SH44A-1705
Flux Transfer Events observations on both side of Magnetopause: from Magnetosphere to Magnetosheath
The successful project Cluster makes it capable to make spatial-temporal studies on space structures. In year 2003 and 2006, the distance between spacecrafts are adjusted to the order of 10000 km, which is larger than the typical thickness of Magnetopause. This gives us the best opportunity to observe FTEs on both side of Magnetopause, respectively and simultaneously. Within this study, we found five events that Cluster observed FTEs on both side of magnetopause and found some differences as follows: 1. The orientations are always obviousely different, sometimes with great angle between them; 2. The 3-D velocity distribution varies between spacecrafts, maybe corresponding to the difference of orientation; 3. The velocity of Magnetosphere FTEs are basically smaller than Magnetosheath ones. Also, with observations from 4 spacecrafts in different locations, we roughly figured out the configuration near the Magnetopause.
SH44A-1706
Recent Plasma Observations Related to Dayside Magnetic Merging and the Low-Latitude Boundary Layer
We have begun an investigation of the nature of the low-latitude boundary layer in the mid-altitude cusp region using data from the Polar spacecraft. This region has been routinely sampled for about three months each year for the periods 1999-2001 and 2004-2006. The low-to-mid-energy ion instruments frequently observed dense, magnetosheath-like plasma deep (in terms of distance from the magnetopause and in invariant latitude) in the magnetosphere. One such case, taken during a period of northward interplanetary magnetic field (IMF), shows magnetosheath ions within the magnetosphere with velocity distributions resulting from two separate merging sites along the same field lines. Cold ionospheric ions were also observed counterstreaming along the field lines, evidence that these field lines were closed. These results are consistent with the hypothesis that double merging can produce closed field lines populated by solar wind plasma. Through the use of individual cases such as this and statistical studies of a braoder database we seek to understand the morphology of the LLBL as it projects from the sub-solar region into the cusp. We will present preliminary results of our ongoing study.
SH44A-1707
Double Star observations of reconnection at the dayside magnetopause.
We present a statistical study on reconnection events performed using the Double Star TC1 plasma and magnetic field data. We examined about 250 magnetopause crossings occurred during the first year of the mission in the 06 - 18 LT interval. The Walèn relation was used to identify reconnection signatures at the magnetopause. We studied the occurrence of reconnection in relation to the local magnetosheath parameters such as the local Alfvèn Mach number, the plasma beta and the magnetic shear angle. Reconnection jet properties are discussed in the framework of the large scale configuration of the merging between the magnetosheath and magnetospheric fields.
SH44A-1708
Ion dynamics in high-latitude reconnection event: Interball-Tail observations.
We analyze ion velocity distributions observed by the Interball Tail spacecraft within reconnection events under northward IMF at the lobe magnetopause. Open flux tubes were observed while the spacecraft crossed the magnetopause. These flux tubes have clear signatures of the rotational discontinuity. Magnetospheric and magnetosheath plasma components exchange along open field lines follow velocity cut-offs (D-shaped distributions) in accordance with Cowley diagram for magnetic flux tube convecting with deHoffman-Teller velocity. Parallel velocity cut-off depends on perpendicular velocity. Magnetosheath ion component entering the magnetospheric part of open flux tube undergoes appreciable heating. We discuss possible mechanisms that are responsible for details of velocity distributions of ion components exchanging along open field lines. Regions with very low magnetic field magnitude were also observed suggesting diffusion region crossing. We use observations to infer the location of reconnection region and to discuss evidence for non-stationary reconnection.
SH44A-1709
Voyager II Observations of Magnetic Reconnection in the Inner and Middle Heliosphere
We use archival Voyager II data, primarily from the interplanetary cruise, to study signatures of magnetic reconnection in the solar wind well beyond 1 AU. Our search algorithm has identified 141 unique instances of pressure-balanced magnetic depressions that are (1) bounded by a pair of antisymmetric rotational discontinuities, and (2) accompanied by an Alfvénic or near-Alfvénic jet in the plane of rotation, as in the traversal of a Petchek-type reconnection exhaust. We present global statistics of these signatures from 1 AU out to 45 AU, and a study of the particle distribution functions of selected events.
