SM53B-0404 1340h
Instability at the leageing edge of a reconnection jet
Magnetic reconnection is the one of the most important phenomena in space plasma physics, which is considered to concern large scale energetic phenomena. Especially, a high-speed plasma jet which occurs with energy release by magnetic reconnection, so called reconnection jet, is considered to play an important role in an energetic process such as plasma particle acceleration in solar flares. In spite of the importance of the reconnection jet, however, little attention has been paid to its dynamics. In this study, we focus on behavior of the reconnection jet and try revealing its property conducting three-dimensional MHD simulations. In our simulations, initial condition of the magnetic field we employed is the one-dimensional Harris current sheet model. Magnetic reconnection is initiated by the time-independent and spatial fixed anomalous resistivity. We add small perturbations, that their wave number vector is directed to initial current direction, to plasma pressure and density, and study how the reconnection jet develops. Under this condition, numerical results show that the leading portion of the reconnection jet is deformed into wavy configuration and grows to show mushroom-like shape on the central current sheet plane, that is, the perturbations are brought up into unstable state with development of the reconnection jet. In the reconnection process, the leading part of a reconnection jet pushes and compresses the standing plasma ahead of it, and a strong pressure and density gradient develops between the reconnection jet and the plasma in front of the jet. Consequently, in this circumstance, we might consider the unstable state at the leading edge of the reconnection jet as the interchange instability. A reconnection jet has more dynamic structure than we have thought. This implies the capability that the structure of the leading part of the reconnection jet influence plasma energetic processes. We discuss this possibility by exhibiting the extremely complicated flow pattern of the jet in the later stage of development of the reconnection jet. And we also mention the influence on the structure of the jet and ambient plasma and field in the more realistic and general situation in which the guide field exists.
SM53B-0405 1340h
Cluster Spacecraft Observations of Antiparallel and Component Magnetic Reconnection Close to the X-line at the High-Latitude Magnetopause
We present Cluster spacecraft observations of two magnetopause crossings close to a magnetic reconnection X-line. The crossings occur during the same event within a time interval of about 10 minutes. We interpret the first crossing as a component reconnection X-line crossing and the second crossing as an antiparallel reconnection X-line crossing. The crossings occur on the duskside high-latitude magnetopause tailward of the cusp under dominant northward interplanetary magnetic field (IMF) B$_{Z}$ and variable IMF B$_{Y}$. That passages are close to the X-line is concluded from the reversals of the ion reconnection jets. A detailed analysis of the two crossings is done using high resolution data from field and particle instruments. We identify and describe different regions close to the X-line e.g. inflow region, outflow region and separatrices. We discuss the results of the observations in terms of differences between component and antiparallel reconnection at the high-latitude magnetopause and compare them with recent simulations of magnetic reconnection.
SM53B-0406 1340h
Laboratory Modeling of Reconnection in the Magnetosphere
The Space Plasma Simulation Chamber (SPSC) at the Naval Research Laboratory is being used for investigation of 3-dimensional magnetic reconnection with ratios of length scale to ion skin depth comparable to those observed in Earth's magnetotail. The SPSC uses a steady-state cylindrical plasma column with a background magnetic field of 200 Gauss. We initiate the reconnection events and plasma dynamics, by pulsing an axial magnetic field rising to about 400 Gauss in about 2 $\mu$s. The experimental observations will be compared to theory and numerical simulations.
