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Observation of Ion Acceleration and Heating during Collisionless Magnetic Reconnection in a Laboratory Plasma

Description: The ion dynamics in a collisionless magnetic reconnection layer are studied in a laboratory plasma. The measured in-plane plasma potential profile, which is established by electrons accelerated around the electron diffusion region, shows a saddle-shaped structure that is wider and deeper towards the outflow direction. This potential structure ballistically accelerates ions near the separatrices toward the outflow direction. Ions are heated as they travel into the high pressure downstream region.
Date: December 10, 2012
Creator: Jongsoo Yoo, Masaaki Yamada, HantaoJi and Clayton E. Myers
Partner: UNT Libraries Government Documents Department

Quantitative Study Of Guide Field Effects on Hall Reconnection In A Laboratory Plasma

Description: The effect of guide field on magnetic reconnection is quantitatively studied by systematically varying an applied guide field in the Magnetic Reconnection Experiment (MRX). The quadrupole field, a signature of two-fluid reconnection at zero guide field, is significantly altered by a finite guide field. It is shown that the reconnection rate is significantly reduced with increasing guide field, and this dependence is explained by a combination of local and global physics: locally, the in-plane Hall currents are reduced, while globally guide field compression produces an increased pressure both within and downstream of the reconnection region. __________________________________________________
Date: April 17, 2012
Creator: Tharp, T. D.; Yamada, M.; Ji, H.; Lawrence, E.; Dorfman, S. & Myers, C.
Partner: UNT Libraries Government Documents Department

Formation of Plasmoid Chains in Magnetic Reconnection

Description: A detailed numerical study of magnetic reconnection in resistive MHD for very large, previously inaccessible, Lundquist numbers (104 ≤ S ≤ 108) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable to super-Alfvenically fast formation of plasmoid (magnetic-island) chains. The plasmoid number scales as S3/8 and the instability growth rate in the linear stage as S1/4, in agreement with the theory by Loureiro et al. [Phys. Plasmas 14, 100703 (2007)]. In the nonlinear regime, plasmoids continue to grow faster than they are ejected and completely disrupt the reconnection layer. These results suggest that high-Lundquist-number reconnection is inherently time-dependent and hence call for a substantial revision of the standard Sweet-Parker quasistationary picture for S>104.
Date: September 9, 2009
Creator: R. Samtaney, N.F. Loureiro, D. A. Uzdensky, A.A. Schekochihin, and S.C. Cowley
Partner: UNT Libraries Government Documents Department

Effects of Global Boundary and Local Collisionality on Magnetic Reconnection in a Laboratory Plasma

Description: The magnetic reconnection process is studied in a wide range of operating conditions in the well-controlled Magnetic Reconnection Experiment. The reconnection rate is observed to be a function of both global (i.e., system size) and local (collisionality) plasma parameters. When only local collisionality is lowered, the current sheet is shortened while effective resistivity is enhanced, both accelerating reconnection rates. At a fixed collisionality, the current sheet length increases with system size, resulting in the reduction of the reconnection rate. These results quantitatively agree with a generalized Sweet-Parker analysis.
Date: July 24, 2007
Creator: A. Kuritsyn, H. Ji, S.P. Gerhardt, Y. Ren, and M. Yamada
Partner: UNT Libraries Government Documents Department

Identification of the Electron Diffusion Region during Magnetic Reconnection in a Laboratory Plasma

Description: We report the first identification of the electron diffusion region, where demagnetized electrons are accelerated to super-Alfvenic speed, in a reconnecting laboratory plasma. The electron diffusion region is determined from measurements of the out-of-plane quadrupole magnetic field in the neutral sheet in the Magnetic Reconnection Experiment. The width of the electron diffusion region scales with the electron skin depth (∼ 5.5-7.5c=ωpi) and the peak electron outflow velocity scales with the electron Alfven velocity (∼ 0.12 - 0.16VeA), independent of ion mass.
Date: June 26, 2008
Creator: Yang Ren, Masaaki Yamada, Hantao Ji, Stefan Gerhardt, and Russell Kulsrud
Partner: UNT Libraries Government Documents Department

