SM31A-1680
Hybrid simulations of magnetic reconnection in asymmetric current sheets : the problem of the initial kinetic equilibrium
As one of the main phenomena allowing energy entry in the magnetosphere, magnetic reconnection has been heavily studied via in situ observations and theoretical investigations, numerical simulations playing an important role. For many years now, Harris' kinetic equilibrium (1962) has been extensively used to initiate current sheets simulations. If it can be considered as an approximation at the magnetotail, it can no longer be valid for magnetopause reconnection where asymmetries due to magnetosheath and magnetospheric plasma populations cannot be ignored. So far, kinetic simulations performed in such an asymmetric configuration have been initiated with fluid equilibrium conditions where the plasma is locally Maxwellian. A certain amount of time is therefore required for the distribution function to thermalize, involving pressure waves emission and slight modification of the first moments, during which magnetic reconnection is triggered via tearing mode instablity, magnetic perturbation or resistivity spot. In this work, we propose a study of magnetic reconnection within an magnetopause-like configuration with a hybrid kinetic equilibrium allowing density and temperature gradients. The Vlasov equilibrium is found via considerations on the constants of motion for the ion species within an antiparallel magnetic tangential discontinuity.
SM31A-1681
Fast Plasma Instrument for MMS: Data Compression Simulation Results
Magnetospheric Multiscale (MMS) mission will study small-scale reconnection structures and their rapid motions from closely spaced platforms using instruments capable of high angular, energy, and time resolution measurements. To meet these requirements, the Fast Plasma Instrument (FPI) consists of eight (8) identical half top-hat electron sensors and eight (8) identical ion sensors and an Instrument Data Processing Unit (IDPU). The sensors (electron or ion) are grouped into pairs whose 6° × 180° fields-of-view (FOV) are set 90° apart. Each sensor is equipped with electrostatic aperture steering to allow the sensor to scan a 45° × 180° fan about the its nominal viewing (0° deflection) direction. Each pair of sensors, known as the Dual Electron Spectrometer (DES) and the Dual Ion Spectrometer (DIS), occupies a quadrant on the MMS spacecraft and the combination of the eight electron/ion sensors, employing aperture steering, image the full-sky every 30-ms (electrons) and 150-ms (ions), respectively. To probe the diffusion regions of reconnection, the highest temporal/spatial resolution mode of FPI results in the DES complement of a given spacecraft generating 6.5-Mb s-1 of electron data while the DIS generates 1.1-Mb s-1 of ion data yielding an FPI total data rate of 7.6-Mb s-1. The FPI electron/ion data is collected by the IDPU then transmitted to the Central Data Instrument Processor (CIDP) on the spacecraft for science interest ranking. Only data sequences that contain the greatest amount of temporal/spatial structure will be intelligently down-linked by the spacecraft. Currently, the FPI data rate allocation to the CIDP is 1.5-Mb s-1. Consequently, the FPI-IDPU must employ data/image compression to meet this CIDP telemetry allocation. Here, we present simulations of the CCSDS 122.0-B-1 algorithm- based compression of the FPI-DES electron data. Compression analysis is based upon a seed of re- processed Cluster/PEACE electron measurements. Topics to be discussed include: (i) Review of compression algorithm; (ii) Data quality; (iii) Data formatting/organization; (iv) Compression optimization; and (v) Implications for data/matrix pruning. We conclude with a presentation of the base-lined FPI data compression approach.
SM31A-1682
Non-Steady Reconnection In Stratified Atmospheres: Applications To The Solar Environment
The magnetic reconnection process has a key-role in plasma physics. Though being over-studied for decades, there are several aspects of this fundamental mechanism that remain unclear. Nevertheless, magnetic field line reconnection is often invoked to explain in part or completely the complex evolution of several astrophysical and heliospheric structures. In particular, fast reconnection mechanisms are required in a huge variety of phenomena, for instance in solar explosive processes where erupting prominences, c.m.e.s' onset and flares' emissions are different actors in a unique scenario where reconnection plays a critical role. A strong interest is focused on the conditions for the transition between different reconnection regimes: in particular, the evolution of a current-sheet can enter a stage where the same process repeats over and over leading to a turbulent phase with the development of small scale structures interacting each other, or to a possible scale-free fractal reconnection process. This work presents a numerical analysis of such a picture in the MHD approximation where an initial current-sheet equilibrium configuration is considered and its dynamics is followed throughout the formation and the interaction of reconnection jets. The evolution of the system is observed to be driven from the initial condition through several states to a final chaotic configuration. The model is hence applied to different structures observed in the solar atmosphere.
SM31A-1683
Ion Cyclotron Waves in the VASIMR
The Variable Specific Impulse Magnetoplasma Rocket is an electric propulsion system under development at Ad Astra Rocket Company that utilizes several processes of ion acceleration and heating that occur in the Birkeland currents of an auroral arc system. Among these processes are parallel electric field acceleration, lower hybrid resonance heating, and ion cyclotron resonance heating. The VASIMR is capable of laboratory simulation of electromagnetic ion cyclotron wave heating during a single pass of the plasma through the resonance region. The plasma is generated by a helicon discharge of about 25 kW then passes through an RF booster stage that shoots left hand polarized slow mode waves from the high field side of the resonance. This paper will focus on the upgrades to the VX-200 test model over the last year. After summarizing the VX- 50 and VX-100 results, the new data from the VX-200 model will be presented. Lastly, the changes to the VASIMR experiment due to Ad Astra Rocket Company's new facility in Webster, Texas will also be discussed, including the possibility of collaborative experiments at the new facility.