SH44A-1710
False Positive Signatures of Agyrotropy Caused by Gradients
Any plasma detector takes time to collect its phase space distribution; during this time the spacecraft moves across (potentially) reorienting magnetic fields, and/or gradients of density, temperature, or flow velocity. This study reports on the magnitude of these false indications of agyrotropy for a spectrum of tangential and rotational discontinuity layer models that have differing degrees of (i) magnetic sheer, (ii) density variation, (iii) temperature variation, and (iv) various possible mixtures compatible with pressure balance for the TD and Walen condition for the RD. A separate study illustrates the sensitivity of the diagnostic to an incorrect determination of the electron bulk velocity, since agyrotropy is determined from the pressure tensor in the electron rest frame. These studies point to the importance of (a) adequate bulk velocity determinations and (b) the need for sub-moment cadence indicators of variation of the moments and the magnetic field direction when trying to use agyrotropy as a tool for discovery. Examples from the Polar Hydra instrument data products will illustrate this strategy for enhancing the integrity of physical agyrotropy detections and reducing the number of false positive ones. Examples of questionable and better detections of agyrotropy will be illustrated so that the method of rejection and acceptance are clarified.
SH44A-1711
Discreteness Noise Floor Signatures of Agyrotropy
Since agyrotropy is a null test for cylindrical symmetry of the pressure tensor about the local magnetic field, it will always be non-zero when the pressure tensor is determined from a finite number of (i) macro-particles in PIC or (ii) counted particles from an electrostatic analyzer. This paper refers to the error signal for false agyrotropy that accrues from a gyrotropic Vlasov plasma when approximated by N "readings" as a "discreteness floor". Such a discreteness floor is present in the N readings even when they are selected randomly from an ambient plasma that is gyrotropic. We have determined the discreteness floor for PIC simulation plasmas and that for the Polar Hydra DDEIS measurement system. Examples of changes in this floor will be illustrated across geophysical plasma boundaries where N changes, such as the magnetopause. As this varying discreteness floor is not physical, we have developed a technique to deconvolve the apparent agyrotropy with N readings to a best inference (with error estimate) of the ambient mediums Vlasov agyrotropy. This method has been shown to recover with high precision the agyrotropy profiles of the Harris sheet equilibrium with background that may be determined analytically. This technique for deconvolution will be illustrated for Hydra data to illustrate the discreteness floor removal, and the persistence of (Vlasov) agyrotropy signals at various current sheet crossings on Polar's orbit.
SH44A-1712
Effect of Oxygen on the Instability of Current Sheets
Recent observations from the CLUSTER mission have shown that during geomagnetically disturbed times, the O+ number density in the plasma sheet close to the reconnection X-line in the Earth's magnetotail can become comparable to, or even higher than, the corresponding H+ number density and that the O+ ions carry most of the particle pressure [Kistler et al., 2005; Wygant et al. 2005]. The presence of O+ also has implications for transport processes at the magnetopause [e.g., Bouhram et al., 2005]. However, there has been no systematic kinetic study of effects of O+ on magnetic reconnection. Even something as basic as the linear properties of tearing mode in three-species plasma is currently lacking. This constitutes a major gap in theoretical understanding of reconnection in the magnetosphere and hampers understanding of spacecraft data. Using our nonlocal linear Vlasov code we have examined the effect of O+ on various current sheet instabilities such as lower hybrid drift instability and tearing mode. These results and their consequences for magnetic reconnection will be discussed.
