SM51A-01
Dynamic Processes at the High-altitude Cusp
All magnetopause field lines concentrate in the cusp and the particles moving along these lines are bringing information on magnetopause processes wherever they occur. On the other hand, last results of Interball, Polar and Cluster missions have shown that the cusp itself and/or its vicinity is the crucial region for solar wind - magnetosphere coupling and that the principal processes are going on there. In the contribution, we would like to present the high-altitude cusp as a very busy and dynamic region that forms the whole magnetosphere and to stress out the role of IMF direction in the cusp formation. The considerations are based on statistics of the cusp identifications as well as on the analysis of dispersion patterns and plasma flows observed by the Interball-1/Magion-4 satellite pair.
SM51A-02
The Cusp for High and Low Merging Rates
The traditional distinction between a northward and southward IMF cusp is deficient, in that northward IMF cusp
encounters often closely resemble southward IMF encounters. Partly for that reason, "southward" IMF cusps
have been far more often examined than "northward." Recently Newell et al. [2007] showed that the
magnetopause merging rate, dFMP/dt, much better predicts cusp latitude than does Bz (or Bs). Here, we
investigate the extent to which high and low merging rate conditions better separate cusp encounters into
mutually distinct classes. Indeed, high merging rate cusps (those with dFMP/dt > 2*<dFMP/dt>) differ
sharply from low merging rate cusps (those with dFMP/dt < 0.5<dFMP/dt>). High energy (tens of keV) ions,
apparently of magnetospheric origin, do not extend into the high merging rate cusp, but frequently do for low
merging rates. The local time extent of the cusp is 2.33 hours MLT for low merging rate conditions and 3.45 hours
MLT for high merging rate conditions. All high merging rate cusps show clear forward dispersion (declining
energy with increasing latitude) with low-energy ion cutoffs within that dispersion. Low merging rates cusps rarely
show forward merging, but about half show reverse dispersion. "Double" cusps, some with very clear latitudinal
separation, occur in some high merging rate cases, but never for low merging rates. The high merging rates
cases also typically have a "shadow" region of electron only precipitation at roughly polar rain energies and
intensities. For low-merging rate conditions, the region equatorward of the cusp contains ions as well as
electrons, and forms a boundary layer.
http:sd-www.jhuapl.edu/Aurora/PressRelease/docs/2006JA012015.pdf
SM51A-03
Agyrotropy: An Observable Probe of Magnetic Topology
Claims for traversal of magnetic separatrices and electron diffusion region transits are many, though few, if any, are theoretically certain. The reason for this on the edge of the MMS era is that there is no clearly agreed upon observable for this type of identification that has a theoretical basis. All too often data signatures are interpreted as, or circularly defined to be, those of the magnetic separatrices or the electron diffusion region. Examples of such identifications are a burst of electric field noise, parallel electric fields, heat flux layers, or "bursts" of energetic particles. The purpose of this presentation is to discuss an observable, electron agyrotropy, that agilely illuminates the boundaries of magnetic topology. For this study fully kinetic simulations of reconnecting layers have been used with open boundary conditions including guide and anti-parallel geometries and multiple island equilibria. Unlike observations, the actual magnetic topology in the simulations can be determined using the vector potential; the same simulation run can be used to compute the observable electron agyrotropy from the pressure tensor of the PIC particles in the code. In this way the patterns of agyrotropy are demonstrated to "paint" the mathematical separatrices of the vector potential. Even in time dependent geometries the electron agyrotropy provides a clear indication of the location of such layers. Since non-zero agyrotropy reflects an electron distribution that is not cylindrically symmetrical about the magnetic field direction, its detection would be a strong local signature that unusually thin layers are being traversed. Because these layers are structured in space, they can support electric fields from the off diagonal elements of the pressure tensor of the type required to explain collisionless magnetic reconnection. As it is a local measurement, the pattern of agyrotropy can be found by orchestrating simultaneous independent measurements using an array of spacecraft such as Cluster or MMS. If detectors are routinely intercalibrated to the level that agyrotropy is routinely small, interesting experimental discoveries can be made by delineating locales where the agyrotropy is too large to be explained by intercalibration errors. As a word of caution, agyrotropy detection need not imply the detection of magnetic separatrices or even the diffusion region, since non-zero electron agyrotropy is a well known property of the Harris sheet in the presence of a background plasma. However, this agyrotropy is not large by the standards of PIC reconnecting sites, so sorting events by the size of agyrotropy would help to guarantee identifications.