SM31A-1684
Hysteresis of the terrestrial magnetosphere as the southward IMF varies : a 3D PIC global simulation analysis
Three-dimensional PIC (Particle-In-Cell) simulations are used to analyze the dynamics of the terrestrial magnetosphere under the impact of a varying IMF. The distance between the dayside magnetopause subsolar point and the "Earth" center, Rmp , is measured, as the intensity of southward IMF |Bz | is slowly varying. Based on the field topology theory, one analyzes the variation of Rmp as a reference index of the dynamics of this interaction, when IMF |Bz | successively increases and decreases to its original value. Striking results are: (i) as the IMF |Bz | increases above a critical value, the variation of Rmp suddenly changes (so called "bifurcation" process in field topology); (ii) the subsolar point recovers its original location by following different paths as the IMF |Bz | value successively increases (from zero to a maximum fixed value) and decreases (from this maximum to zero) passing through some critical values. These different paths are the signature of an "hysteresis" effect, and are characteristic of the so-called "subcritical-type bifurcation".This hysteresis signature indicates that dissipation processes take place via an energy transfer from the solar wind to the magnetosphere by some irreversible way, which leads to a drastic change in the magnetospheric field topology. New other signatures will be described to evidence this hysteresis effect, which is interpreted herein as a consequence of the magnetic reconnection taking place at the dayside magnetopause.
SM31A-1685
On The Development Of Spectrum Of Streaming Instabilities In Strongly Magnetized Plasma
Using linear kinetic theory and 3D PIC simulations, we explore the late time development of electron streaming instabilities driven during magnetic reconnection with a strong guide field. Some interesting result are discovered by both theory and simulations. We perform a 3D simulation starting with buneman instability in the strongly magnetized plasma. Electrons holes quickly formed. We find that the parallel Buneman instability can progress into two distinct forms of turbulence at late time: electron two-stream instability sustaining electron holes in the parallel direction, and lower hybrid instability driving turbulence in the perpendicular direction; The parallel phase speed of both instabilities increase with time enabling the energy of high velocity electrons to be redistributed; Both theory and simulations show that the lower hybrid turbulence only develops after strong parallel electron heating by the buneman instability.
SM31A-1686
On the scaling of the subsolar magnetopause parallel electric field: Resistive MHD theory
Recently, Cassak and Shay [Phys. Plasmas, 14, 2007] applied two-dimensional MHD conservation laws to derive an analytic expression for the reconnection rate at Earth's dayside magnetopause. Borovsky [JGR, in press, 2008] used the Cassak-Shay formula as a starting point to derive a first principles solar wind-magnetosphere coupling function. Based on 3D MHD numerical experiments (using the BATSRUS global MHD code), Borovsky argued that dayside reconnection is not driven by the solar wind. Rather, the reconnection rate is determined by the local plasma densities and magnetic field magnitudes on the two sides of the magnetopause current sheet, consistent with the Cassak-Shay formula. However, due to the three-dimensional nature of the subsolar magnetopause flow, the relevance of the Cassak-Shay formula to dayside magnetopause reconnection is questionable. In this talk, we revisit the problem of determining the subsolar magnetopause reconnection electric field in the context of the resistive MHD equations. We derive an analytic expression for the parallel electric field at Earth's subsolar magnetopause, demonstrating that neither the popular Sonnerup-Gonzalez expression [Sonnerup, B. U. O., JGR, 79, 1974; Gonzalez, W. D. and F. S. Mozer, JGR, 79, 1974] nor the Cassak-Shay formula is relevant in 3D resistive MHD. In particular, our expression predicts that if the plasma resistivity is constant, the subsolar parallel electric field should scale like the fourth root of the resistivity. In contrast, the Cassak-Shay formula predicts a square root scaling when the resistivity is constant. In principle, THEMIS could be used to address this question by determining the amount of magnetic flux pileup upstream of the magnetopause current sheet under various conditions.
SM31A-1687
A New Kinetic Based Fluid Approach to Collisionless Magnetic Reconnection
A previous analysis of data from the Wind spacecraft has revealed that electrons were trapped in the
reconnection region encountered in the deep magnetotail [1]. Here we present a new analytical theory for the
electron distribution function, which successfully accounts for the anisotropic electron features observed by
Wind. The anisotropy is related to extensive trapping of electrons in parallel electric fields. The electron
density and equations of state for the parallel and perpendicular electron pressures are obtained by taking
moments of the derived electron distribution function. The resulting model is successfully benchmarked
against a kinetic simulation. Thus, the new approach allows for extensive fluid modeling of impulsive
reconnection while retaining the important kinetic effects of the electrons.
[1] J.~Egedal, M.~ Oieroset, W.~Fox, and R.~P.~Lin., Phys.~Rev.~Lett., 94, 025006 (2005).