SM53B-0407 1340h
Hall Magnetic Reconnection: Guide Field Dependence
Two (2D) and three (3D) dimensional Hall magnetohydrodynamic (MHD) simulations are used to study the dependence of a guide field on magnetic reconnection. The 2D simulations are run until a steady state is achieved for $B_{\rm gf}/B_0$ = 0, 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 where $B_{\rm gf}$ is the guide field and $B_0$ is the reversed field. It is found that the reconnection rate and plasma energization are reduced for increasing guide field strength. This is caused by a ${\bf J} \times {\bf B}$ force associated with Hall currents and the guide field that reduce the inflow and outflow velocities. However, the reconnection rate and plasma energization are only reduced by a factor of 2 for $B_{\rm gf} = 5\,B_0$. Additionally, the quadrupole field associated with Hall reconnection is eliminated for $B_{\rm gf} \simeq B_0/3$. The 3D simulations demonstrate asymmetric development of a reconnection line [similar to the no guide field case reported by Huba and Rudakov (2002)], as well as the development of a (braided) magnetic flux tube. Applications to magnetospheric plasmas are discussed. \medskip \noindent Huba, J.D. and L.I. Rudakov, Phys. Plasmas {\bf 9}, 4435 (2002). \medskip \noindent Research supported by NASA and ONR.
SM53B-0408 1340h
Simulation Study of Rapid Onset of Magnetic Reconnection
The issue of reconnection onset remains a challenge to the plasma physics community. For most physical systems of interest, reconnection does not proceed in steady manner, but rather there are periods of time in which magnetic flux is accumulated, followed by other periods in which the energy is rapidly dissipated. In current sheet geometry, one of the most well-known onset mechanisms is the collisionless tearing instability. However, for systems such as the magnetotail, it appears the tearing instability is stabilized by the magnetic geometry. Furthermore, even if the tearing mode is unstable, fully kinetic 2D simulations indicate the instability saturates at small amplitude and does not generally trigger large-scale reconnection. Recent results from Los Alamos suggest a possible resolution to the onset problem by considering the role of current aligned plasma instabilities such as the lower-hybrid drift instability (LHDI). The nonlinear development of the LHDI leads to a variety of nonlinear modifications which can promote reconnection onset even in complex magnetic configurations such as the Earth's magnetotail. We report results of 2D and 3D kinetic simulations where the fast onset of reconnection in presence of current aligned modes is documented.
SM53B-0409 1340h
Role of Electron Temperature Anisotropy in the Onset of Magnetic Reconnection
Predictions of tearing saturation in a neutral sheet range from small amplitude saturation to explosive growth. Using fully kinetic simulation it is demonstrated that in 2D single island tearing saturates at very small amplitudes due to preferential electron heating in the parallel direction. However, the presence of multiple unstable modes allows the system to get past the stabilization and grow to ion scales. In 3D there are two modes that can affect the nonlinear evolution of tearing. One is the Weibel instability driven by $T_{\perp e} /T_{\parallel e} <1$ and the other is the lower hybrid drift instability (LHDI). In this work the non-local linear Vlasov theory is computed for the perpendicular Weibel instability which is driven by $T_{\perp e} /T_{\parallel e} <1$. The resulting growth rate is much smaller than previous calculations and can at most increase the saturation amplitude up to the singular layer thickness. On the other hand, the LHDI can affect tearing through the generation of $T_{\perp e} /T_{\parallel e} >1$ within the current layer rather than through anomalous resistivity. When $T_{\perp e} /T_{\parallel e} >1$, the parallel Weibel/mirror instabilities merge with tearing to give rise to anisotropic tearing, which becomes the dominant instability with a broad angular spectrum. The levels of $T_{\perp e} /T_{\parallel e} >1$ generated by the LHDI significantly enhance the tearing growth rate and extend the spectrum to very short wavelengths. This expedites the transition to ion scales compared to cases where there is no LHDI.
SM53B-0410 1340h
3D Magnetic reconnection: evolution of x-lines.