SM51A-04
Reconnection Onset in the Magnetotail: Kinetic Theory and Particle Simulations
The mechanism of the onset of magnetic reconnection in collisionless plasmas in the tails of planetary magnetospheres and similar processes in the solar corona is one of the most fundamental and yet not fully solved problems of space plasma physics. Nonlocal kinetic linear stability analysis of the tearing mode, which is responsible for the onset of spontaneous reconnection, reveals the key role of passing electrons in the mode destabilization. However, the linear theory taking into account passing electrons is cumbersome and difficult to independently verify. Also, it does not preclude the nonlinear destabilization of the tearing mode before it reaches amplitudes sufficient to change the initial tail topology of magnetic field lines and form X-lines. Therefore, the destabilization mechanism must be verified by particle simulations. Modeling the onset with particle codes requires either extremely large simulation boxes or open boundary conditions. We show that in a simulation setup with open boundaries bursts of spontaneous reconnection are detected in the outflow regions of the initial X-point geometry. These bursts strongly resemble the ion tearing instability predicted by Schindler [1974] as a mechanism for magnetospheric substorms in the tail of Earth's magnetosphere. Quenching the onset by replacing open boundary conditions for particles with their reintroduction reveals the key role of passing particles in the tearing destabilization. The theory and simulations are consistent with Geotail statistics on the size of plasmoids and their origin in the tail. They have important implications for Cluster and THEMIS observations.
SM51A-05
Adiabatic Plasma Equilibria With Applications to Magnetic Reconnection
A novel numerical code for computing plasma equilibria with adiabatic (entropy) inputs is introduced. As opposed to prescribing the current or pressure profiles for plasma equilibrium (which leads to the conventional Grad- Shafranov elliptic partial differential equation), prescribing the entropy results in a nonstandard differential equation whose solution by usual iterative techniques is not guaranteed. To solve this equation we have developed a numerical code based on the "alternating dimensions method" (ADM) suggested by Grad et al., [1975]; the code finds adiabatic equilibria by solving iteratively the 2-D equilibrium partial differential equation and a 1-D ODE obtained from averaging it over magnetic flux surfaces. The code is applied to the "Newton Challenge" forced reconnection geometry, and the resulting equilibria with various boundary deformations are compared with snapshots from dynamic MHD simulations of the forced reconnection problem. The method has wide applications in space plasmas, and can be used to compute magnetospheric and solar corona configurations, as well as the "quasi-static" evolution of such equilibria under entropy conservation constraints.
SM51A-06
Effects of Electron Pressure Tensor and Heat Flux on Magnetic Reconnection from PIC and Hybrid Simulations
Thin current sheets lead to rapid magnetic reconnection and conversion of magnetic energy to particle energy. Two-dimensional (2D) simulations performed with different physical models and an initial planar current sheet (the GEM and Newton challenge studies) showed similar fast reconnection rates. In this paper, we discuss in detail simulations of 2D reconnection carried out with a full particle-in-cell (PIC) code and a hybrid (particle ions, massless fluid electrons) code that was part of the Challenge study (Birn et al., GRL, 32, L06105, 2005). In the hybrid code, the electron model contains the full electron pressure tensor in the electron momentum equation to break the frozen-in condition. We compare quantitatively the effects of the electron pressure tensor in the two types of simulations and show both how they evolve in time and where in the thin current sheet the electron off-diagonal pressure tensor terms become important. In addition, we make quantitative comparisons between reconnection rates and flow velocities obtained from the two codes. It is still an open question how best to evolve the pressure tensor and include the effects of electron heat flux in the hybrid model. The evolution equation for the pressure tensor has several terms and the effects of some of these terms on the reconnection dynamics will be examined. In particular, PIC simulations will be used to examine the role of heat flux in reconnection events in the absence of a guide field.