This work is supported by
DOE Junior Faculty Award DE-F602-06ER54878 and
NSF/DOE Award PHY-0613734
SM31A-1688
A Statistical Model of Reconnecting Magnetic Islands in a Current Layer
The formation of secondary magnetic islands in electron current layers is well-documented by 2-D full particle simulations of magnetic reconnection and by observational data in the magnetotail and magnetopause. However, the dynamics of these magnetic islands in very large-scale current layers, such as those in the corona or the magnetopause, are not yet well-understood. To retain the small-scale physics of particle-in- cell codes in these large-scale systems, a statistical model is developed. The island distribution is characterized by two independent parameters: the flux ψ contained in the island and the area A enclosed by it. An evolution equation for this distribution function is derived, based on rules (verified by PIC simulations) which govern the small-scale generation of secondary islands, the rates of island growth, and island merging. The steady state distribution of islands reveals that the largest islands, which result from multiple mergers, contain distinctly reduced magnetic fields compared with the ambient value. The distribution of islands is used to predict observable quantities such as the maximum island size, typical magnetic field strengths, and power laws. These predictions are compared with observations from magnetospheric satellites such as THEMIS.
SM31A-1689
Experimental Studies of Electrostatic Turbulence, Energetic Electrons, and Electron Phase-space Holes During Magnetic Reconnection on the Versatile Toroidal Facility
We report experimental observations of electrostatic turbulence during spontaneous magnetic reconnection
events on the VTF experiment [1]. Reconnection is studied in a regime with strong guide field
(Bguide/Brec ~ 10) and high density (ωpe/ωce ~ 10). Electrostatic
fluctuations are observed by small, high-bandwidth, impedance-matched Langmuir (RF) probes using high-
speed (up to 16~GHz) digital oscilloscopes. Fluctuations correlate in time with reconnection events and
include broadband fluctuations extending from ωLH to ωce, wave packets near
ωce, and large-amplitude, positive potential spikes, identified as electron phase space holes [2].
With arrays of RF probes we have measured the phase velocity of the fluctuations, and in the case of the
holes, the speed and shape of the propagating structures. For the holes, the parallel and perpendicular
sizes are roughly equal, approximately 1-2 mm (50-100 λD, or 5-10 ρe). Finally, we will
present studies of the relationship between the fluctuations, reconnection electric fields, and creation of
energetic particles during the reconnection process. The latter are studied with a novel electron energy
analyzer which integrates 7 grid/collector pairs into a small, 1.5~cm area. We are in the process of correlating
cause and effect between the reconnection process and the formation of turbulence, holes, and electric
fields (reconnection rate).
This work was funded in part by DOE Grant DE-FG02-06ER54878 and CMPD Grant DEFC02-04ER54786.
[1] J. Egedal, et al., PRL 98 015003 (2007).
[2] M.V. Goldman, D. Newman, and A. Mangeney, PRL 99, 145002 (2007).
SM31A-1690
Nonlinear dependence of anomalous ion-acoustic resistivity on electron drift velocity
Collisionless magnetic reconnection requires the violation of ideal MHD by various kinetic-scale effects. Recent research has highlighted the potential importance of wave-particle interactions by showing that Vlasov simulations of unstable ion-acoustic waves predict an anomalous resistivity that can be significantly higher in the nonlinear regime than the quasi-linear estimate. Here, we investigate the dependence on the initial electron drift velocity of the current driven ion-acoustic instability and its resulting anomalous resistivity. We examine the properties of statistical ensembles of 10 Vlasov simulations with real mass ratio for a range of drift velocities and for electron to ion temperature ratios 0.9, 1 and 2, relevant to both solar and magnetospheric physics. We show that the ion-acoustic anomalous resistivity depends nonlinearly on the electron drift velocity for the low temperature ratios examined, in contrast to the linear dependence predicted by theory and commonly assumed in models of magnetic reconnection. Specifically we find that a) anomalous resistivity is a power law function of the electron drift velocity (vde / θem), approximately with exponent β ~ 8-10, and b) anomalous resistivity is a power law function of the normalized drift velocity (vde-vcrit)/θem, approximately with exponent α ~ 2.5-6. An anomalous resistivity model consistent with our results could be important for simulations of magnetic reconnection in astrophysical plasmas.
SM31A-1691
Retreat and reformation of X-line during quasi-continuous reconnection at the high- latitude magnetopause
It is known that magnetic reconnection can persist for hours, e.g., at Earth's magnetopause. However, it remains unknown in what manner reconnection can be continuous in regions where flow tangential to the current sheet is significant on one side of the current sheet, such as at the high- latitude or flank magnetopause; whether a single X-line sits still and remains active for an extended period of time, or an X-line retreats with the tangential flow and then a new X-line reforms at or near the original reconnection site. Here we present observations on 19-20 November 2006 by the Cluster spacecraft that were skimming the high-latitude dusk-flank magnetopause, which are consistent with more than one reconnection X-line present on the tailward side of the polar cusp under northward interplanetary magnetic field. Evidence of quasi-continuous reconnection over 16 hours exists in the form of Alfvenic acceleration of magnetosheath ions found almost always when either of the satellites traversed the magnetopause current layer. The data indicate that for most of the time, a dominant X-line was between Cluster and the cusp (sunward of Cluster), but ion velocity distributions consisting of two magnetosheath (shocked-solar-wind) populations demonstrate that for part of the time, more than one X-line existed. Further, the motion of reconnected field lines shows that some X-line(s) retreated tailward and was anti-sunward of Cluster. It is inferred that following the X-line retreat, another X-line reformed sunward of Cluster, leading to multiple X- lines.