SH44A-1713
Magnetic reconnection with multiple X-lines in an open system: Two fluid simulations with finite electron inertial effects
To understand large-scale development of magnetic reconnection, it is necessary to consider reconnection triggered at multiple X-points in a long current sheet and associated coalescence of magnetic islands. While a number of numerical simulations have been performed to study multiple X-point magnetic reconnection, most of these simulations are done in periodic systems, in which the large-scale development of magnetic reconnection tends to be disturbed at the boundary. Thus, in this study, we perform two-fluid simulations of multiple X-point magnetic reconnection using an open boundary condition. Here the X-points, which are located with a spatial interval of 12.8D, are initially imposed by adding magnetic perturbations. First, when the initial perturbations having the same amplitude are added at three X-points, the two X-lines at the ends preferentially survive and the middle X-line is vanished by a pair of converging flows from the both ends. Then the two X-lines at the ends retreat from each other. Next, when we enhance the amplitude of initial perturbation at one of the ends by a factor of 1.2 or more, only the enhanced X-line can survive and two other X-lines disappeared. When the X-line at one of the ends is enhanced by a factor of 1.1, however, X-lines at both ends survive and retreat from each other. We have confirmed that such coexistence of both X-lines at the ends takes place at larger amplitude difference as the distance between the end-X-lines are increased. Furthermore, we have also confirmed that when the amplitude of the initial perturbation at the middle X-point is enhanced, the coexistence of two end X-lines occur more easily. These results suggest that more than one X-line can coexist even in the final stage of the development of magnetic reconnection driven in a long current sheet. In addition, we have found more varieties to be introduced to the final stage when the open boundary on one of the sides is changed to a closed boundary. In our presentation, these new simulation results will be shown.
SH44A-1714
Breakdown of the Frozen-in Condition and Plasma Acceleration: Dynamical Theory
The magnetic reconnection hypothesis emphasizes the importance of the breakdown of the frozen-in condition, explains the strong dependence of the geomagnetic activity on the IMF, and approximates an average qualitative description for many IMF controlled effects in magnetospheric physics. However, some important theoretical aspects of reconnection, including its definition, have not been carefully examined. The crucial components of such models, such as the largely-accepted X-line reconnection picture and the broadly-used explanations of the breakdown of the frozen-in condition, lack complete theoretical support. The important irreversible reactive interaction is intrinsically excluded and overlooked in most reconnection models. The generation of parallel electric fields must be the result of a reactive plasma interaction, which is associated with the temporal changes and spatial gradients of magnetic and velocity shears (Song and Lysak, 2006). Unlike previous descriptions of the magnetic reconnection process, which depend on dissipative-type coefficients or some passive terms in the generalized Ohm's law, the reactive interaction is a dynamical process, which favors localized high magnetic and/or mechanical stresses and a low plasma density. The reactive interaction is often closely associated with the radiation of shear Alfvén waves and is independent of any assumed dissipation coefficients. The generated parallel electric field makes an irreversible conversion between magnetic energy and the kinetic energy of the accelerated plasma and the bulk flow. We demonstrate how the reactive interaction, e.g., the nonlinear interaction of MHD mesoscale wave packets at current sheets and in the auroral acceleration region, can create and support parallel electric fields, causing the breakdown of the frozen-in condition and plasma acceleration.
SH44A-1715
Fully Kinetic 3D Simulations of Collisionless Reconnection in Large-Scale Pair Plasmas
Fully kinetic simulation offers a first principles approach to the understanding of the key physics of magnetic reconnection in planetary magnetospheres and solar flares. However, the large separation between electron and ion spatial and temporal scales has severally limited these computational studies in 3D at large scales that are relevant to macroscopic physical systems. In contrast, kinetic simulation of reconnection in an electron-positron plasma is feasible in 3D at large scales. Reconnection in pair plasma is relevant to the understanding of several astrophysical problems, such as jets in active galactic nuclei, pulsar winds, and gamma-ray bursts. Furthermore, this limit provides useful insight into the dynamics of large-scale 3D reconnection, including role of the kink mode, in hydrogen plasmas. In this work, reconnection in pair plasma in large-scale 3D systems is examined using the highly optimized PIC code VPIC developed at LANL. Both the anti-parallel limit and the influence of the guide field is investigated. It is found that 3D reconnection involves initial growth of tearing islands and island coalescence to large scales followed by outward expansion of the diffusion region and repeated formations of secondary islands, consistent with results from recent 2D simulations [Daughton and Karimabadi, Phys. Plasmas 14, 072303, 2007]. However, from linear Vlasov theory, the unstable eigenmodes in a pair plasma in 3D also include the drift-kink mode in addition to the tearing mode. We will discuss how the reconnection dynamics is influenced by the nonlinear interaction of these modes with the formation of complicated 3D structures.