Final Report: Laboratory Studies of Spontaneous Reconnection and Intermittent Plasma Objects

Description: The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF) at the MIT Plasma Science and Fusion Center and the MIT Physics Department. The NSF/DOE award No. 0613734, supported two graduate students (now Drs. W. Fox and N. Katz) and material expenses. The grant enabled these students to operate the VTF basic plasma physics experiment on magnetic reconnection. The first configuration was characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be relevant to specifically to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In the Report we review the general motivation of this work and provide an overview of our experimental and theoretical results enabled by the support through the awards.
Date: May 31, 2011
Creator: Egedal-Pedersen, Jan & Porkolab, Miklos
Partner: UNT Libraries Government Documents Department

Laboratory Study of Hall Reconnection in Partially Ionized Plasmas

Description: The effects of partial ionization (ni/nn ≤ 1%) on magnetic reconnection in the Hall regime have been studied systematically in the Magnetic Reconnection Experiment (MRX). It is shown that, when neutrals are added the Hall quadrupole field pattern and thus electron flow is unchanged while the ion outflow speed is reduced due to ion-neutral drag. However, in constrast to theoretical predictions, the ion diffusion layer width does not change appreciably. Therefore, the total ion outflow flux and the normalized reconnection rate are reduced.
Date: May 15, 2012
Creator: Eric E. Lawrence, Hanto Ji, Masaaki Yamaada and Jongsoo Yoo
Partner: UNT Libraries Government Documents Department

Experimental Study of Current-Driven Turbulence During Magnetic Reconnection

Description: CMPD Final Report Experimental Study of Current-Driven Turbulence During Magnetic Reconnection Miklos Porkolab, PI, Jan Egedal, co-PI, William Fox, graduate student. This is the final report for Grant DE-FC02-04ER54786, “MIT Participation in the Center for Multiscale Plasma Dynamics,” which was active from 8/1/2004 to 7/31/2010. This Grant supported the thesis work of one MIT graduate student, William Fox, The thesis research consisted of an experimental study of the fluctuations arising during magnetic reconnection in plasmas on the Versatile Toroidal Facility (VTF) at MIT Plasma Science and Fusion Center (PSFC). The thesis was submitted and accepted by the MIT physics Department, “W. Fox, Experimental Study of Current-Driven Turbulence During Magnetic Reconnection, Ph.D. Thesis, MIT (2009)”. In the VTF experiment reconnection and current-sheet formation is driven by quickly changing currents in a specially arranged set of internal conductors. Previous work on this device [Egedal, et al, PRL 98, 015003, (2007)] identified a “spontaneous” reconnection regime. In this work fluctuations were studied using impedance-matched, high-bandwidth Langmuir probes. Strong, broadband fluctuations, with frequencies extending from near the lower-hybrid frequency [fLH = (fcefci)1/2] to the electron cyclotron frequency fce were found to arise during the reconnection events. Based on frequency and wavelength measurements, lower-hybrid waves and Trivelpiece-Gould waves were identified. The lower-hybrid waves are easiest to drive with strong perpendicular drifts or gradients which arise due to the reconnection events; an appealing possibility is strong temperature gradients. The Trivelpiece-Gould modes can result from kinetic, bump-on-tail instability of a runaway electron population energized by the reconnection events. We also observed that the turbulence is often spiky, consisting of discrete positive-potential spikes, which were identified as “electron phase-space holes,” a class of nonlinear solitary wave known to evolve from a strong beam-on-tail instability. We established that fast electrons were produced by magnetic reconnection. Overall, these instabilities were ...
Date: August 31, 2010
Creator: Porkolab, Miklos; Egedal-Pedersen, Jan & Fox, William
Partner: UNT Libraries Government Documents Department