SM31A-1692
Reconnection at the Magnetopause of Jupiter and Saturn: Evidence From FTEs
Flux transfer events are interconnected flux tubes seen at the terrestrial magnetopause that contribute to the transfer of magnetic flux from the dayside magnetopause to the geomagnetic tail. At Earth these events occur principally when the IMF is southward and their occurrence rate decreases with increasing solar wind Mach number. Because they have a size and flux content that can be estimated these events can provide a lower limit of the magnetic flux transferred from the dayside to the tail in a planetary magnetosphere. We know that FTEs were observable in the Pioneer data at the jovian magnetopause but have not yet been reported for the saturnian magnetopause. In the paper we report on a survey of the rate of occurrence of FTEs at the two planets and use that survey to determine a lower limit to the rate of magnetic reconnection at Jupiter and Saturn.
SM31A-1693
Cluster Observations of Non-steady Reconnection on the Magnetopause Under the Influence of Mirror Modes in the Magnetosheath
We report on two Cluster magnetopause crossing events under the following circumstances: the crossings took place near the subsolar point, the interplanetary magnetic field had a southward Z component, and mirror modes were present in the magnetosheath. In both events fast plasma flows are observed several times at the magnetopause. The typical duration of the flow observations is one minute, which agrees with the mirror mode timescale observed in the sheath. We interpret the flows as being outflow jets of non-steady reconnection, and the coincidence of timescales suggests that mirror modes may contribute to making magnetopause reconnection non-steady. Furthermore, in one of the two events the outflow jet direction also shows reversals of the velocity component in the antiparallel field direction, suggesting that in this case reconnection is non-steady not only temporally, but also spatially. We also present preliminary results from hybrid simulations on the effect of mirror modes on magnetopause reconnection.
SM31A-1694
Magnetic Reconnection Across A Dusty Harris-like Current Sheet
Simulations of magnetic reconnection in a dusty plasma were conducted across a Harris-like current sheet. These simulations were conducted with the DENISIS 4-fluid dusty magnetoplasma code. The simulations begin with a Harris-like current sheet which goes through a ballistic relaxation process. This allows the system to achieve suitable pressure profiles. Multiple resistive models were employed. These included global resistivity, a locally enhanced resistivity, parameter dependent dynamic resistivity, and explicit use of collision frequencies in the induction equation. Magnetic topology, mass fluxes, and magnetic fluxes were examined throughout the simulations. These simulations indicate that while the assumptions of the Sweet- Parker and Petschek models of reconnection may be correct on small timescales, the overall process of reconnection across a current sheet can be highly dynamic. This includes reformation of current sheets on smaller scales after reconnection and multiple reconnection events across a dynamic sheet.
SM31A-1695
Onset and non-steady state evolutions of collisionless magnetic reconnection
We study the evolutions of collisionless magnetic reconnection with full kinetic particle-in-cell simulations. Two scenarios of forced reconnection, one for steady state and the other for non steady state, are setup with open boundary conditions. The fast reconnection onset is discovered to be an nonlinear electron self- reinforcing process. Accelerated by the reconnection electric field (Ey), the small portion of energetic electrons in the vicinity of the X point enhance the reconnection rate through the off-diagonal terms of the electron pressure tensor. For the overall evolution of reconnection, there are different stages: the onset or early growing stage when the Ey structure is a monopole at the X point, the bipolar stage when the Ey structure is bipolar and the outer electron diffusion region (EDR) is being elongated over time, and the possible final steady state stage when Ey is uniform in the reconnection plane. We find the change of reconnection rate is not empowered or dependent on the length of the EDR. During the early growing stage, the EDR is elongated while the reconnection rate is growing. During the later stage, the reconnection rate may significantly decrease but the length of the inner EDR is largely stable. The results indicate that reconnection is not controlled by the downstream physics, but rather by the availability of plasma inflows from upstream. The Hall current induced by the quadrupole magnetic field is discovered to play an important role in the elongation of the EDR. The electron super-Alfvénic outflow jet structure could be elongated when the structure of Ey is bipolar, and remains stable during steady state.
SM31A-1696
Comparisons of Simulations With Space Observations of Asymmetric Magnetic Field Reconnection on Ion and Electron Spatial Scales
Sub-solar magnetopause measurements of asymmetric magnetic field reconnection on the Polar and THEMIS satellites are compared with a PIC simulation having open boundary conditions and a guide field to show the excellent agreement between the two data sets on both ion and electron spatial scales. Examples involving parallel electric fields, violations of the frozen-in condition, and regions where electron scale physics are important will be discussed.