SH44A-1716
Self-regulation of the reconnecting current sheet in relativistic pair plasmas
A thin current layer plays an important role in magnetic reconnection, because the microphysics in the region controls the magnetic dissipation rate. We investigate the detail structure of a reconnecting current sheet (RCS) in relativistic pair plasmas. Such RCS sometimes changes its current intensity through the reconnection electric field, so that it meets the outer physical conditions. However, in the relativistic regime, as soon as the current-carrying velocity reaches to its upper limit, typically an order of the light speed c, the mechanism breaks down and then the RCS finds the other way to compensate the required current. The evolution of the RCS and its self-regulating mechanism will be presented.
SH44A-1717
The Weibel Instability in Electron-Positron Plasma Reconnection
Full particle simulations of magnetic reconnection reveal the development of turbulence in the outflow jet during magnetic reconnection in pair plasma. This turbulence facilitates fast reconnection by opening the outflow nozzle, thereby shortening the current layer. We demonstrate that the temperature anisotropy that develops in the outflow plasma is the driver for this turbulence. Using particle simulations and analytic theory we explore the behavior of the Weibel-like instability that grows within a narrow current layer with anisotropic temperature - a geometry designed to mimic the current layers seen in electron-positron reconnection. The theoretically predicted wavelength and growth rate explain the structure of the observed turbulence and determine the length of the outflow nozzle seen in reconnection simulations.
SH44A-1718
A Statistical Model of Reconnecting Magnetic Islands in a Current Layer
Full particle 2-D simulations of magnetic reconnection with an ambient guide field show that secondary magnetic islands develop in long electron current layers. Observational evidence in the magnetotail and in the magnetopause also supports the formation of magnetic islands. However, the dynamics of these magnetic islands in large-scale current layers, such as those seen in the magnetopause or inferred in the corona, are not easily determined by simulations. To retain the small-scale physics of PIC codes for these large-scale systems, a statistical model is developed for the island distribution. Magnetic islands are characterized by the flux ψ contained in the island and the area A enclosed by it. These are used as the independent parameters for a distribution function of magnetic islands which evolves in time. The time evolution is governed by certain rules (verified by particle-in-cell simulations) concerning the generation of secondary islands, the rates of island growth, and island merging. An evolution equation for the distribution function of magnetic islands based on these rules is derived. Because of the complexity of the island merging rules, the evolution equation includes integral terms which prevent an analytic solution. The distribution of magnetic islands is therefore solved numerically.
SH44A-1719
Fast Reconnection in Electron-Positron Plasmas via Turbulent Outflow Jets
Numerical simulations of reconnection in electron-positron (pair) plasmas provide an interesting test of current theories of whistler-mediated Hall reconnection. Because of the system's mass symmetry the Hall term vanishes from the generalized Ohm's law, suggesting that perhaps pair reconnection is slow, as in the classic Sweet-Parker picture. But, as has already been documented, pair reconnection is, in fact, fast. We present large particle-in-cell simulations of pair reconnection confirming that the reconnection rate remains constant and fast even as the system size changes by almost an order of magnitude. The current layer extends to almost the system size in the smallest boxes but stops growing as the box size increases. For the largest systems we identify a Weibel-like temperature anisotropy instability in the outflow from the X-line that causes the current layer to broaden and permits fast reconnection. We discuss the implications of this instability, in particular whether it allows pair reconnection to be fast in all parameter regimes.