SM51A-07
Kinetic Simulations of Collisionless Reconnection in Pair Plasmas
Despite a great deal of research effort, many basic issues regarding collisionless reconnection remain poorly understood. Although fully kinetic simulations offer a first principles approach, the large separation between electron and ion spatial and temporal scales has severally limited these computational studies. In contrast, the problem of magnetic reconnection in an electron-positron plasma is feasible in both 2D and 3D. It has been suggested that reconnection in pair plasmas may be of interest to certain astrophysical problems. Furthermore, this limit is useful to consider in the effort to understand the essential physics of magnetic reconnection, since there is no scale separation and it can be shown analytically that there are no whistler waves. In this work, we first examine the limit of large-scale 2D systems. Consistent with recent publications, it is demonstrated that reconnection rates remain fast for very large systems but a steady-state is never achieved. Instead the reconnection dynamics proceeds by the repeated formation of secondary islands as the diffusion region expands. From linear Vlasov theory, the unstable eigenmodes in a pair plasma include collisionless tearing and the drift-kink mode. In 3D simulations, the box length in the direction of the current can be selected to either permit or exclude the drift-kink. In the limit where the box length is short enough to exclude the drift-kink mode, reconnection proceeds essentially in the same manner as the 2D simulations. However, when the simulation box is long enough to allow the drift-kink mode, the reconnection dynamics change dramatically with complicated 3D structures resulting from the nonlinear interaction between collisionless tearing and drift-kink. These results may have some relevance to reconnection in hydrogen plasmas since the drift-kink mode is replaced by the ion-ion kink mode at high mass ratio.
SM51A-08
A Dynamic Theory of the Breakdown of the Frozen-in Condition and Plasma Energization at the Current Sheet
In the last few decades, observations at current sheets have mostly been explained by quasi-steady reconnection model, often in a 2-D laminar current configuration, having a X-line and including the non-ideal terms in the generalized Ohm's law. The most accepted theoretical explanation of the generation of parallel electric field in the past years relies mainly on the generalized Ohm's law, which, in fact, yields only a force balance, not the generation of parallel electric field itself. As a result, previous understanding of the dynamics of current sheets has been limited due to the lack of a dynamical theory for the generation of electric fields and poor consideration of 3D mesoscale Alfvenic dynamical processes occurring at magnetopause and tail current sheets. Based on a recently-developed dynamic theory of the generation of electric fields, here we present a dynamical current sheet model, where the propagation, generation and nonlinear interaction of mesoscale Alfvenic disturbances play a crucial role in breaking down the frozen-in condition locally, radiating shear Alfvén waves, forming local magnetic structures, and causing plasma energization at current layers. Our dynamical model not only explains some observational facts explained by the traditional reconnection model but also gives new and very different interpretations and predictions for aspects of physical processes occurring at current sheets that cannot be provided by previous models, or are not given in previous models.
SM51A-09
Global Magnetic Structure and Convection of a Northward IMF Reconnection Event
In this study we explore the global structure of a northward IMF reconnection event observed by the Cluster and IMAGE spacecraft on March 18, 2002. In a previous paper we analyzed magnetic field and electron and ion moments to infer the instantaneous structure and size or a reconnection site local to the null region. Using simulation studies, DMSP data, and Iridium data, we examine the same event not at Cluster spacecraft separation and magnetopause thickness length scales, but at global separator length scales. The simulations posit a separator at the position of the Cluster spacecraft and 3-D nulls near the position of Cluster. We also find that the low-altitude satellite data support sunward ionospheric convection in a region that maps along field lines to both the position of Cluster as well as the magnetopause location of the large-scale separator seen in resistive MHD simulations. From this we are able to support the simulation results and determine a lower limit to the size of the reconnection region. Having established faith in the simulation results, we can establish an upper limit on the size of the reconnection site and we tie the observed convection pattern to the separator structure. We will then compare the simulated separator structure to previous simulations and observations of global anti- parallel and component reconnection.