SM31A-1697
Non-linear coupling between magnetic reconnection and MHD-scale Kelvin-Helmholtz instability: Full particle simulations
We have performed two-dimensional full particle (PIC) simulations to investigate the non-linear coupling between magnetic reconnection (RX) and MHD-sale Kelvin-Helmholtz instability (KHI). Recently, the results from two-dimensional two-fluid (ion and electron) simulations have revealed that when the magnetic field components along the k-vector of KHI are anti-parallel across the velocity shear layer (anti-parallel case), RX driven by the flow of KHI at the non-linear stage strengthens the flow of KHI and a highly rolled-up KH vortex is formed. The non-linear coupling between RX and KHI can lead to the plasma mixing across the shear layer. In the two-fluid system, however, since the motion of particles is not considered the mixing process of plasmas cannot be accurately understood. The kinetic effects may also deliver additional dynamics to the coupling between RX and KHI. Thus, in this study we use full particle PIC simulations of MHD-scale KHI for the anti-parallel case. As a result, we found that both ions and electrons are mixed in the KH vortex on a time- scale of the growth of the vortex. Furthermore, we found that the RX process in the vortex produces the strong acceleration of electrons parallel and anti-parallel to the out-of-plane magnetic field. These plasma mixing and acceleration tend to appear around the edge of the vortex, because the edge of the vortex is filled with the reconnected field lines. This is the first study that investigates the non-linear coupling between magnetic reconnection and MHD-scale KHI using kinetic particle simulations.
SM31A-1698
Dayside Reconnection and Plasma Transport in the LFM Code
In this paper we will present a new series of observations of magnetic reconnection at the magnetopause using the Lyon-Fedder-Mobarry (LFM) three dimensional MHD simulation code. In previous work we were able to identify the reconnection rate, average magnetic shear angle, and distribution of reconnected flux at the dayside magnetopause as a function of the angle of the solar wind magnetic field. Our latest work explores the locations where plasma flows beyond the dayside magnetopause and how these regions are connected to those where reconnection is a dominant process. We also present an update on the progress of incorporating Hall physics into the simulation code.
SM31A-1699
Characteristic Features of Asymmetric Magnetic Reconnection: Particle-in-Cell Simulations
Asymmetric magnetic field reconnection refers to reconnection in conditions where either or both the magnitude of the reconnecting magnetic field and the plasma density are unequal on the two sides of the current layer. A prime example is reconnection at the dayside magnetopause. The modifications to the usual symmetric description of reconnection on ion and electron spatial scales indroduced by these asymmetries are investigated using 2D particle-in-cell simulations. The starting configuration is a fluid equilibrium with overall pressure balance containing a density drop of typically a factor of ten and a magnetic field magnitude jump of a factor of three. Simulations are performed both without a driving convection electric field (which should be relevant to sub-solar observations outside of the average magnetopause position) and with such a driving field at the magnetopause boundary (which should model the case where the magnetopause is driven closer to the Earth). The usual Hall physics signatures of a bipolar normal electric field and quadrupolar out-of-plane magnetic field are replaced by an enhanced normal electric field on the low density side and a bipolar magnetic field on the high density side. The dependence of these structures on the strength of the guide field will be discussed as well as results concerning the spatial extent of regions associated with the breaking of the electron frozen-in condition and the existence of parallel electric fields.
SM31A-1700
Relativistic Electron Acceleration Associated with Magnetic Reconnection
Recent in-situ observations from the Wind and Cluster spacecraft have provided detailed information regarding the energetic electron spectrum near reconnection X lines. The Wind [Oieroset et al., 2002] data showed power law spectra with the hardest spectrum and highest electron flux near the center of the diffusion region. The Cluster data found that the electron energy spectrum became harder in the downstream region [Imada et al., 2007] and found a direct correlation between the enhanced electron fluxes and density spikes and bipolar Bz signatures [Chen et al., 2008], suggesting a direct link to the presence of magnetic islands. The mechanisms for the production of relativistic electrons are investigated using 2D particle-in-cell simulations which are initiated with a 10-island configuration with and without an initial guide field. Significant energization occurs only when the number of islands is reduced to two or three with wavelength satisfying kx L < 0.2. The energization proceeds in two distinct stages. In the first stage, a small number of electrons are accelerated to relativistic energies at the X line by the inductive electric field. The second stage is associated with the final coalescence into one large island and produces a considerably larger number of relativistic electrons. With a guide field, this stage is dominated by the formation of elongated density cavities along one pair of separatrices and continued direct acceleration at the X line. Without the guide field, the direct X line acceleration becomes unimportant, and the acceleration is localized in the flux pile-up regions [Hoshino et al., 2001] and results from the curvature drift interacting with the localized inductive electric field. Thus the presence or absence of a guide field may explain the different in-situ observations. In either case, some 15-20% of the decrease in magnetic field energy is transferred to the electrons, with a few percent appearing in relativistic (> 150 keV) electrons.
SM31A-1701
Demagnetization of Thermal Ions and Electrons in Current Layers in the Magnetosphere
Experimental assays of the demagnetization of thermal electrons and ions at the bowshock and throughout the magnetosphere are demonstrated with a new single spacecraft technique that uses plasma, magnetic and electric field data. Routine demagnetization of the ions at the bowshock and Chapman-Ferraro layer are demonstrated while electrons remain magnetized for the strongest bowshocks of the Polar era. Electron signatures at the magnetopause occasionally hint at the presence of electron scale current channels and demagnetization, although strongly demagnetized thermal electrons are rare in a manner that we statistically quantify using a 3 year Polar data set. The demagnetized electron events are preferentially found in typical magnetopause locales and a restricted region of magnetic local time.
