Magnetic Reconnection: Theory and Observations I Posters
Presiding: J Birn, Los Alamos National Laboratory; M Hesse, NASA Goddard Space Flight Center
SM23B-01 1330h
Alfven Wave Resonance Cone Propagation
Using a 2.5-D code, we studied the propagation of inertial Alfven (IA) waves radiated from a localized oscillating source. Structure of the resonance cone in the fields, current, and density including nonlinear effects is investigated. Electron parallel drifts in the large current region generate secondary electrostatic waves affecting significant bulk heating as well as formation of elongated tail in the electron velocity distribution function. Alfven waves are known to transport energy from distant part of the outer space to near-Earth geo-space. The propagation of such waves over long distance and the microscopic mechanisms for their dissipation affecting plasma heating and acceleration in the auroral ionosphere remains one of the outstanding problems in space plasma physics. It is emerging now that such waves initiates from the distant sites of magnetic reconnections when current sheets thickness transverse to the magnetic field lines become small of the size of electron and ion skin depths. Low frequency perturbations in the current of such small transverse dimensions radiate inertial or kinetic Alfven waves. The radiated waves propagate over distances of several Earth radii in the mode of Alfven wave resonance cone dumping enrgy in the auiroral ionosphere. The entire problem of the generation, propagation and dissipation in the realistic configuration of the magnetosphere is a formidable task. In this paper we report results from our initial attempt to model the basic plasma processes driven in the course of the propagation of Alfven wave resonance cone emitted from a localized source. The currents in the resonance cone structure are seen to generate electrostatic waves in the frequency band of ion-acoustic and lower-hybrid waves, which heat the electrons. This provides an effective dissipation mechanism.
SM23B-02 1330h
Ion and electron dissipation in oblique slow shocks
Ion and electron dissipation in collisionless slow-mode shocks at highly oblique shock angles (>80°) are examined using one-dimensional hybrid (kinetic ions, massless fluid electrons)and full particle (kinetic ions and electrons) simulations. In the hybrid code, an improved full electron pressure tensor model is used to enable the formation of highly oblique slow shocks in which effects of the downstream electron temperature anisotropy with respect to the local magnetic field direction (Te∥ >Te⊥), as seen in spacecraft observations and full particle simulations, are retained. Unlike the slow shocks at moderately oblique angles (<80°) in which the shock dissipation is provided primarily by the ions, additional electron physics is needed to set up shocks at very oblique angles: The electron temperature anisotropy results from both the large mirror effects and the electron acceleration/heating by the parallel electric field of very obliquely propagating kinetic Alfvén waves excited by ion-ion streaming in the shock. The additional electron dynamics lead to spiky structures in the shock ramp in the density, and the ion and electron parallel temperature/pressure. We present simulations of very oblique slow shocks and discuss both the single particle effects and contributions from resonant waves to the shock dissipation.
SM23B-03 1330h
Three-dimensional MHD Simulation Study of the Structure of the Leading Part of a Reconnection Jet
We have performed three dimensional MHD simulations of magnetic reconnection to investigate the structure the fast plasma flow generated by reconnection, so called reconnection jet. Our simulation results show the significantly complicated structure of the front of the reconnection jet. In the randomly perturbed plasma environment, the smaller scale structure develops rapidly and finally, the front of the reconnection jet becomes turbulent state. Then, in the more simple cases that the initial perturbation is single sinusoidal configuration, we found that the smaller wavelength mode grows faster and the magnetic bubbles are generated. The magnetic bubbles elongate as the time development. From the properties of the structure of the jet front, we interpreted basically this deformation of the structure as the excitation of the Rayleigh-Taylor and/or the ballooning mode.