SH44A-1720
The Influence of Sheared Parallel Flows on the Onset of Magnetic Reconnection in Collisionless Regimes
The influence of sheared parallel flows on the onset of magnetic reconnection has been the subject of many investigations, majority of which employed various MHD models. At the same time, relatively little is known about the influence of parallel flows on magnetic reconnection in collisionless plasmas. As a first step in a broader program aimed at tackling this complicated problem, the linear stability of a newly developed exact kinetic equilibrium configuration is investigated. This model incorporates sheared parallel flows into the well-known Harris current sheet configuration, and allows considerable flexibility in the choice of the flow profile. The results from fully kinetic 2D PIC simulations are compared against numerical solution of a linearized complete Vlasov-Maxwell system, and against predictions of a simple semi-analytical model. MHD analysis typically predicts existence of an instability driven by the flow shear when the magnitude of the flow exceeds a critical speed which is of the order of Alfvén speed. In the kinetic regime the dispersion relation of the tearing instability is seen to be continuously modified by the flow. For some parameters the growth rate of the unstable reconnecting mode in the presence of a sheared flow may become larger than the growth rate of the tearing instability in the absence of flow, indicating that the mode has an additional driving mechanism. However, for the range of parameters considered in the present study, the growth rate of this "hybrid" instability is comparable to that of the tearing mode. When the magnitude of the flow is further increased, the growth rate significantly decreases. Consistent with these results, preliminary nonlinear PIC simulations demonstrate that if it proceeds, the onset of fast magnetic reconnection develops in the manner that is very similar to the situation when the flow is absent. In particular, no simulation has produced highly wrapped vortices that are observed in MHD simulations.
SH44A-1721
A First-Principles Analytical Theory for 2D Magnetic Reconnection in Electron and Hall Magnetohydrodynamics
Although the relevance of two-fluid effects in fast magnetic reconnection is well-known, (J. Birn et al., J. Geophys. Res., 106 (A3), 3715 (2001) a first-principles theory -- akin to Sweet and Parker's in resistive MHD -- has been elusive. Here, we present such a first principles steady-state analytical theory for electron MHD, (L. Chacón, A. N. Simakov, A. Zocco, Phys. Rev. Lett., submitted) and its extension to Hall MHD. (A. N. Simakov, L. Chacón, in preparation) The theory discretizes the extended MHD equations at the reconnection site, leading to a set of time-dependent ODEs. Their steady-state analysis, which describes the system at or around the point of maximum reconnection rate, provides predictions for the scaling of relevant quantities with the dissipation coefficients (e.g, resistivity and hyper-resistivity) and other relevant parameters. In particular, we will show that EMHD admits both elongated and open-X point configurations of the reconnection region, and that the reconnection rate can be shown not to scale explicitly with the dissipation parameters. This result is, to our knowledge, the first analytical confirmation of the possibility of fast magnetic reconnection in EMHD. In Hall MHD, the transition between resistive MHD and EMHD is studied, and scalings with the ion inertial length are obtained.
SH44A-1722
Fully Kinetic Simulations of Driven Magnetic Reconnection With Boundary Conditions Relevant to MRX
Many simulations of magnetic reconnection use periodic boundary conditions which limit the physical relevance of the results when comparing with large open systems that occur in nature or with laboratory reconnection experiments. To address this issue, more realistic boundary conditions are employed to model the Magnetic Reconnection Experiment (MRX) using a fully kinetic, two-dimensional code. The simulation is made up of a box with conducting boundary conditions and two current-carrying wires. As the current is ramped down over the time scale of the simulation, a current sheet forms and elongates. Scaling comparisons for the length and width of the electron layer as well as the reconnection rate are presented. In both the experiment and simulation the thickness of the electron layer scales linearly with the electron skin depth when the plasma density is varied. However, in the experiment, the layer is four to five times thicker [1]. The period of the driving waveform is a key parameter that must be matched for these comparisons; the layer is typically longer and thinner when the drive is stronger. For strong drives, magnetic islands similar to previous undriven, open-boundary simulations [2] are sometimes observed in the present work but have not been conclusively identified in the experiment. Boundary conditions near the wires also play an important role; when a fraction of particles are reflected rather than lost, the elongation of the layer late in the simulation is limited. The simulation may also be used to study aspects of driven reconnection for which measurements are not currently available in the experiment. For example, the problem of energy transfer from fields to particles is investigated by measuring the ion and electron distribution functions within the layer. The results may inform future experimental studies of ion and electron acceleration. [1] Y. Ren, Princeton PhD Thesis (2007). [2] W. Daughton, et all., Phys. Plasmas. {13}, 072101 (2006).
