause reconnection. Two-dimensional particle-in-cell simulations are used to probe reconnection in an asymmetric current sheet with a strong density gradient and unequal asymptotic magnetic field strengths. The starting configuration is a fluid equilibrium with overall pressure balance. This basic configuration is well maintained on average in the kinetic treatment, but the detailed structure at the center of the sheet is altered: a relatively strong E_x electric field forms on the high-field side of the layer to help confine the ions, the electron and ion drifts are not simply related by the temperature ratio, and the electrons dominate the current density in a thin region near the center of the current layer where the density gradient is strongest. Initial-value simulations of reconnection in this asymmetric layer produce smaller reconnection rates and lower saturation levels than for the symmetric case. If the asymmetric layer is subject to driving by an externally-imposed electric field, then an extended interval of quasi-steady reconnection is observed, which leads to larger islands. The effects of varying the magnetic shear angle across the layer will be reported.

SH43A-08 

Magnetic Reconnection: New Results from Cluster and Double Star Measurements

* Pu, Z (zypu@pku.edu.cn), Institute of Space Physics and Applied Technology, Peking University, Room N419, Physics Building, Peking University, No 5 Yiheyuan Road, Haidian District, Beijing, 100871, China Xiao, C (cjxiao@ourstar.bao.ac.cn), The National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012, China Wang, X (wangxg@pku.edu.cn), School of Physics, Peking University, Beijing, 100871, China

Magnetic reconnection (MR) is a fundamental process in plasmas by which magnetic field topology changes and connections of plasma particles with the magnetic field are re-arranged. MR is believed to play a key role in explosive energy release events in the solar-terrestrial system In the past half a century, great progresses in understanding of MR process has been gained through theoretical analysis, numerical simulations, and experimental and satellite observations. Nevertheless, many fundamental questions remain to be answered. Though 3D reconnection theories, specially the kinematic and topological models, have been developed in plasma and fluid physics as well as mathematics, it is lack of in situ measurements to confirm if they correctly reveal the nature of the physical world. A direct observation of 3D MR geometry was never able to make until launching of ESA Cluster constellation. The Cluster mission provides the first opportunity to detect the 3D magnetic structure through 4-point measurement as the spacecraft traverse the heart of the MR region. In addition, coordinated Double Star (DS) and Cluster observations enable us to see, for the first time, the evolution of structures at small scales within the Cluster tetrahedron, and then at large scales with Cluster and DS conjunction. The comparative Cluster-DS measurements also make it possible to observe the global pattern of MR at the magnetopause. This lecture devotes to review a part of new results from Cluster and DS measurements, which help to gain better underrstanding of MR on the bases of observations.

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.