SM23B-04 1330h
Magnetosphere-ionosphere coupling initiated at the duskside boundary layer during impulsive IMF rotations
Observations from ACE, Wind, Cluster, FAST and ground magnetometers and radars, complemented by theory, are presented to illustrate various aspects of Magnetosphere-Ionosphere (M-I) coupling during an event characterized by a sequence of interplanetary directional discontinuities (DDs) impinging on the magnetopshere. The slow (several hours) northward rotation of the IMF is punctuated by these DDs, which include abrupt and simultaneous east-west rotations of the field and flow. Cluster was traversing the duskside magnetopause boundary layer at geomagnetic latitudes of ~30°. Calculations confirm the presence of magnetopause surface waves of Kelvin-Helmholtz origin, in addition to other undulatory motions of large amplitude. The continuous ground coverage from magnetometer arrays and radars allow us to document the ionospheric imprint of this activity. Finally, the M-I coupling during this event is compared to that in a previous Cluster-FAST-Sondrestrom conjunction when Cluster was in the duskside high-latitude boundary layer. This work is supported partially by NASA Grant NAG5-13116
SM23B-05 1330h
Sheared Buneman Instabilities in Current-Driven Plasmas
Simulation studies of magnetic reconnection in strongly magnetized plasmas1 have indicated that electron phase-space holes evolve out of current-driven Buneman instabilities and that these holes play an important role in supplying the needed dissipation in the reconnection process by acting as electron scattering centers. Drake and collaborators have shown in simulations that the evolution of electron holes is mediated by lower hybrid waves. We have shown independently, via 2-D simulations of the evolution of Buneman instabilities, that electron phase space holes evolve and interact with lower hybrid waves in a manner similar to that seen in the reconnection simulations2 Perpendicular shear in the parallel velocity and the current will be present near the edges and elsewhere in realistic current sheets associated with magnetic reconnection. We have studied the effects of such shear on the nature and evolution of Buneman instabilities and the resulting electron phase space holes. Both linear theory and 2-D Vlasov simulations are employed. It is shown that even a small amount of velocity shear can have a large effect on the nonlinear evolution of holes and lower hybrid waves. 1Drake, J. F., M. Swisdak, C. Cattell, M. A. Shay, B. N. Rogers, and A.~Zeiler, Formation of Electron Holes and Particle Energization During Magnetic Reconnection, Science, 299, (2003). 2Martin V. Goldman, D. L. Newman, A. Mangeney, F. Califano, Theory and Simulation of Sheared Electron Beam Instabilities in Strongly Magnetized Plasmas, COSPAR04-A-02395; D3.5-0015-04, 35th COSPAR Scientific Assembly Paris, France, 18 - 25 July 2004. This research was supported by DOE, NSF and NASA.
SM23B-06 1330h
Particle acceleration at a 3D reconnection site
We study test particle trajectories in the vicinity of a three-dimensional (3D) magnetic null point during spine reconnection. Particles are injected into the steady-state non-uniform magnetic and electric fields derived by Priest and Titov (1996), and the equations of motion numerically integrated. We use input parameters typical of the solar corona, for which reconnection has been suggested as the fundamental mechanism responsible for particle acceleration in flare events. We show that substantial acceleration is possible in the 3D spine reconnection configuration, in the strong electric field regime. The energy gain is strongly dependent on the location of injection into the simulation box, as was the case in 2D X-point configurations. In our 3D geometry, we first vary the location of injection within a plane through the spine, and derive an analytical value for the injection angle for which maximum energy gain is achieved. Secondly we vary the azimuthal location of particle injection and show that as one moves away from the plane with maximum electric field magnitude, higher final energies can be achieved, though this requires substantially longer times. We also discuss application of our trajectory code to the study of particle acceleration during reconnection in the Earth's magnetotail.
SM23B-07 1330h
A Mechanism for the Loading-Unloading Substorm Cycle Missing in MHD Global Magnetospheric Simulation Models
Loading and consequent unloading of magnetic flux is an essential element of the substorm cycle in Earth's magnetotail. We are unaware of an available global MHD magnetospheric simulation model that includes a loading-unloading cycle in its behavior. Given the central role that MHD models presently play in the development of our understanding of magnetospheric dynamics, and given the present plans for the central role that these models will play in ongoing space weather prediction programs, it is clear that this failure must be corrected. A 2-dimensional numerical driven current-sheet model has been developed that incorporates an idealized current-driven instability with a resistive MHD system. Under steady loading, the model exhibits a global loading-unloading cycle. The specific mechanism for producing the loading-unloading cycle will be discussed. It will be shown that scale-free avalanching of electromagnetic energy through the model, from loading to unloading, is carried by repetitive bursts of localized reconnection. Each burst leads, somewhat later, to a field configuration that is capable of exciting a reconnection burst again. This process repeats itself in an intermittent manner while the total field energy in the system falls. At the end of an unloading interval the total field energy is reduced to well below that necessary to initiate the next unloading event and, thus, a loading-unloading cycle results. It will be shown that, in this model, it is the topology of bursty localized reconnection that is responsible for the appearance of the loading-unloading cycle.