SM31A-1702
Non-Linear Whistler Waves in Magnetic Reconnection
SM31A-1703
Magnetic reconnection in two-dimensional MHD turbulence
The nonlinear dynamics of magnetic reconnection in broadband turbulence is investigated through direct numerical simulations of decaying, compressible and incompressible, two-dimensional magnetohydrodynamics (MHD). Complex processes of reconnection between magnetic islands (or magnetic vortices), of different size and field strengths, locally occur. Matching classical turbulence analysis with the Sweet-Parker theory, the main statistical features of these multi-scale reconnection events are identified. Locally, magnetic reconnection can be described through a steady state, asymmetric, Sweet-Parker model, in which parameters controlling reconnection rate are determined by the turbulence. Research supported in part by NASA (NNX07AR48G, NNG06GD47G, NNX08AI47G) and NSF (ATM0752135 ATM 0539995).
SM31A-1704
A Theory of Whistler Regime Reconnection Structure and Rate Based on Group Velocity Cones:Comparison With Simulations
Based on group velocity of the whistler mode, we theoretically predict the range of reconnection rate (R) in the whistler regime depending on the half width (w) of the current sheet (CS) in the diffusion region. The maximum value of R is Rmax =0.35 and it occurs for w ~0.45di, where di is the ion inertial length. A quantitative comparison between the theory and simulations reveals that the steady-state reconnection rates seen in recent simulations with large sizes of simulated plasmas are the result of the group-velocity cones of whistler waves emanating from thin electron current layer (ECL) in the electron diffusion region.
SM31A-1705
Reconnection Dynamics in Semi-Collisional Plasmas
The physics of magnetic reconnection is examined in the semi-collisional regime using fully kinetic particle-in- cell (PIC) simulations in which binary collisions are included by a Monte-Carlo technique [T. Takizuka and H. Abe, J. Comput. Phys., v. 25, p. 205, 1977]. This approach describes a full Fokker-Planck collision operator and thus permits a first-principles study of the transition between collisionless and collisional reconnection. For sufficiently collisional regimes, this approach recovers the well-known Sweet- Parker scaling from resistive MHD. In relatively small systems with neutral sheet geometry, a clear transition to fast reconnection is observed when the thickness of the Sweet-Parker current layer falls below the ion inertial length. In larger systems, the highly elongated Sweet-Parker current layers are unstable to a tearing-like instability leading to an abrupt transition to faster reconnection. Although certain aspects of these results are consistent with expectations from two-fluid theory, there are significant differences in the structure and time dependence within the weakly collisional regime. In particular, there is a basic tendency for the diffusion region to form an elongated current sheet for all collisionality regimes. For large-scale systems, these elongated current layers are unstable to plasmoid formation leading to a time-dependent reconnection process in both the collisionless and semi-collisional parameter regimes.
SM31A-1706
Reconnection Between Magnetic Flux Ropes in a Laboratory Plasma
The behavior and interaction of magnetic flux ropes have long been a topic of interest to solar and space plasma physicists, but few laboratory experiments have been performed as it is necessary to have a relatively collisionless plasma and currents with significant self-generated fields. Movable lanthanum hexaboride (LaB6) cathodes have been developed to study the 3D dynamics of flux ropes in the Large Plasma Device (LaPD). Each 2.5~cm LaB6 cathode can produce current densities of 5-20 A/cm2 and Δ B/B ~ 10%. The background plasma (n ~ 2 × 1012 cm-3, d ~ 60 cm, L ~ 18 m, and τrep= 1 s) is produced with a DC discharge using a pulsed barium oxide-coated cathode. The two current channels are created by biasing the LaB6 cathodes with respect to a grid anode at the opposite end of the chamber. They are emitted parallel to each other and the background B field. J × B forces cause the currents to move across the field and interact. Reconnection has been observed at multiple locations between the two currents. The role of reconnection in these interactions will be investigated through detailed volume measurments of the magnetic field and current density (65000 time steps at 20000 spatial points). Data from Langmuir probes and microwave horn antennas will also be presented.
SM31A-1707
Dynamics of two colliding flux ropes in a three dimensional experiment
Coalescence of flux ropes is an important process in both nature and laboratory plasmas. How axial magnetic field affecting this process is a critical issue for understanding the magnetic reconnection in three dimensions. We experimentally demonstrate for the first time that the axial magnetic field may impede the merging process and thus prevent the occurring of reconnection. The interesting finding is an out-of-plane quadrupole-like magnetic structure, which is formed by diffusion of axial magnetic fields and diamagnetic fields.