New Insights into Dissipation in the Electron Layer During Magnetic Reconnection

Description: ELECTRON DISSIPATION IN RECONNECTION 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 insuffcient 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.
Date: July 18, 2008
Creator: H. Ji, Y. Ren, M. Yamada, S. Dorfman, W. Daughton and S.P. Gerhardt
Partner: UNT Libraries Government Documents Department

Three-dimensional, Impulsive Magnetic Reconnection in a Laboratory Plasma

Description: Impulsive, local, 3-D reconnection is identified for the first time in a laboratory current sheet. The events observed in the Magnetic Reconnection Experiment (MRX) 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.
Date: May 3, 2013
Creator: S Dorfman, et al
Partner: UNT Libraries Government Documents Department

Experimental Evaluation of Multi-spacecraft Data Analysis Techniques in a Laboratory Plasma

Description: The Magnetic Reconnection Experiment (MRX)[1] has been utilized to assess the effectiveness of minimum variance analysis on the magnetic field (MVAB) and boundary-crossing time analysis (BCTA). The neutral sheet is swept, or jogged, in a controlled manner with respect to the stationary probes by pulsed internal coil currents. Magnetic field data from measurement points resembling data from multi-spacecraft flying though a reconnecting current sheet is used to check both techniques to deduce a proper normal vector. We examine discharges with the two-dimensional (2-D) X-line structure as well as cases in which a flux rope forms within the layer. All discharges are in a two-fluid regime in which electrons are magnetized but not ions. Boundary-crossing time analysis with four sample measurement points forming a tetrahedron generates a reasonable unit normal vector and relative velocity along the normal vector for all of the tested cases. On the other hand, MVAB sometimes fails to predict a proper normal direction. This is because the X-line magnetic geometry is fundamentally 2-D or 3-D. However, the direction along the reconnecting field determined by MVAB does not deviate much from the real magnetic geometry documented by 2-D magnetic probe arrays and one additional probe at a different toroidal location. Based on these observations, we suggest a procedure for determining a local coordinate system for data from the Magnetospheric Multi-Scale (MMS) mission when spacecraft passes through a reconnecting current sheet. The distance between measurement points on the order of the ion skin depth (c/{omega}{sub pi}) is pertinent to determination of the magnetic geometry.
Date: March 27, 2012
Creator: Yamada, Jongsoo Yoo and Masaaki
Partner: UNT Libraries Government Documents Department

Gyrokinetic Electron and Fully Kinetic Ion Particle Simulation of Collisionless Plasma Dynamics

Description: Fully kinetic-particle simulations and hybrid simulations have been utilized for decades to investigate various fundamental plasma processes, such as magnetic reconnection, fast compressional waves, and wave-particle interaction. Nevertheless, due to disparate temporal and spatial scales between electrons and ions, existing fully kinetic-particle codes have to employ either unrealistically high electron-to-ion mass ratio, me/mi, or simulation domain limited to a few or a few ten's of the ion Larmor radii, or/and time much less than the global Alfven time scale in order to accommodate available computing resources. On the other hand, in the hybrid simulation, the ions are treated as fully kinetic particles but the electrons are treated as a massless fluid. The electron kinetic effects, e.g., wave-particle resonances and finite electron Larmor radius effects, are completely missing. Important physics, such as the electron transit time damping of fast compressional waves or the triggering mechanism of magnetic reconnection in collisionless plasmas is absent in the hybrid codes. Motivated by these considerations and noting that dynamics of interest to us has frequencies lower than the electron gyrofrequency, we planned to develop an innovative particle simulation model, gyrokinetic (GK) electrons and fully kinetic (FK) ions. In the GK-electron and FK-ion (GKe/FKi) particle simulation model, the rapid electron cyclotron motion is removed, while keeping finite electron Larmor radii, realistic me/mi ratio, wave-particle interactions, and off-diagonal components of electron pressure tensor. The computation power can thus be significantly improved over that of the full-particle codes. As planned in the project DE-FG02-05ER54826, we have finished the development of the new GK-electron and FK-ion scheme, finished its benchmark for a uniform plasma in 1-D, 2-D, and 3-D systems against linear waves obtained from analytical theories, and carried out a further convergence test and benchmark for a 2-D Harris current sheet against tearing mode and other instabilities in ...
Date: August 11, 2009
Creator: Lin, Yu; Wang, Xueyi; Chen, Liu & Lin, Zhihong
Partner: UNT Libraries Government Documents Department