SM23B-08 1330h
Stretching and Collapse of the Nightside Magnetosphere at 9Re
During the fall of 2001, 2002 and 2003, the apsides of Polar spacecraft precessed through the equatorial plane. The plasma sheet at 9 Re was probed near local midnight. Sometimes the magnetic field magnitude in this region was very low, below 5 nT. However, no reconnection was observed at this distance in our investigation. But when the field was so weak, we saw a variety of dynamic signatures associated with magnetic reconnection including stretching and collapse of magnetic field. In this study, we present an event during which the magnetic field at about 9 Re was strongly and rapidly stretched and then suddenly collapsed. During the collapse of magnetic field strength, there was an immediate dipolarization which suggests that the onset of magnetic reconnection occurred very near this region.
SM23B-09 1330h
3-D Particle-in-Cell Simulation of Current sheets with real electron to ion mass ratio: Bifurcation and Magnetic Reconnection
Just a few years ago it was commonly questioned that thin current sheets (CSs) with thickness as small as electron skin depth could form in space plasmas. There is observational evidence now that such thin CSs do form. It appears that as the measurements in space are progressively refined, observation of such thin CSs could be a norm not an exception. Using three-dimensional electromagnetic particle-in-cell (EMPIC) simulations of a thin CS, we recently reported that electrostatic instabilities consisting of Buneman and inflectional shear instabilities trigger an explosive magnetic reconnection event at time scales much faster than that for the electromagnetic instabilities. The simulation reported previously was based on an artificial ion to electron mass ratio M/m=25. We have repeated the simulation with the real mass ratio for H+ ions (M/m=1836) and performed a systematic set of study by varying the CS thickness. We have found that a combination of the electrostatic instabilities, one of which is the inflectional shear instability, plays the crucial role in triggering the explosive magnetic reconnection process. We will report the initial electrostatic instabilities depending on the CS thickness; such instabilities begin the process of current disruption in the central part of the CS. Once the process of current disruption begins, causing shear in the electron flow velocity, inflectional instability sets in leading to the explosion. One significant consequence of the explosion is the bifurcation of the original CS with nearly total current disruption in the central region. Another equally significant consequence is the large acceleration of both electrons and ions in the central region of the CS. Our simulations show that the primary cause of the magnetic reconnection is the fragmentation of the current by the electrostatic instabilities.
SM23B-10 1330h
The Current Sheet Structure in Incompressible Hall Magnetic Reconnection
Theoretical models of magnetic reconnection have been traditionally developed within the framework of magnetohydrodynamics (MHD). However, for reconnection events taking place at the Earth's magnetopause and magnetotail, the importance of kinetic effects such as the Hall current has recently been acknowledged. In this work, we present results from parallel simulations of the incompressible Hall MHD equations in 2½ dimensions, and quantitatively investigate the relevance of the Hall effect by performing a set of simulations with different values of the Hall parameter. We compute the corresponding reconnection rates as a function of time, and explore the spatial structure of the fields in the surroundings of the diffusion region. We also present a family of exact stationary solutions for the Hall MHD equations in 2½ dimensions. We compare the analytically derived spatial structure of the electric current density (and of other physical quantities) against the results obtained from the simulations.