SM51A-10
Low Energy Particle Observations Associated With a Solar Wind Reconnection Exhaust on July 22, 1999
We examine particle signatures associated with a solar wind reconnection exhaust on July 22, 1999 using experiments on the Wind satellite. The passage of the exhaust past the satellite took only 40 seconds and thus it was essential that the 3D-P instrument on Wind was operating in a ‘burst mode' that provides full three- dimensional particle distributions. In this work we analyze the characteristics of thermal electrons (EESA-L) and protons (PESA-L) before, during and after the exhaust. EESA-L and PESA-L measurements were first converted into units of distribution function. Then the data was transferred to the solar wind rest frame and corrected for the spacecraft potential. The observations show a proton beam coming from the direction of the X-line and increases in the electron fluxes at the exhaust boundaries. These are likely associated with the ion-acoustic and Langmuir waves identified at the exhaust boundary. Despite of the large rotation in the magnetic field direction the solar wind heat flux remained parallel to the magnetic field throughout the event implying no change in the IMF polarity.
SM51A-11
Analysis of perturbations near FTEs in the magnetopause region
The moment and energy transferred by the solar wind to Earth's magnetosphere is controlled mainly by the magnetic reconnection occurring in the vicinity of the subsolar point and the amount of interplanetary magnetic field with south Bz component. A manifestation of the variant reconnection in space and time are Flux Transfer Events (FTEs). We present an analysis of possible wave perturbations found near or within FTE's observed by Polar in the years 2000-2003. The disturbances were characterized using FFT, minimum variance analysis and maximum entropy method. Wave mode identification is performed based on wave properties.
SM51A-12
Comparison of Hall MHD and the non-gyrotropic resistivity model in the global magnetohydrodynamic code BATSRUS
We have recently added Hall MHD to the global magnetohydrodynamic code BATSRUS. Compared to ideal or resistive MHD, Hall MHD provides a more realistic modeling of the interaction of the solar wind with unmagnetized bodies and the reconnection process in magnetospheres. On the other hand accurate modeling of reconnection is rather expensive due to the required resolution and the stiffness of the Hall MHD equations. The non-gyrotropic resistivity model of Kuznetsova et al. is a phenomenological approximation based on the results of particle simulations. It provides an inexpensive but surprisingly accurate model for the reconnection process. We will systematically compare the two approaches within the BATSRUS code.
SM51A-13
Group Velocity Cones in Diverging Magnetic Reconnection Structures
In a typical geometry of magnetic reconnection, the so-called exhaust regions diverge out of the localized diffusion region. The diverging reconnection structure is conical in shape with its apex near the x-line. Such conical regions have been seen in both MHD and kinetic simulations of reconnection as well as in satellite observations. We demonstrate here that depending on the reconnection regime, the cone angle of the exhaust regions found in numerical simulations and as well as in satellite observations compare well with the maximum angles of group- velocity cones associated with the slow MHD mode, or the whistler mode, or the kinetic Alfvén wave (KAW). For each of these three reconnection regimes, we give a quantitative description of the group velocity cones. In the MHD case the cone angle is small and increases almost linearly with the plasma b < 2. In the whistler regime the cone angle is a constant of 19.5 degrees. In the KAW regime the cone angle depends on plasma b as well as on the time scale associated with the diffusion process in the current sheet. The close agreement between the observed and simulated exhaust cone angles and the group-velocity cone angles in all three regimes is highly suggestive that at least the geometrical feature of the exhaust region is determined by the group velocity of the disturbances, which propagate out of the diffusion region.
SM51A-14
Observable features of reconnection and bursty bulk flow associated turbulence in the Earth's plasma sheet
A better knowledge of the statistical features of reconnection and bursty bulk flow associated turbulence is essential for the understanding of multi-scale redistribution of energy in the Earth's plasma sheet. Statistical analysis of multi-scale properties of turbulence facilitates to understand the interaction of the plasma flow with the dipolar magnetic field and to recognize the remote or nearby temporal and spatial characteristics of reconnection. The main emphasis of this presentation is on studying the specific statistical features of reconnection and plasma flow associated magnetic fluctuations. The transiency of driving and the associated non-stationarity of multi-scale fluctuations makes the estimation of statistical features difficult. Additional complexity is introduced by the presence of waves, boundaries or/and large-scale gradients which induce anisotropic fluctuations and increase intermittency. Nevertheless, the scaling ranges associated with MHD turbulent cascade, Hall -MHD and large-scale fluctuations can be identified.