SH44A-1723
Detection of an Electron-scale Dissipation Layer near the X-line during Magnetic Reconnection in Laboratory
Despite its disruptive influences on the large-scale structures of space and solar plasmas, the crucial topological changes and associated dissipation during magnetic reconnection take place only near an X-line within thin singular layers. The collisional Sweet-Parker model, where electrons and ions flow together through a single geometrically thin and long dissipation layer, fails to explain the observed fast reconnection rates. When electrons and ions are allowed to move separately in modern collisionless models, it has been predicted that ions, despite their heavy mass, exhaust efficiently through a thicker, ion-scale dissipative layer while mobile electrons can evacuate through a thinner, electron-scale dissipation layer, allowing for efficient release of magnetic energy. While ion dissipation layers have been frequently detected, the existence of election layers near the X-line and the associated dissipation mechanism is still an open question, due to extremely rare encounters in space, despite their crucial importance in determining magnetic topology and dissipations. Here we report the first definite evidence of electron dissipation layers near the X-line detected in a reconnecting laboratory plasma. The measured electron layers display properties strikingly similar to predictions by 2D PIC simulations, including their geometrical shape, insensitivity to ion mass, and sensitivity to the boundary conditions, but disagreeing on the electron layer thickness. The electron layers are 4 to 6 times thicker in the laboratory than those in simulations, beyond thickening due to residual electron-ion collisions. These results effectively rule out all 2D mechanisms operative in the simulation model, including the usually hypothesized electron inertia effects, as a main mechanism for dissipations within the electron layer, and thus strongly, although indirectly, support that 3D effects, such as wave-particle interactions, are responsible for fast magnetic reconnection.
SH44A-1724
X-line Retreat During Magnetic Reconnection
In many astro- and space- physics situations, we observe energy release processes in vicinities of an obstacle. In the Earth's magnetotail, an Earthward plasma flow hits the magnetosphere which might end up with magnetic field pile-up and/or dipolarization of the magnetosphere. During solar flares, in view of magnetic reconnection model, a downward plasma jet collides with a magnetic flux tube in the lower part of the solar corona. In order to study energy release processes in these situations, we have set up a hard wall (symmetric boundary) to model either dipole magnetic field in the Earth's magnetosphere or magnetic loops in the solar corona. For simplicity, we have neglected any density gradient or magnetic field structures. The initial configuration of the current sheet is therefore of a Harris-type. After initiating magnetic reconnection by a magnetic field perturbation, its time evolution is observed by means of full particle, PIC simulation. The prominent feature in our simulation is a slow motion of the X-point directing away from the wall, which we call `X-line Retreat'. Our preliminary results show retreat speed to be about 0.1 times the Alfven speed measured at the boundaries. In our talk, we will discuss the mechanism and cause of the X-line retreat as well as the structures of the diffusion region. Implication on particle acceleration will also be addressed.
SH44A-1725
Asymmetric Magnetic Reconnection: General Theory and Collisional Simulations
Theories of magnetic reconnection usually assume that the plasmas on either side of the dissipation region have identical densities and magnetic field strengths. However, this canonical description is rarely realized in nature. There has been wide interest in the shock structure of fast reconnection, particularly at the dayside magnetopause, but a general theory of the structure of the dissipation region and the rate of reconnection during asymmetric reconnection has not been addressed. We derive analytical expressions from first principles using a Sweet-Parker type scaling analysis. Most of the scaling results are independent of the dissipation mechanism and, therefore, apply to asymmetric reconnection in general. Furthermore, we show that a generic feature of asymmetric reconnection is that the X-line and stagnation point are not colocated. This implies that there is a bulk flow of plasma across the X-line, as has been seen in many numerical studies and observations at the dayside. The theory is verified using two dimensional collisional magnetohydrodynamic simulations. Applications to the magnetosphere are discussed.