In 3D reconnecting systems, the presence and evolution of x-points and x-lines play a crucial role in determining the topological transitions of the magnetic field [1]. Recently the attention has turned towards understanding the evolution of an initial perturbation in 3D where a current sheet is compressed in a localized region. Reconnection develops in the compressed region, forming an x-line. Subsequently, the x-line extends showing an anisotropic behaviour. Based on Hall-MHD theory, the electrons accelerated in the reconnection region and still tied to the field lines, bend the field lines in the current aligned direction leading to a domino effect that propagates the x-line in the electron direction (i.e. against the current) [2]. In contrast with the electron-dominated scenario, in presence of thicker sheet and for ion dominated current sheet, the process of x-line extension is observed to propagate in the opposite direction, following the ion motion. We will present a number of kinetic, hybrid, Hall-less hybrid and Hall-MHD simulations to clarify the issue and to determine the relative importance of the two effects. [1] Lau and Finn, Ap.J., 350, 672 (1990). [2] Huba and Rudakov, PoP, 9, 4435 (2002); Shay et al, GRL, 30, 1345 (2003)
SM53B-0411 1340h
Test particle simulation of MHD shock structure in fast magnetic reconnection
Test fluid particles are traced on the time-developed MHD simulation field data, in order to study MHD shock waves formed in fast magnetic reconnection. In general, when fast magnetic reconnection is steadily caused in the uniform current sheet, high speed plasma jet is generated by a pair of slow shocks and a plasmoid is formed in the downstream. According to our MHD simulations, when the plasmoid continue to propagate in the current sheet, the slow shock may be separated into two regions, i.e. reconnection jet region and plasmoid region. The reconnection jet is generated by the former slow shock. While, the propagation of the plasmoid is driven by the latter slow shock around the plasmoid. In addition, if the reconnection jet is supersonic, the slow shock in the jet region partially collapses, and then, intermediate waves and expansion waves appear in the jet region. The details of these MHD wave structures which are new features for the fast magnetic reconnection on the basis of MHD theory are reported by using test particle simulation technique.
SM53B-0412 1340h
Effect of different initialization methods on reconnection physics: A critical study using Hall MHD, hybrid, Hall-less hybrid and full particle simulations
Two commonly used techniques for performing reconnection simulations have been to either localize the resistivity (in simulations where electron kinetic effects are not included) or impose an external perturbation on the system. In MHD, it has been shown that depending on the spatial extent of resistivity, the nature of the solution changes. When resistivity region occupies a large fraction of simulation domain, one obtains a Sweet-Parker solution whereas in a localized resistivity case, the solution is Petschek-like. We demonstrate that this is due to the fact that in MHD the size of the diffusion region coincides with the region of localized resistivity. In non-MHD limit, this is no longer the case. In order to gain a better understanding of the effect of initialization on the physics of reconnection, we compare and contrast different initialization schemes, including new initialization procedure. We examine the reconnection details and the size of the diffusion region in four types of simulations codes: Hall MHD, hybrid (electron fluid, kinetic ions), Hall-less hybrid, and full particle simulations.
SM53B-0413 1340h
Questioning common wisdom on issues of (i) cause of fast magnetic reconnection, (ii) origin and scale of quadrupole magnetic structure, and (iii) physical reality of Sweet-Parker regime
Our recent work brings to question three generally accepted concepts on reconnection: (1) It has been demonstrated that Hall MHD, hybrid, and full particle simulations yield comparable reconnection rates whereas MHD leads to much smaller rates. The cause of fast reconnection observed in non-MHD models has been attributed to the dispersive properties of whistler mode. We show, using Hall-less hybrid simulations, that fast reconnection is achieved due to ion kinetic effects even when whistlers are non-dispersive. (2) It is well known that Hall currents lead to quadrupole magnetic structure in the reconnection process and the observations of such structures in spacecraft data is attributed to Hall physics. We show that (i) linear kinetic theory predicts the presence of the quadrupole structure, (ii) ion kinetics play a significant role in the generation of the quadrupole structure and the resulting quadrupole structure in a collisionless plasma is always wider than that predicted by Hall MHD. (c) Resistive MHD studies have predicted conditions for the occurrence of Sweet-Parker and Petschek regimes. We show that in a kinetic plasma Sweet-Parker is not a stable configuration.