Experimental Study of the Hall Effect and Electron Diffusion Region During Magnetic Reconnection in a Laboratory Plasma

Description: The Hall effect during magnetic reconnection without an external guide field has been extensively studied in the laboratory plasma of the Magnetic Reconnection Experiment (MRX) [Yamada et al., Phys. Plasmas 4, 1936 (1997)] by measuring its key signature, an out-of-plane quadrupole magnetic field, with magnetic probe arrays whose spatial resolution is on the order of the electron skin depth. The in-plane electron flow is deduced from out-of-plane magnetic field measurements. The measured in-plane electron flow and numerical results are in good agreement. The electron diffusion region is identified by measuring the electron outflow channel. The width of the electron diffusion region scales with the electron skin depth (~ 8c/ωpe) and the peak electron outflow velocity scales with the electron Alfven velocity (~ 0:11VeA), independent of 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. It is found that the classical and anomalous dissipation present in the experiment can broaden the electron diffusion region and slow the electron outflow. As a consequence, the electron outflow flux remain consistent with numerical simulations. The ions, as measured by a Mach probe, have a much wider outflow channel than the electrons, and their outflow is much slower than the electron outflow everywhere in the electron diffusion region.
Date: July 2, 2008
Creator: Yang Ren, Masaaki Yamada, Hantao Ji, Seth Dorfman, Stefan Gerhardt, and Russel Kulsrud
Partner: UNT Libraries Government Documents Department

Equilibria and Stability of JET Discharges with Zero Core Current Density

Description: Injection of Lower Hybrid Heating and Current Drive (LHCD) into the current ramp-up phase of JET [Joint European Torus] discharges can produce extremely reversed q-profiles characterized by a core region of near zero current density (within Motional Stark Effect diagnostic measurement errors). Non-inductive, off-axis co-current drive induces a back electromotive force inside the non-inductive current radius that drives a negative current in the plasma core. The core current density does not go negative, although current diffusion calculations indicate that there is sufficient LHCD to cause this. The clamping of the core current density near zero is consistent with n=0 reconnection events redistributing the core current soon after it goes negative. This is seen in reduced MHD simulations and in nonlinear resistive MHD simulations which predict that these discharges undergo n=0 reconnection events that clamp the core current near zero.
Date: October 15, 2002
Creator: Stratton, B.C.; Hawkes, N.C.; Huysmans, G.T.A.; Breslau, J.A.; Zakharov, L.E.; Alper, B. et al.
Partner: UNT Libraries Government Documents Department

Two-Fluid and Resistive Nonlinear Simulations of Tokamak Equilibrium, Stability, and Reconnection

Description: The NIMROD and M3D / M3D-C1 codes now each have both a resistive MHD and a two-fluid (2F) capability including gyroviscosity and Hall terms. We describe: (1) a nonlinear 3D verification test in the resistive MHD regime in which the two codes are in detailed agreement , (2) new studies that illuminate the effect of two-fluid physics on spontaneous rotation in tokamaks, (3) studies of nonlinear reconnection in regimes of relevance to fusion plasmas with peak nonlinear reconnection rates that are essentially independent of the resistivity, and (4) linear two-fluid tearing mode calculations including electron mass that agree with analytic studies over a wide range of parameter regimes.
Date: September 1, 2008
Creator: Jardin, S.; Sovinec, C.; Breslau, J.; Ferraro, N.; Hudson, S.; King, J. et al.
Partner: UNT Libraries Government Documents Department