SM31A-1708
Magnetic reconnection in the heliosphere: a global picture
We use interplanetary plasma data from Voyagers 1 and 2 in conjunction with ACE and Wind to study signatures of magnetic reconnection in the solar wind over the applicable duration of each mission. We study the evolution of current sheets and pressure-balanced exhaust structures with distance from the sun as we seek to determine the significance, if any, of fast magnetic reconnection to solar wind dynamics on a global scale. Thermal Alfvénic jets are shown to originate at fluid-like discontinuities in the solar wind at distances of up to 34 AU. We will present strong evidence of slow-mode shock acceleration in merging streams and exceptionally high reconnection rates in ICMEs at large distances (r≫1 AU) from the sun. These observations support the hypothesis that persistently driven reconnection in the solar wind develops towards a Petchek-like equilibrium on Alfvénic timescales.
SM31A-1709
The Diffusion Region of Asymmetric Magnetic Reconnection
The reconnection on the dayside magnetopause can have very disparate inflow plasma conditions on either
side of the x-line, with high density/low magnetic field on the magnetosheath side and low density/high
magnetic field on the magnetospheric side. Learning the properties of this asymmetric reconnection will
greatly improve our understanding of the dayside magnetosphere as well as help the implementation of the
Magnetospheric Multiscale Mission (MMS). Although a simulation analysis of diffusion region structures has
been performed for resistive MHD simulations (Cassak and Shay, 2007), our understanding in the
collisionless case is much less complete. We perform Hall MHD and kinetic PIC simulations of asymmetric
reconnection, and present the resultant diffusion region structures. It is found that, in accordance with basic
scaling theory, the stagnation point and x-line are not co-located. Most interesting (Cassak and Shay, 2007),
however, is that the electron and ion stagnation points are also not co-located due to their disparate masses.
The Hall MHD and PIC simulations are compared to shed light on the underlying physics responsible for the
diffusion region structures. The simulations are also compared with the symmetric case to determine if the
non-steady character of reconnection has any dependence on asymmetries.
P. A. Cassak and M. A. Shay,
Physics of Plasmas, 14, 102114, 2007.
SM31A-1710
Driven Magnetic Reconnection in Semi-Collisional Regimes: Fully Kinetic Simulations with Boundary Conditions Relevant to MRX
Recent 2D collisionless kinetic simulations of driven magnetic reconnection with boundary conditions relevant to the Magnetic Reconnection eXperiment (MRX) have successfully reproduced overall dynamics and the global (i.e. the ion-scale) geometry of the reconnection region (Dorfman et al., to appear in Physics of Plasmas, 2008). At the same time, the structure of the electron diffusion layer was different between the experiment (Ji et al.,GRL, v. 35, L13106 , 2008) and the simulations. This discrepancy may indicate that the actual reconnection mechanism in the experiment differs from that in the 2D collisionless simulations. The two leading possibilities to explain this inconsistency are binary collisions and 3D effects such as current aligned instabilities. We present the initial results of a systematic analysis of the role of binary collisions on driven magnetic reconnection in MRX-relevant geometry. This study utilizes ultra-high performance 3D kinetic simulation code VPIC (K. J. Bowers et al. Phys. Plasmas, v. 15, p.~055703, 2008), which treats Coulomb collisions using a well-known Monte-Carlo technique (T. Takizuka and H. Abe, J. Comput. Phys., v. 25, p. 205, 1977). In 2D simulations, the inclusion of binary collisions is observed to modify the structure of the electron diffusion region compared to the collisionless case. In particular, for the levels of collisionality comparable to the experimental ones, the layer may become significantly broader at the location of maximum electron outflow (the position where its width is measured experimentally). The implications of these results for interpreting the experimental observations are discussed.
SM31A-1711
Laboratory Study of Solar Flare Dynamics in MRX
We will present an experimental study of the dynamics of half-toroidal plasma arcs relevant to solar coronal activities utilizing the existing MRX facility [1,2]. A set of electrodes are inserted in MRX to generate a variety of plasma flux loops which contain variable toroidal guide field. Three dimensional evolution of the simulated flares is monitored by an ultra fast framing camera. The time evolution of discharges with Argon, Helium and Hydrogen with currents of 5-15 kA show the stability condition for a line-tied plasma flux loop similar to those on the solar surface. With the initial data it is shown that the q value, which describes the rotational transform of field lines, is the key for characterizing the global stability. Our experimental results will contribute to the understanding of evolution of magnetic topology of the solar flare including concepts such as current sheets, stability of current carrying flares, and line-tying, which are vitally important for understanding the Solar/Heliospheric and Interplanetary Environment. [1] M. Yamada, H. Ji, S. Hsu, T. Carter, R. Kulsrud, N. Bretz, F. Jobes, Y. Ono, and F. Perkins. Study of driven magnetic reconnection in a laboratory plasma. Phys. Plasmas, 4:1936, 1997. [2] V.S. Titov and P. D'emoulin. Basic topology of twisted magnetic configurations in solar flares. Astron.and Astrophys., 351:707, 1999.
SM31A-1712
X-line sliding in an asymmetric ion flow
We have carried out two-dimensional full-particle simulations of an asymmetric current sheet (like the Earth's magnetopause) with a reconnected layer. When the ion flows cross obliquely the X-line, the X-line starts to slide with its speed comparable to the ion flow at the X-line. If (1) Stagnation point of the ion flows are decoupled from the X-line, and (2) Those flows form an oblique pattern about the X-line, the sufficient condition of the X-line sliding is concluded. The presence of an anti-parallel shear flow along the current sheet, or an out-of-plane guide field is found to settle that condition.
