SM31D-0659
Electron-magnetohydrodynamic Simulations of Collisionless Reconnection in Thin Current Sheets
Recent simulations of collisionless reconnection and spacecraft observations in the magnetotail and magnetopause have shown the existence of very thin electron current sheets, with scale lengths of the order of a few electron skin depths. The stability of such current sheets is crucial to the understanding of the onset of reconnection. A two-dimensional electron-magnetohydrodynamic (EMHD) model is used to simulate the dynamics on such short space and time scales. The simulations of a thin electron current sheet with anti-parallel magnetic field show the development of whistler-like perturbations, leading to magnetic reconnection. In the EMHD model, reconnection of field lines is facilitated by electron inertia which provides the non-ideal effect in Ohm's law and breaks the frozen-in condition. The whistler mode structure and growth rate are obtained from the numerical solutions of the eigen-mode equations derived from the linearized EMHD model. These agree well with the full simulations, confirming the instability of the whistler-like mode. The linear eigen-mode analysis shows that modes with wavelengths smaller than the equilibrium scale length are stable, while those of the order of or greater than the equilibrium scale length are unstable. The growth rate reduces monotonically with the ratio of equilibrium scale length and electron skin depth, indicating that the instability is driven by finite electron inertia. The simulation shows that the instability initiates the reconnection of the field lines, with the reconnection rate determined by the growth rate of the instability. As the reconnection progresses the out of plane magnetic field develops a quadrupole structure over the reconnection region. The reconnection slows down with the saturation of the instability, and the initial single peak of the electron current sheet develops multiple peaks and its magnitude is reduced, yielding a bifurcated current sheet. Three-dimensional studies are in progress and will be compared with these results.
SM31D-0660
Ballooning stability of near-Earth plasma sheet in presence of magnetospheric convection*
Most investigations of ballooning instabilities in the near-Earth magnetotail have been carried out under the assumption of a magnetostatic magnetosphere. In reality, the magnetosphere exhibits persistent convection in the tail region, which is often turbulent, as evidenced by the presence of bursty bulk flows in both observations and simulations. The convection across magnetic flux surfaces has the effect of limiting the ballooning growth by reducing the interaction time as the perturbation passes through the destabilizing region. Crudely, this yields a window in the pressure gradient required for ballooning instability. In this work, we analyze the ballooning properties of the near-Earth plasma sheet in the presence of magnetospheric convection. The configurations of the near-Earth magnetotail are obtained from global MHD simulations using the OpenGGCM code, using idealized, as well as observed solar wind conditions as input. An approximate local dispersion relation for ballooning instability in the presence of flow is evaluated for the tail region when the configuration attains quasi steady-state conditions. Under simple, idealized solar wind conditions with steady southward IMF, the near-Earth tail region is analyzed and the stability boundaries are determined. Using solar wind data from recent observed substorm events, we developed a sequence of global magnetospheric configurations by means of OpenGGCM simulations, and the role of the ballooning instability in these events is delineated. *Research supported by NSF Grant No. ATM-0542954.
SM31D-0661
Demonstration of X-line retreat by a large scale full particle simulation
The X-line retreat problem is investigated by a large scale two-dimensional full particle simulation. In the case where the obstacle sits close to the X-line, the collision between the reconnection flow and the obstacle may affect on the process right at the X-point. Carrying out a two-dimensional full particle simulation with an asymmetric condition for the reconnection flow, Oka et al. (presented in this meeting) found that the X-line moves away from the obstacle with the speed of ~ 0.1 Alfven speed. To examine more long-time evolution of the X- line retreat, we have performed a simulation of the X-line retreat with a larger simulation box whose scale size is more than ~ 100 ion inertia length. In this presentation, we will demonstrate how the X-line retreat evolves in the macroscopic view and will discuss its relation to the microscopic process in the X-line region.