Final Report for DoE Grant DE-FG02-06ER54878, Laboratory Studies of Reconnection in Magnetically Confined Plasmas

Description: The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF). The first configuration is characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be specifically relevant to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In this Report we review the general motivation of this work, the experimental set-up, and the main physics results.
Date: January 29, 2010
Creator: Egedal-Pedersen, Jan
Partner: UNT Libraries Government Documents Department

New Insights to the Sawtooth Oscillation (m/n=1/1 mode) in Hot Plasmas based on High Resolution 2-D Images of Te Fluctuations

Description: Two dimensional (2-D) images of electron temperature fluctuations with high temporal and spatial resolution have been employed to study the sawtooth oscillation (m/n=1/1 mode) in Toroidal EXperiment for Technology Oriented Research (TEXTOR) tokamak plasmas. 2-D imaging data revealed new physics which were not available in previous studies based on the 1-D electron temperature measurement and X-ray tomography. Review of the physics of the sawtooth oscillation is given by comparative studies with prominent theoretical models suggest that a new physics paradigm is needed to describe the reconnection physics of the sawtooth oscillation. The new insights are: A pressure driven instability (not a ballooning mode) leads to the X-point reconnection process. The reconnection process is identified as a random 3-D local reconnection process with a helical structure. The reconnection time scale is similar for different types of sawtooth oscillation ("kink" and tearing type) and is significantly faster than the resistive time scale. Heat flow from the core to the outside of the inversion radius during the reconnection process is highly collective rather than stochastic.
Date: November 26, 2007
Creator: H.K. Park, N.C. Luhmann, Jr, A.J.H. Donné, C.W. Domier, T. Munsat, M.J. Van de Pol, and the TEXTOR Team
Partner: UNT Libraries Government Documents Department

New perspectives on substorm injections

Description: There has been significant progress in understanding substorm injections since the Third International Conference on Substorms in 1996. Progress has come from a combination of new theories, quantitative modeling, and observations--particularly multi-satellite observations. There is now mounting evidence that fast convective flows are the mechanism that directly couples substorm processes in the mid tail, where reconnection occurs, with substorm processes the inner magnetosphere where Pi2 pulsations, auroral breakups, and substorm injections occur. This paper presents evidence that those flows combined with an earthward-propagating compressional wave are responsible for substorm injections and discusses how that model can account for various substorm injection signatures.
Date: December 1, 1998
Creator: Reeves, G.D.
Partner: UNT Libraries Government Documents Department

Magnetic reconnection in sheared solar magnetic arcades

Description: The evolution of solar magnetic arcades is investigated with the use of MHD simulations imposing resistivity on sheared magnetic fields. It is found that there is a critical amount of shear, over which magnetic reconnection can take place in an arcade-like field geometry to create a magnetic island. The process leading to reconnection cannot be solely attributed to a tearing instability, but rather to a reactive evolution of the magnetic arcade under resistivity. The natures of the arcade reconnection are governed by the spatial pattern of resistivity. A fast reconnection with a small shock angle can only be achieved when the diffusion region is localized. In this case, a highly collimated reconnection outflow can tear the plasmoid into a pair, and most of principal features in solar eruptive processes are reproduced.
Date: December 31, 1996
Creator: Choe, G.S.
Partner: UNT Libraries Government Documents Department

Experimental Test of the Sweet-Parker Model of Magnetic Reconnection

Description: We report a quantitative experimental test of the Sweet-Parker model of magnetic reconnection in a controlled laboratory plasma. It is found that the observed reconnection rate cannot be explained by the Sweet-Parker model unless the model is generalized to incorporate compressibility, downstream pressure, and the effective resistivity. The latter is significantly enhanced over its classical values in the collisionless limit.
Date: October 3, 1997
Creator: Ji, Hantao; Yamada, Masaaki; Hsu, S. & Kulsrud, R.
Partner: UNT Libraries Government Documents Department