SM53B-0414 1340h
Questioning common wisdom on magnetic reconnection on issues of (i) anti-parallel vs component merging, (ii) 3D effects, and (iii) role of lower-hybrid drift instability
Our recent work brings to question three generally accepted concepts on reconnection. (1) It has been thought that tearing mode always has its maximum growth at parallel propagation independent of the size of the guide field. We show that this is not the case. Further, we re-examine all previous theories of tearing saturation, both in the presence of absence of a guide field. We find no agreement with any of the previous theories. We present a new theory for nonlinear evolution of tearing that is verified by simulations. (2) There has been a rush to perform 3D full particle simulations and certain conclusions have been drawn regarding the relative role of various instabilities in reconnection onset. We show the issues with such simulations, including some of the conclusions that have been made. (3) Lower-hybrid drift instability (LHDI) has been considered for a long time as contributing to reconnection as a source of anomalous resistivity. As it turns out, LHDI can affect reconnection much stronger by creating a temperature anisotropy in the electrons within the sheet. A new model for reconnection is demonstrated.
SM53B-0415 1340h
A gyrokinetic electron and fully kinetic ion particle simulation model for collisionless plasma dynamics
A novel new kinetic simulation model has been developed to investigate dynamics in collisionless plasmas, such as magnetic reconnection with a finite guide magnetic field. In this model, the electrons are treated as gyrokinetic (GK) particles and ions are treated as fully kinetic (FK) particles. 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 electron-to-ion mass ratio, wave-particle interactions, and off-diagonal components of the electron pressure tensor. The model is particularly suitable for plasma dynamics with wave frequencies lower than the electron gyrofrequency, and for problems in which the wave modes ranging from Alfv\'en waves to lower-hybrid/whistler waves need to be handled on an equal footing. Using this model, the computation power can be significantly improved over that of the existing full-particle codes. The GKe/FKi model also provides important improvements in physics over the full particle models. It can handle physics with realistic electron-to-ion mass ratio and dynamic processes on the global Alfv\'en time/spatial scales. Compared with the hybrid (i.e., FK ion and fluid electron) codes, the advantage of the GKe/FKi code is that it includes the electron kinetic physics, such as wave-particle resonances and finite electron Larmor radius effects. The simulation model has been successfully benchmarked for linear waves in uniform plasmas against analytic dispersion relation. In this talk, the simulation model and its linear benchmark are presented.
SM53B-0416 1340h
Electron Dynamics in the Dissipation Region of Guide-Field Magnetic Reconnection
In the case of a significant guide magnetic field in the reconnection region, electrons are magnetized unless gradient scale lengths become comparable to the local electron Larmor radius. In this situation, electron scattering by the reconnection magnetic field components can occur. We will demonstrate that electron scattering is necessary to avoid unphysical singularities in the electron flow velocity. Furthermore, we will trace particles in self-consistent particle-in-cell simulations to demonstrate the interaction between electron Larmor motion and the steep gradients of the reconnecting magnetic field. We will show that oblique heat-flux tensor components play a critical role in the electron dynamics near the stagnation point. Finally, we will present the results of analytical scaling arguments that show how and when the electron Larmor scale emerges as a natural scale for the inner, electron, diffusion region.
SM53B-0417 1340h
Externally Driven Magnetic Reconnection in the Presence of a Normal Magnetic Field
The possibility that the collisionless tearing instability could explain the sudden onset of magnetic reconnection associated with the expansion phase of substorms in the terrestrial magnetotail has been the subject of extensive investigation. Two-dimensional treatments have generally concluded that such a spontaneous instability is not possible due to the stabilizing compressibility effects associated with the electron response to the tearing perturbation. In the magnetotail, however, reconnection typically occurs in configurations that have evolved under driving by a convection electric field imposed by the solar wind and which may feature structure in the east-west direction. A massively-parallel 3D PIC code is used to study the evolution of a plasma-sheet configuration under the influence of such a driving field. The simulations start from a moderately thick current sheet whose half width is $1.6 c/ \omega_{pi}$ and which is only weakly unstable to the lower hybrid drift and drift kink instabilities. Imposition of the driving electric field leads to thinning of the current sheet as a result of the formation of an embedded electron current structure; the half width is reduced to as little as $\sim 0.4 c/\omega_{pi}$. Once the local $B_z$ field is driven through zero, rapid reconnection ensues, leading to the formation of a large island. No evidence of strong excitation of finite-$k_y$ modes is observed, although the outflow regions do develop an interchange-like structure in the equatorial plane. It appears that the reconnection produced by a continuous external driver in a 2D normal field configuration plasma sheet is basically similar to that in a 1D current sheet.