SH44A-1726
Explosive Instability and Coronal Heating
The observed energy loss rate from the solar corona implies that the coronal magnetic field has a critical angle at which energy is released. It has been hypothesized that at this critical angle an "explosive instability" would occur, leading to an enhanced conversion of magnetic energy into heat. In earlier investigations we have shown that a shear-dependent process called "secondary instability" could account for many of the distinctive features of the hypothetical "explosive instability." Here we show that this "secondary instability" can occur in a system with line-tied magnetic fields and boundary shearing. We also show that, as the disturbance due to secondary instability attains finite amplitude, there is a transition to turbulence which leads to enhanced dissipation of magnetic and kinetic energy. Furthermore, after each dissipative burst, the system is able to reform itself so that a subsequent burst can occur. These results are obtained from numerical simulations performed with a new parallelized, viscoresistive, three-dimensional code that solves the cold plasma equations. The code employs a Fourier collocation -- finite difference spatial discretization, and uses a third-order Runge-Kutta temporal discretization.
SH44A-1727
Simulation of the similarity of reconnection processes in the RSX device and in magnetospheric and solar plasmas
We consider the configuration typical of the RSX device [1]: multiple flux ropes are tied at one end to the plasma gun producing them and are relatively free to move at the other end. We consider the evolution of one or multiple ropes focusing primarily on reconnection and of the role of electrostatic fields in enabling the processes observed experimentally. The results of MHD [2] and kinetic simulations [3] are compared with the experiment, focusing primarily on the linear and non-linear stage of the kink and tearing instabilities present in the experiment. Furthermore, the results of kinetic simulation elucidate the role of drift instabilities in the large scale momentum creation and in the small scale anaomalous processes. [1] Intrator T. P., et al., J. Geophys. Res., 112, A05S90, (2007) [2] Lapenta, et al, J. Geophys. Res., 111, A12S06 (2006). [3] Lapenta et al., Phys. Plasmas, 13, 055904 (2006)
SH44A-1728
Universal Method for Describing Magnetic Reconnection
A quantitative description of three-dimensional (3D) magnetic reconnection still remains a challenge. This is because the variety of 3D structural features, where reconnection may occur in magnetic configurations, is much wider than in 2D case. And, what is particularly important, some of them, for example, quasi-separatrix layers, do not provide an exact partition of configurations on distinct magnetic fluxes, which makes it difficult to identify in this process the reconnected flux itself. Nevertheless, Hesse et al. (2005, ApJ 631) have recently shown that the latter is possible if the electric field parallel to magnetic field is known at the reconnecting flux tubes. We present here an alternative method for describing magnetic reconnection based solely on the field-line mapping technique that has recently been formulated in its most general form by Titov (2007, ApJ 660). We extend this technique from the analysis of magnetic structure at a given moment to the analysis of its evolution by introducing a new quantity called comparative squashing factor Qc. The large values of Qc define the magnetic surfaces enclosing the reconnected flux. The proposed method is universal in the sense that it assumes only the evolution of magnetic field and boundary flows to be known. This method admits also a straightforward numerical implementation, which allows one to develop an efficient diagnostics of the reconnection process in numerical simulations of realistic 3D magnetic configurations. Work supported by NASA and the Center for Integrated Space Weather Modeling (an NSF Science and Technology Center).
SH44A-1729
Patchy Reconnection Below Solar Coronal Mass Ejections
Observational evidence indicates that the flare reconnection which occurs behind solar coronal mass ejections (CME's) is often bursty and patchy in nature. This interpretation is supported both by coronal observations of numerous small scale downflows behind CME's, and by in situ observations of patchy magnetic connections to the corona in near-Earth CME's. We will present magnetohydrodynamic simulations of reconnection in Y-type current sheets representative of the current sheets created behind CME's. We will discuss how fast, Petschek-like reconnection can be excited in these simulations, and how this can lead to the generation of numerous patches of reconnection distributed across the sheet. The simulations will be related to the solar observations, and to observations and simulations of multiple island formation in magnetotail reconnection. This research was supported by grants from NASA and ONR.