SM31D-0662
Forced Magnetic Reconnection with Open Boundary Conditions
We present kinetic simulations of collisionless forced magnetic reconnection driven by different models of magnetic flux inflows, with open boundary conditions applied in the outflow directions. We use the implicit Particle-in-Cell code CELESTE3D [1], which retains kinetic effects for both electrons and ions. Different from results of fluid simulations, the reconnection rate is intermittent rather than steady even when the driving inflow is constant. Similar to the pervious discoveries by W. Daughton et al. [2], we find secondary islands grow as the electron diffusion region is elongated over time. For the well-studied Newton Challenge reconnection problem, compared to results with periodic boundary conditions, here we find that with the open boundaries, at the same driving amplitude, fast reconnection starts earlier and reaches a bigger maximum reconnection rate. We will study the dependence of the maximum reconnection rate on the driving amplitude and other factors. References: [1] G. Lapenta, J. U. Brackbill, and P. Ricci, Phys. Plasmas 13, 055904 (2006) [2] W. Daughton, J. Scudder and H. Karimabadi, Phys. Plasmas 13, 072101 (2006)
SM31D-0663
The Role of Electron Heat Flux in Magnetic Reconnection
Particle-in-Cell (PIC) and hybrid simulations (kinetic ions, fluid massless electrons) have been used to investigate magnetic reconnection in 2-D with no guide field. Both simulations are initialized with a Harris sheet equilibrium and the magnetic field is perturbed in order to excite a linear tearing instability. The electron momentum equation is used to calculate the electric field in the hybrid simulation, and the divergence of the full pressure tensor is included in order to break the frozen-in condition at the X-point. In order to evolve the full pressure tensor, we multiply the Vlasov equation by vivj in order to obtain an evolution equation for Pij. However, this scheme requires knowledge of the divergence of the heat flux (Q), which leads to the well known closure problem in plasma fluid theory. The Hybrid code currently solves the full evolution equation of the electron pressure tensor with the divergence of the heat flux term set to zero. In this paper, we compare the results from the hybrid code with results from the PIC code. Generally the results of the two codes agree, consistent with earlier work. However, we find differences in the evolution of the electron heating and also in the location of the heating. For example, in the hybrid code, the electrons mainly heat in the center of the diffusion region, whereas in the PIC code, the electrons heat at the edge of the diffusion region. To show the effects of the electron heat flux, we calculate it directly from a PIC code and compare with the other source terms in \frac{∂ P}{∂ t}. We show that the heat flux term is as important in determining the electric field as all the other source terms, and therefore cannot be neglected in the calculation of the pressure tensor. Distribution functions are then presented which demonstrate kinetically the source of the heat flux. A scheme is then presented for including heat flux in the hybrid code which does not rely on taking the third moment of the Vlasov equation. Preliminary hybrid results are presented which includes heat flux in the calculation of the momentum equation.
SM31D-0664
A GKE/FKI Particle Simulation of Current Sheet Instabilities With Finite Guide Field and Comparison with Theory
The instability of current sheet under finite guide field (By) is investigated using our new gyrokinetic (GK) electron and fully kinetic (FK) ion particle simulation code, which resolves wave modes ranging from Alfvén waves to lower-hybrid/whistler waves. Compared with full-particle codes, the rapid electron cyclotron motion is removed in this model, wave-particle interactions preliminary simulation of Harris sheet is carried out in the 2D yz plane, with z being along the current sheet normal and anti-parallel Bx perpendicular to the simulation plane. The simulation has been performed with both a linearized (δ f) GKe/FKi code and the nonlinear code, for By/Bx=0.1-10. Under very small By, our results show LHDI modes at the current sheet edge propagating mainly in the y direction, as seen in previous simulations. As By increases, k\perp and diamagnetic drift direction shift away from the current flow direction y. The LHDI modes become weaker while high frequency modes stronger. In the cases with a large By, the LHDI modes evolve to a globally propagating instability, and multiple ion cyclotron modes are excited. The simulations are performed for both purely electrostatic cases as well as electromagnetic cases. The mode properties obtained from the linear simulation are compared with those from theoretical calculations based on an electrostatic model. A more complete 3D simulation is planned to investigate the new physics introduced by the large guide field.
SM31D-0665
Reconnection initiated in the magnetosheath: Comparing hybrid simulations and Cluster observations
Recent observations by the Cluster spacecraft have established the occurrence of magnetic reconnection in the magnetosheath. It was shown that reconnection occurs due to the interaction of a thick tangential discontinuity, with northward IMF at its leading edge and southward IMF at the trailing edge, with the bow shock. This interaction involves compression of the discontinuity by the bow shock to a thickness of about 10 ion skin depth. In this study, we use 2.5-dimensional global hybrid (kinetic ions, fluid electrons) simulations to show that the interaction of a thick tangential/rotational discontinuity with the bow shock leads to its compression and eventual onset of reconnection in the magnetosheath. The observed and simulated thickness of the discontinuity as well as reconnection rates and plasma and field signatures compare favorably. The reconnection in the simulation is quasi-steady with minimal level of island formation. These results are also compared to simulations with thinner discontinuities and different internal structures (e.g. polarization) which show highly time-dependent behavior with magnetic islands forming and evolving on varying scales. Implication of these results regarding steady state and time dependent reconnection is discussed. In addition, global scale consequences of this interaction and transformation of the magnetopause from northward to southward IMF is discussed.
SM31D-0666
Dayside Magnetic Reconnection under the Conditions of Dipole Tilt and Northward IMF
We have performed a high-resolution and time-dependent three-dimensional MHD simulation of interaction between the solar wind and the Earth's magnetosphere in order to study dayside magnetic reconnection when the dipole tilt and northward IMF are simultaneously included in the whole volume of the simulation box. In the present study, for the case of positive dipole tilt, three different characteristics of the reconnection region appear at the dayside magnetopause during the northward IMF conditions; 0° ≤ θ ≤ 60°, 60° ≤ θ ≤ 120° and 120° ≤ θ ≤ 180°, where θ is a counterclockwise angle in a view from the sun starting from the Y-axis. In the northern and southern hemispheres, both the electric field in the perpendicular direction to convection and the resistive electric field are largest in the region where we would expect occurrence of antiparallel reconnection. Moreover the electric field in the northern hemisphere is almost 3 times larger than that in the southern hemisphere for 45°. However the parallel component of the current in the southern hemisphere is larger than that in the northern hemisphere when the dipole tilt is positive. The feature is different from that for southward IMF condition [Park et al., 2006]. In addition, the perpendicular vorticity has large value in the reconnection regions.