SM53B-0418 1340h
Multiple-spacecraft Detection of the Diffusion Region of Magnetic Reconnection by Cluster Mission
Magnetic reconnection (MR) is the process by which the magnetic field topology changes, accompanying with release and conversion of magnetic energy into kinetic and thermal energy of plasmas. MR occurs in the small localized diffusion region, its global consequence plays a central role in solar wind-magnetosphere interactions. For more than 30 years MR has been the most challenging frontier of space plasma physics. One of unsolved key questions concerns the physics and spatial scales of the diffusion region. Until recently in situ observations of MR have all been taken outside the diffusion region. Measurements inside the diffusion region would help to test and develop the models and theories of reconnection. Four spacecraft Cluster mission provides a unique opportunity to detect small-scale structures in three dimensions in space, including MR area. In the present paper a comprehensive study on the diffusion region in the magnetotail for a typical MR event (09-15-2001) is carried out with the Cluster multiple satellite data. The following new results from this case study are obtained: 1. The magnetic field topology change across the neutral point is observed by simultaneous measurements of 4 Cluster spacecraft. The related plasma flow reversals and the distinct quadrupolar Hall magnetic field components are clearly identified. 2. The thickness of the tail current sheet in the tail is best fitted as Lz=1.3di (di stands for the ion inertial length) based on Cluster multiple-point high resolution (0.05s) magnetic field measurements; the scale length of the diffusion region in the x-direction is determined to be Lx=3.7-5.0di. 3. The MR separatrix angle E is derived to be 15<E<20. The reconnection rate is estimated to be R=0.09-0.12. 4. Energization of energetic ions and electrons is found during the diffusion region crossing. The lower hybrid waves are detected in the vicinity of the diffusion region.
SM53B-0419 1340h
Strong guide field effect on three-dimensional quick magnetic reconnection triggering
Influence of the guide field on quick magnetic reconnection triggering (QMRT) is investigated by three-dimensional (3-D) full-particle simulations with the mass ratio of Mi/Me = 400. A systematic survey of the QMRT has been made for two initial guide fields of B0y/B0x = 0.75, 1.3 (B0x: asymptotic magnitude of the reconnecting component). QMRT has been found in a null guide-field case of D=0.5 (D: the current sheet half thickness normalized by the ion inertial length) and is quick because reconnection onset is directly coupled to the quickly growing lower-hybrid drift instability (LHDI) [Shinohara et al., 2004]. This work has revealed that the thickness of D = 0.5 with B0y = 0.75 shows no QMRT signature such as current density intensification or significant reconnected magnetic field at the neutral sheet, and crucially required thickness to reach QMRT has been shown at most D = 0.35. For B0y = 1.3, thickness of D = 0.35 turns to be too thick to achieve QMRT, and required thickness to reach QMRT appears for D < 0.3. This work has shown one of the difficulties in the QMRT in the presence of a strong guide field.