SH44A-1730
Semi-Automatic Detection Software for Supra-Arcade Downflows
Detectable signatures of magnetic reconnection, a possible source of solar coronal heating, aid in constraining flare energetics. Supra-arcade downflows (SADs), first detected during the Yohkoh mission, are an example of an observable consequence of magnetic flux tube reconnection. These sunward-traveling voids above arcade loops are consistent with outflows resulting in standard 3D reconnection models of solar flares. We have developed semi-automated detection software to detect downflows and analyze their trajectories, speeds, sizes, and magnetic flux in order to constrain parameters for flare modeling. We will present these measurements as observed primarily by SXT and TRACE supplemented with new XRT observations and discuss their implications.
SH44A-1731
The Evolution of the Electric-Field Distribution During the CME/Two-Ribbon-Flare Event
The two-ribbon-flare (TRF) associated electric field near the solar surface can be estimated from the observed footpoint motion of the two-ribbon flares and the observed magnetic field on the solar surface. Based on the steady reconnection model, the estimated electric field on the solar surface is equal to the electric field near the reconnecting current sheet (RCS). The electric field near the RCS can be used to calculate the magnetic flux change rate during the CME/TRF event. The electric field near the RCS is also responsible for the generation of the relativistic particles and the acceleration of the coronal mass ejection (CME). Therefore, it is important to know how to correctly estimate the electric field at the RCS during a CME/TRF event. In this investigation, we use a two-dimensional magnetohydrodynamic simulation to study the evolution of the electric-field distribution during the CME/TRF event. Our preliminary results indicate that the electric field is not always uniform during the CME/TRF event. Particularly, the electric field above and below the reconnection site are usually different from each other. Our results might provide an explanation to the poor correlation between the estimated electric field on the solar surface and the acceleration of the CME in the previous observations (Jing et al., 2005).
SH44A-1732
Magnetic Reconnection Site Suggested by a Double Coronal X-ray Source Observed by RHESSI
In the classical reconnection model, magnetic field annihilation in a current sheet generates turbulence and outflows of high speed plasmas in opposite directions. The turbulence accelerates particles and heats the background plasma. Observational signatures, such as radio emission and hard and soft X-rays produced by these high-energy particles and hot plasmas, are thus expected to show the two oppositely directed outflows. However, such observations have rarely been reported in the past. In this talk, we present data analysis and interpretation of an M1.4-class flare observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on April 30, 2002. This event, with its footpoints occulted by the solar limb, exhibits such a rarely observed double-source morphology in the corona over the 6--30~keV range. The two coronal sources, appearing at different altitudes, show energy-dependent structures with the higher-energy emission being closer together. Spectral analysis implies that the emission at higher energies in the inner region between the two sources is mainly nonthermal, while the emission at lower energies in the outer region is primarily thermal. The two sources are both visible for about 12 minutes and have similar light curves and power-law spectra (assumed nonthermal) above about 20~keV. These observations suggest that the magnetic reconnection site lies between the two sources. Bi-directional outflows of the released energy in the form of turbulence and/or particle beams away from the reconnection site can be the source of the observed radiation. The spatially resolved thermal emission below about 15~keV, on the other hand, indicates that the lower coronal source has a larger emission measure but a lower temperature than the upper source. This is likely the result of the different magnetic topologies of the two sources. Implications of these results for particle acceleration and plasma heating in theoretical flare models will be discussed. http://hesperia.gsfc.nasa.gov/~weiliu/research/publications/2007_doubleXR
SH44A-1733
Magnetic Reconnection Rate in X- and M-class Solar Flares
The magnetic reconnection is generally believed to be responsible for the energy release and particle acceleration in solar flares. Since the magnetic reconnection is difficult to be observed directly, the apparent motions of associated hard X-ray (HXR) kernels or H-alpha ribbons can be regarded as the chromospheric signatures of magnetic reconnection process and thus provides an important way to measure the magnetic reconnection rate in the corona. By combining the MDI (Michelson Doppler Imager) magnetograms, the reconstructed HXR images from RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Image), the H-alpha images from global high-resolution H£\ network, and the TRACE 160 nm images, the reconnection electric field and magnetic flux change rate in X- and M-class flares can be estimated. We will compare the results of magnetic reconnection rate from different measurements (HXR, H-alpha, EUV/UV). Comparison of reconnection rate between observation and theory will also be discussed.