SM31A-1713
Slip-Squashing Factors as a Measure of Three-Dimensional Magnetic Reconnection
A general method for describing magnetic reconnection in arbitrary three-dimensional magnetic
configurations is proposed. The method is based on the field-line mapping technique previously used only for
the analysis of magnetic structure at a given time. This technique is extended here so as to analyze the
evolution of magnetic structure.
Such a generalization is made with the help of new dimensionless quantities called "slip-squashing factors".
Their large values define the surfaces that border the reconnected or to-be-reconnected magnetic flux tubes
for a given period of time during the magnetic evolution. The proposed method is universal, since it assumes
only that the time sequence of the evolving magnetic field and the tangential boundary flows are known.
The application of the method is illustrated for simple examples, one of which was considered previously by
Hesse and coworkers in the framework of the general magnetic reconnection theory. The examples help to
compare these two approaches; they reveal also that, just as for magnetic null points, hyperbolic and cusp
minimum points of a magnetic field may serve as favorable sites for magnetic reconnection.
The new method admits a straightforward numerical implementation and provides a powerful tool for the
diagnostics of magnetic reconnection in numerical models of solar-flare-like phenomena in space and
laboratory plasmas.
Research partially supported by NASA and NSF.
http://arxiv.org/abs/0807.2892
SM31A-1714
Fast Plasma Instrument for MMS: Simulation Results
Magnetospheric Multiscale (MMS) mission will study small-scale reconnection structures and their rapid motions from closely spaced platforms using instruments capable of high angular, energy, and time resolution measurements. The Dual Electron Spectrometer (DES) of the Fast Plasma Instrument (FPI) for MMS meets these demanding requirements by acquiring the electron velocity distribution functions (VDF's) for the full sky with high-resolution angular measurements every 30 ms. This will provide unprecedented access to electron scale dynamics within the reconnection diffusion region. The DES consists of eight half-top-hat energy analyzers. Each analyzer has a 6° × 180° field of view (FOV) with a single pixel resolution of 6° × 11.25°. Full-sky coverage is achieved by electrostatically stepping the FOV of each of the eight sensors through four discrete deflection look directions. Data compression and burst memory management will provide approximately 30 minutes of high time resolution data during each orbit of the four MMS spacecraft. Each spacecraft will intelligently downlink the data sequences that contain the greatest amount of temporal structure. Here we present the results of a simulation of the DES analyzer measurements, data compression and decompression, as well as ground- based analysis using as a seed re-processed Cluster/PEACE electron measurements. The Cluster/PEACE electron measurements have been re-processed through virtual DES analyzers with their proper geometrical, energy, and timing scale factors and re-mapped via interpolation to the DES angular and energy phase- space sampling measurements. The results of the simulated DES measurements are analyzed and the full moments of the simulated VDF's are compared with those obtained from the Cluster/PEACE spectrometer using a standard quadrature moment, a newly implemented spectral spherical harmonic method, and a singular value decomposition method. Our preliminary moment calculations show a remarkable agreement within the uncertainties of the measurements, with the results obtained by the Cluster/PEACE electron spectrometers. The data analyzed was selected because it represented a potential reconnection event as currently published.
SM31A-1715
Study on obliquities of reconnection X-line on the dayside Magnetopause
Reconnection on the dayside magnetopause is widely accepted to be a major access of mass and energy change. Still, the divarication of anti-parallel and component reconnection remains unsettled and attracts great interest. In this study, we surveyed the fast flow events observed by Cluster and DSP/TC1 between January and April, 2004 and 2005, trying to figure out the global configuration of dayside magnetopause reconnection. It is obtained from the study that the X-line obliquity strongly depends on the IMF clock angle. The relationship can be approximately represented as follows: [θ=-0.34(AIMF-180)] with θ represents the X-line obliquity and AIMF the IMF clock angle, all in degrees. Noteworthily, we didn't include the effect of X componsent or magnetitude of the IMF. The differences of magnetosphere magnetic field at different locations was not considered either.
SM31A-1716
Two-Fluid MHD Simulation of Relativistic Magnetic Reconnection
Magnetic reconnection is a driver of explosive events in space- and astrophysical plasmas. It also plays a role in high-energy astrophysical settings such as pulsar winds and magnetar flares, where plasmas are composed of relativistic electrons and positrons. Kinetic-scale properties of relativistic magnetic reconnection in these environments have been recently investigated by PIC simulations. In addition, a good relativistic MHD model is needed in order to model further large-scale problems. However, due to the extreme complexity of the equation system, the number of MHD studies has been quite limited. We have recently developed a relativistic multi-fluid code to study large-scale reconnection problems. We consider a symmetric two-fluid system (positron fluid and electron fluid) and introduce an inter-species friction term to represent an effective resistivity. With a spatially localized resistivity profile, we successfully simulated magnetic reconnection in a relativistic two-fluid system. In this paper we will present the nonlinear, large- scale, long-term evolutions, and parameter dependencies (e.g. reconnection rate vs inflow energy composition) of the relativistic MHD reconnection system.