SM53B-0420 1340h
Using Exact Particle Paths in the Calculation of Linear Ion-Electron Tearing and Dependence on the Equilibrium State Parameters
We've developed an algorithm to iteratively solve the linearized Vlasov equation for growth rates and first-order potentials of instabilities such as collisionless ion-electron tearing in a neutral sheet. It requires no assumptions about the nature of the equilibrium trajectories and can give exact (numerical) results. In this work we describe the algorithm, and its application to study the behavior of the tearing-mode starting from a Harris equilibrium over a range of the control parameters: the current sheet half-width $w$, $\frac{T_e}{T_i}$, $\frac{M_e}{M_i}$. The method starts with computing and saving several thousand equilibrium particle paths to sample the phase space for a chosen equilibrium state. Then for a given $k$, the coupled equations for $\vec{A_1}$ and $\gamma$ are solved iteratively with $\vec{J_1}$ computed from time-integrals over the saved particle paths. A new $\vec{A_1}$ is found using a Green's function to ensure the boundary- conditions are satisfied, while the correction to $\gamma$ is found from a relation involving integrals of the currents and $\vec{A_1}$. $\phi_1$ is found by assuming quasi-neutrality. The method is stable, converges for any starting values we've tried, requires less than a MB of memory, and 10MB--10GB of disk space for the paths. CPU time on a desktop PC is from $\sim 10$ minutes to an hour for each $k$. The algorithm is similar in spirit to previous work [1], but was developed independently so the implementation is different. Typical dispersion curves, plots of potentials and response functions will be shown. We've found the growth-rates can be fit quite well (correlations of 0.999) by the expression $\gamma(k)= a_x k^b(1/w-k) $ where parameters $a_x, b$ are determined from the peak growth $\gamma_x$ and wavenumber $k_x$. Preliminary results are that for parameter values $\frac{M_e}{M_i}=1$, $\frac{T_e}{T_i}=1$ the dependence of $\gamma_x$, $k_x$ on $w$, over the range $3$--$20$, is given by $k_x=0.527 w^{-1.07}$ and $\gamma_x=0.219 w^{-2.32}$ where $\gamma_x$ is scaled by $\Omega_i$, $k$ and $w$ are scaled by $\rho_i$. For $\frac{M_e}{M_i}= 1/1836$, $\frac{T_e}{T_i}=1/2$ the dependence is $k_x=0.451 w^{-1.11}$ and $\gamma_x=0.0567 w^{-2.379}$. [1] M. Brittnacher, K. B. Quest, and H. Karimabadi, J. Geophys. Res., 100, 3551, (1995); W. Daughton, J. Geophys. Res., 103, 29429, (1998).
SM53B-0421 1340h
Reproducing the bi-polar magnetic signature at the jet leading edge by 3-D reconnection with non-zero guide field
A bi-polar (southward-then-northward) signature of Bz component at the leading edge of earthward jets in the magnetotail has been reported [Slavin et al. 2003]. Here we try to reproduce this magnetic field feature by three-dimensional reconnection (finite extent in the dawn-dusk direction) in the presence of non-zero guide field (dawn-dusk magnetic component). Setting the guide field intensity by referring to Slavin et al. and setting the dawn-dusk extent of the reconnection region to an often quoted value [Angelopoulos et al., 1997], we managed to reproduce the bi-polar signatures by a 3-D (Hall) MHD simulation with anomalous resistivity. Redirecting the argument, the bi-polar signature may be taken as the evidence that the dawn-dusk width of the reconnection region is less than several Earth radii.
SM53B-0422 1340h
Forcing the onset of magnetic reconnection?
The onset of magnetic reconnection in the near magnetotail is one of the most crucial problems in understanding magnetospheric activity. It is now clear, from observations, theory, and computer simulations, that the formation of a thin current sheet, or the thinning of the tail current sheet to a scale of less than the ion inertia length is necessary to cause fast reconnection. However, the way in which this thinning is achieved remains unclear. MHD theory and simulations support a view that external deformations of the magnetotail, imposed by the solar wind, can lead to the formation of a thin current sheet embedded in the near tail plasma sheet. In this presentation we explore the response of a thick current sheet to such external deformations, comparing ideal and resistive MHD results with PIC simulations. The study also includes investigation of the final state from reconnection in a localized test configuration. It represents the first of a series of studies concerning the physics of magnetic reconnection in an approach akin to the ``GEM reconnection challenge.''