SM23B-11 1330h
Clock Angle Dependence of Magnetic Reconnection at Earth's Subsolar Magnetopause
We use global magnetohydrodynamics (MHD) simulations to investigate the clock angle dependence of magnetic reconnection at Earth's subsolar magnetopause. Under steady solar wind conditions, and when the plasma resistivity is constant, we find that reconnection occurs at the subsolar magnetopause via a flux pileup mechanism for both northward and southward interplanetary magnetic field (IMF) conditions. Magnetic energy accumulates upstream of a Sweet-Parker current sheet to accommodate the sub-Alfvénic magnetosheath flow, and the upstream plasma density decreases (forming a plasma depletion layer) in order to satisfy momentum conservation. We argue, therefore, that in the context of resistive MHD, plasma depletion upstream of the magnetopause is a signature of magnetic flux pileup reconnection which occurs at the subsolar magnetopause under both northward and southward IMF conditions. We discuss the implications of these results for the "component" vs. "antiparallel" merging debate as well as for the interpretation of spacecraft observations of the plasma depletion layer.
SM23B-12 1330h
Hysteresis and the Onset of Fast Magnetic Reconnection
Magnetic reconnection is vastly different depending on the collisionality of the system in question. Collisionless (Hall-mediated) reconnection is fast, with Alfvenic reconnection rates, while collisional (resistive MHD) reconnection is much slower. The transition between the two with varying resistivity may be important for understanding the onset of magnetic reconnection in physical systems such as solar eruptions and laboratory fusion experiments. We investigate the transition using basic theoretical arguments, and provide support using two-fluid numerical simulations. We show that, for intermediate values of the resistivity, the reconnection is bistable, i.e., the system can relax to either of the two configurations depending on the history of the system. We map the hysteresis curve and demonstrate that the transition from collisional to collisionless reconnection is catastrophic. We present a scaling analysis for the resistivities at which transitions occur and compare the predictions to parameters in the solar corona and for a sawtooth crash.
SM23B-13 1330h
Current Structure and Motion of a Northward IMF X-Line
On March 18, 2002, between 14:50 and 15:03 UT, Cluster passed from the tail lobe northward and sunward into the magnetosheath. The IMAGE FUV instrument observed a proton emission, northward of the auroral zone, that endured for a period of hours, including the time of the Cluster crossing. Some researchers mapped the location of cluster during its magnetopause crossing to the location of the ionospheric footprint observed by IMAGE. We argue from Cluster PEACE electron and magnetic field data that the Cluster spacecraft pass to within at least 5 km of an active x-line, the spacecraft actually entering the Hall zone where ions demagnetize. We map the position of the x-line at the points of closest approach to each spacecraft through inversion of a low-order expansion for the magnetic field near the x-line, allowing us to infer the current structure, velocity, orientation, and size of the x-line. The functional fit to the data also gives an estimate of the thickness of the current sheet. The x-line earth and IMF fields form a large angle from the GSE x-direction and while undergoing enhancement in the GSE y-direction, consistent with anti-parallel reconnection, but with a twist in the x-y plane. We propose that the x-line, though it does waver across the spacecraft several times, is largely stable over the times of Cluster observations.
SM23B-14 1330h
Production of Energetic Electrons During Magnetic Reconnection
The production of energetic electrons has been documented in observations of solar flares, reconnection in the Earth's magnetosphere and in laboratory experiments yet the understanding of these widespread observations remains poor. In reconnection with a guide field electron acceleration takes place in acceleration cavities, deep depressions in the electron and ion density with finite parallel electric field that extend through the x-line along one of the magnetic separatrices. The structure of these cavities and electron acceleration has been explored in full particle simulations. The simulations reveal distinct high energy tails, extending well above the electron rest energy. These very energetic electrons are found to arise from multiple encounters with acceleration cavities. The simulations provide evidence that reconnection with a guide field is dominated by the formation of many islands and that electron energization results from multiple accelerations. In this picture the surprising amount of energy going into electrons in comparison with ions is first because of the significant length of the acceleration cavities (large numbers of electrons enter the cavities) and because of their high mobility -- they can rapidly interact with many cavities to reach high energy. Ongoing efforts to understand the powerlaw energy distributions of electrons seen in recent Wind satellite observations will be discussed.