Magnetic reconnection in space plasmas

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Magnetic reconnection produces fundamental changes in the magnetic field topology of plasmas and leads ultimately to substantial plasma heating and acceleration. The transfer of stored magnetic field energy to the plasma occurs primarily at thin conversion layers that extend outward from the reconnection site. We performed a comparative study of the structure and nature of these conversion layers as observed during reconnection at Earth`s magnetopause and in the geomagnetic tail. Our research utilized plasma and magnetic field data from the Earth-orbiting ISEE satellites during crossings of the conversion layers at the magnetopause and in the geomagnetic tail, as well as data obtained during a long-duration balloon flight in Antarctica and simultaneously from satellites in geosynchronous orbit. We have found that the reconnection layer at the magnetopause usually does not contain a slow mode shock, contrary to earlier theoretical expectations. Through a coordinated analysis of data obtained from balloon altitudes and at geosynchronous orbit, we obtained evidence that reconnection can occur simultaneously in both hemispheres at the magnetopause above the polar caps. The final year of our study was oriented primarily towards the question of determining the magnetic topology of disturbances in the solar wind associated with coronal mass ejections (CMEs) and understanding how that topology is affected by magnetic reconnection occurring near the Sun.
Date: April 1, 1996
Creator: Gosling, J.; Feldman, W. & Walthour, D.
Partner: UNT Libraries Government Documents Department

Cusp and Y-type magnetic structures and volocity fields at the endpoint of the reconnection layer

Description: We study the two-dimensional global scale magnetic field structure for a system of two merging cylindrical plasmas in a steady state. In the limit of very large magnetic Reynolds numbers the reconnection process is slow, and the plasma almost everywhere finds itself in magnetostatic equilibrium. We show that under certain conditions the classical Syrovatskii-type Y-point configuration, with surface current concentrated only in the reconnection layer, is not possible. Instead, a cusp configuration is formed, with finite surface current in the separatrix. The equilibrium condition, together with constraints on the volume per flux, enables us to determine the shape of the separatrix and the magnetic field in the vicinity of the cusp point. Our solution is characterized by a singular power law dependence of current density on the flux coordinate ({psi}) near the separatrix: j({Psi}) {approx} |{Psi}|{sup -1/2}. This solution gives us the boundary conditions that are needed to find the flow in the reconnection and the separatrix regions.
Date: June 12, 1997
Creator: Uzdensky, D.A. & Kulsrud, R.M.
Partner: UNT Libraries Government Documents Department

A model solar flares and their homologous behavior

Description: A model describing physical processes of solar flares and their homologous behavior is presented based on resistive MHD simulations of magnetic arcade evolution subject to continuous shear-increasing footpoint motions. It is proposed in the model that the individual flaring process encompasses magnetic reconnection of arcade field lines, generation of magnetic islands in the magnetic arcade, and coalescence of magnetic islands. When a magnetic arcade is sheared, a current sheet is formed and magnetic reconnection can take place to form a magnetic island. A continuing increase of magnetic shear can trigger a new reconnection process and create another island in the underlying arcade below the magnetic island. The newborn island rises faster than the preceding island and merges with it to form one island. Before merging with the upper island is completed, the newborn island exhibits two different phases of rising motion: the first phase with a slower rising speed and the second phase wit h a faster rising speed. This is consistent with the Yohkoh observation by Ohyama and Shibata (1998) of X-ray plasma ejecta motion. The first phase, in which reconnection of line-tied field in the underlying arcade is important, can be regarded to be related with the preflare phase. In the second phase, the island coalescence takes place, which creates an elongated current sheet below and enhances the reconnection rate of the line-tied arcade field. This phase is interpreted as the impulsive phase or the flash phase of flares. The obtained reconnection electric field is large enough to accelerate electrons to an energy level higher than 10 keV, which is necessary for observed X-ray emissions. After merging of the islands is completed, magnetic reconnection continues in the current sheet under the integrated island for rather a long period, which can be considered as the main phase of flares. ...
Date: January 27, 2000
Creator: Choe, G.S. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department