SM53B-0423 1340h
Coupling of waves and reconnection in guide field, magnetopause-like current sheets
Using a three-dimensional Vlasov code we investigate the unstable generation of waves and reconnection in thin, collisionless current sheets with guide fields and in magnetopause-like configurations. The amplitude of the saturated waves appears to critically control the moment of initiation of reconnection, it determines the mode of reconnection, whether it is essentially two-dimensional or three-dimensional as well as the reconnection rate. In the guide field case of non-antiparallel reconnection a transition from regular, well structured, to patchy reconnection and percolation is possible, but in the asymmetric magnetopause-like case we recover the formation of islands reminding flux transfer events.
SM53B-0424 1340h
Characteristics of Electrostatic Solitary waves near the diffusion region associated with reconnection: Cluster and Geotail observations
We report on recent observations of Electrostatic solitary waves (ESWs) by the Wideband PlasmaWave Receiver of Cluster and the Wave From Captures of Geotail near the diffusion region at both sides of X-line. By carefully checking the data of particle, field and wave, we studied the temporal and spatial evolution of ESWs, their relative locations with reconnection layer and the relationship with the distribution functions of electrons. By comparison with full particle simulations, the consistence and inconsistence of the oberservations with simulations will be shown, and we will discuss the role of ESWs in collisionless reconnection.
SM53B-0425 1340h
Two-Dimensional Quasi-Steady and Impulsive Reconnection: A Comparative Study Using Particle-in-Cell and Hall MHD Simulations
We present two 2D studies of collisionless magnetic reconnection dynamics obtained from our new fully electromagnetic PIC code (ExPIC), and compare them with results obtained earlier from Hall MHD simulations using the same initial conditions. Our studies include realistic values of me/mi. The first study involves the scaling of the maximum electron outflow velocity from the reconnection region in the GEM challenge problem, which, according to Hall MHD models, scales as the electron Alfven speed. Our PIC simulations show flows that are uniformly smaller than the electron Alfven speed, with deviations that increase in magnitude as the mass ratio reaches its actual physical value. The second study involves forced magnetic reconnection in a plasma sheet driven continuously by inward boundary flows. It is observed in the PIC simulations that the reconnection rate in the linear regime increases algebraically in time, and is followed by a sudden impulsive enhancement in the nonlinear regime, qualitatively similar to that seen in Ma and Bhattacharjee's earlier Hall MHD simulation (1996). However, the current sheet produced is more singular and the impulsiveness greater in the Hall MHD simulation than it is in the PIC simulation. Quantitative comparisons between PIC and Hall MHD simulation results will be given, and kinetic mechanisms that produce differences between Hall MHD and PIC models will be discussed.
SM53B-0426 1340h
Debye Length Structures and Slow Shocks in Earth's Reconnection Layer: Observations and Simulations
Frequently abrupt (7.5ms FWHM), strong (150mV/m) electrostatic structures are detected nearly perpendicular to {\bf B}, near local density depletions in magnetopause current layers. These electrostatic structures have been suggested [Mozer, et al., Geophys. Res. Lett., 31, L15802, doi:10.1029/2004GL020062,2004] to have either electron inertial or electron Debye length scales. Using a broader suite of observational diagnostics, tests of Rankine-Hugoniot conservation laws, and new, full particle simulations, we demonstrate that these structures are unanticipated Debye length substructures ($L \simeq 5\lambda_{De}$ ) adjoining slow switch-off shock transition layers within the magnetopause proper. The electrostatic structures are found just inside the local density minimum and are perpendicular to the coplanarity plane of the adjacent slow shock layer. Electric layers of scale $r\lambda_{De}$ will cause the demagnetization of thermal electrons of temperature $T_e$ , by pushing down the threshold for the electron demagnetization to $\beta_e > Min({{r^2kT_e}\over{m_ec^2}}, 1)$ , thereby assisting topology change and the formation of Petschek slow shocks away from the separator where $\beta_e$ is usually much less than unity.
SM53B-0427 1340h
Nonlinear Alfven Wave Interactions and Three-Dimensional Alfvenic Reconnection
Observations in magnetospheric physics show that reconnection is a three-dimensional dynamical process often occurring in a patchy and sporadic manner and involving the energization of charged particles and the generation of Alfven waves. When fast mode wave packets or wave fronts impinge on a current sheet at the magnetopause or in the magnetotail, the fast mode wave packets can be nonlinearly converted into shear Alfven wave packets. Thus, reconnection is 3D and Alfvenic. Due to the local change of mechanical and/or magnetic stresses during the nonlinear wave mode conversion, the parallel electric fields required by the breakdown of the frozen-in condition are generated. It must be noted that the generalized Ohm's law does not reveal how parallel electric fields are generated. The generation of the parallel electric fields required by reconnection is derived from a complete set of dynamical equations including (i) Newton's law for ions and electrons and (ii) Maxwell's equations including the displacement current. The 3D Alfvenic reconnection process corresponds to a reactive, rather than a resistive, transport process, where the parallel electric field has inductive nature, causing the energization of charged particles. Radiation of kinetic Alfven waves provides the impedance allowing fast reconnection. The Poynting flux of electromagnetic energy flowing into the reconnection region is converted not only into Joule heating, the kinetic energy of plasma flows and accelerated particles, but also into electromagnetic energy associated with the Alfven wave packet.
SM53B-0428 1340h
Quick reconnection triggering in a non-Harris type current sheet
Recently, we proposed a new possible scenario to achieve quick magnetic reconnection triggering (QMRT) even in an ion-scale current sheet. A 3D full kinetic simulation of an ion-scale current sheet with $m_i/m_e=400$ is performed and found that the non-linear evolution of the lower-hybrid-drift instability excited at the edge region of the current sheet plays a crucial role to enable QMRT without significant anomalous transport around the neutral sheet. However, this scenario is limited to the situation under the simple 1D Harris current sheet. Yoon (2004) found that a equilibrium of the non-Harris type current sheet where the modified two stream instability is possibly excited around the neutral sheet. We are carrying out a series of 2D simulation of the Yoon's equilibrium and examine competitive processes of the lower hybrid and modified two stream instabilities. QMRT in the non-Harris current sheet will be discussed in the presentation.
SM53B-0429 1340h
An Electromagnetic Drift Instability In the Lower Hybrid Frequency Range In Reconnection Region
By using a local two-fluid theory, we investigate an electromagnetic instability in the lower hybrid frequency range driven by cross-field current or relative drifts between electrons and ions. The theory self-consistently takes into account local cross-field current and accompanying pressure gradients. It is found that the instability is caused by reactive coupling between backward propagating whistler (fast) waves and forward propagating sound (slow) waves when the relative drifts are large. The unstable waves have mixed polarization with significant electromagnetic components, propagating obliquely to the unperturbed magnetic field. A physical picture of the instability emerges as a result of further simplifications of the model. The primary feedback mechanism is based on reinforcement of initial electron density perturbations by induced Lorentz force due to both field line bending and (de)compression. The resultant waves are qualitatively consistent with the measured electromagnetic fluctuations in reconnecting current sheet in a laboratory plasma. Detailed wave characteristics are also similar to measurements at reconnection sites of the magnetosphere. This work is supported by DOE, NASA, and NSF.
SM53B-0430 1340h
On linear theory of anisotropy instabilities in a current sheet
Our recent computer simulations have shown that the presence of even a small electron anisotropy can strongly influence both the linear and nonlinear development of reconnection. It is therefore of great importance to determine the evolution of such anisotropies and whether or not they are rapidly reduced by short-wavelength instabilities. Here we re-examine the linear stability of a bi-Maxwellian Harris equilibrium. In particular, we derive approximate analytic expressions for bound tearing-like perturbations, and show that the predicted eigenfunctions and growth rates are in good agreement with those of our linearized Vlasov solver. Limits of propagation parallel and perpendicular to the magnetic field are considered. We also examine the stability of the sheet to three distinct, short-wavelength modes driven unstable by an electron temperature anisotropy: the whistler anisotropy instability, the electron mirror instability and the electron Weibel instability, and discuss their relative importance with emphasis on their roles in isotropizing electrons.