1 Magnetic field topologies related to reconnections in the near-Earth magnetotail with southward IMF studied by three-dimensional electromagnetic particle code D. Cai, Institute of Information Sciences and Electronics, The University of Tsukuba, Tsukuba 305-8573, Japan K.-I. Nishikawa Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019 USA RECONNECTION BY A SOUTHWARD IMF Short title:
2 Abstract. We report three-dimensional topological magnetic field structures determined by the eigenvalues of magnetic null points (critical points) in the reconnection region near the Earth Magnetotail. The evolution of reconnection and associated particle ejections from the neighbors of null points, (critical points), with a southward IMF are important to understand the substorm onset. Recently, the timing of auroral breakup and its expansion are considered to be related to the kinetic instability, the onset of reconnection, and the associated high-speed ion flow. We have investigated the null points and the associated particle ejection near the reconnection region. We have found that the structure of null points is different from the schematic neutral line created by the reconnection which consists of a straight line from the dawn to the dusk. This comes from the fact that a kinetic (drift-kink) instability takes place prior to the reconnection. This instability creates the structure along the dawn-dusk direction and excites reconnection. Four different types of null (critical) points exist for the magnetic field with the condition, ∇· B = 0. At the null points which have complex eigenvalues, the magnetic field has a spiral structure near the null point. Some paired null points are connected with the magnetic field lines in the sense that they are on the same separation-surfaces. The connected magnetic field lines may correspond to a part of the neutral line. However, the null points are not exactly lined up along the dawn-dusk direction. Instead null points are being created and destroyed. Due to this magnetic (electric) field, electrons (ions) are ejected from the reconnection region in a complex manner. Bursty bulk flows (BBFs) may be generated by these intermittent null points. Further investigation is necessary to understand the temporal and spatial evolution of null points and the associated phenomena better.
3 1. Introduction The interaction of the solar wind with the Earth’s magnetic field gives rise to a number of important and intriguing phenomena, many of which are only partially understood. These include reconnections between the solar wind magnetic field and geomagnetic field lines at the dayside magnetopause (including flux transfer events), reconnection in the near-Earth magnetotail, and associated phenomena such as substorms. The mechanism of reconnection [e.g., Park , 1991] has been studied by MHD and particle simulations. In MHD codes (e.g., Fedder et al. [1995]) the microscopic processes can be represented by statistical (macroscopic) constants such as diffusion coefficients, anomalous resistivity, viscosity, temperature, equation of state, and the adiabatic constant. The near-Earth magnetotail is one of the regions where kinetic effects are critical and particle simulations become very important. Recent evidence suggests that the breakup of the northern edge of the aurora is caused by energetic electrons ejected by the reconnection. The equatorial expansion of the aurora is related to the high-speed ion flow [ Nagai et al. , 1998]. Based on this idea, we have investigated the global simulation of the solar wind-magnetosphere interaction with time-varying interplanetary magnetic fields (IMFs) using a particle code that contains, in principle, the complete particle physics [ Nishikawa 1997, 1998a,b; Nishikawa and Ohtani , 1999]. With a southward IMF due to the thinning of the plasma sheet, an increased current density excites a kinetic (drift-kink) instability [ Nishikawa , 1997, 1998a, and references therein]. This instability kinks the current sheet along the dawn-dusk direction which excites reconnection [ Nishikawa , 1998a]. In order to investigate the complicated evolution of the reconnection, the detailed three-dimensional topological structure of magnetic fields with null points and characteristic magnetic field lines and surfaces are plotted as shown later. Here the characteristic field line is in the direction of one eigenvector that is in a different direction towards the null point from the other two eigenvectors. The characteristic surface is spanned by the other two eigenvectors where both of them have the same direction towards the null point. The characteristic field line and surface are called γ -line and Σ-surface, respectively [ Lau and Finn , 1990; Parnell et al. , 1996; Cai , 1998]. Contrary to the schematic neutral line model [e.g., Fig. 8 in Baker et al. 1996], the several paired or connected null points are formed in the reconnection region which will be shown later. Note that these connected null points are usually discussed as the three-dimensional X-points [ Cowley , 1973; Lau and Finn , 1990; Greene , 1988; B¨ uchner , 1999]. The simulation method is discussed briefly in section 2. The three-dimensional topological magnetic field structures are investigated in section 3. In section 4, discussions on the topological magnetic field before and after the reconnections are described.
4 2. Three-Dimensional Electromagnetic Particle Simulation Model For the simulation of solar wind-magnetosphere interactions, the following boundary conditions were used for the particles [ Buneman et al. , 1992, 1995; Nishikawa et al. , 1995; Nishikawa , 1997, 1998a,b]: (1) Fresh particles representing the incoming solar wind (unmagnetized in our test run) are continuously injected across the y − z plane at x = x min with a thermal velocity plus a bulk velocity in the + x direction; (2) thermal solar particle flux is also injected across the sides of our rectangular computation domain; (3) escaping particles are arrested in a buffer zone, redistributed there more uniformly by making the zone conducting in order to simulate their escape out of the boundary, and finally written off. We use a simple model for the ionosphere where both electrons and ions are reflected by the Earth’s dipole magnetic field. Effects of a conducting ionospheric boundary will be developed in future simulations. The effects of the Earth’s rotation are not included. For the fields, boundary conditions were imposed just outside these zones [ Buneman et al. , 1992, 1995; Nishikawa et al. , 1995; Nishikawa , 1997, 1998a,b]: radiation is prevented from being reflected back inward, following Lindman’s ideas [ Lindman , 1975]. The lowest order Lindman approximation was found adequate: radiation at glancing angles was no problem. However, special attention was given to conditions on the edges of the computational box. In order to bring naturally disparate time scale and space scale closer together in this simulation of phenomena dominated by ion inertia and magnetic field interaction, the natural electron mass was raised to 1/16 of the ion mass and the velocity of light was lowered to twice the incoming solar wind velocity. This means that charge separation and kinetic phenomena are included qualitatively but perhaps not with quantitative accuracy. Likewise, radiation-related phenomena (e.g., whistler modes) are included qualitatively. 3. Simulation Results The first test exploring the solar wind-magnetosphere interaction was run on the CRAY-YMP at NCAR using a modest 105 by 55 by 55 grid and only 200,000 electron-ion pairs [ Buneman et al. , 1992]. We also have reported on our second test run on the CRAY-2 at NCSA using a larger 215 by 95 by 95 grid and about 1,500,000 electron-ion pairs [ Buneman et al. , 1995]. Initially, these fill the entire box uniformly and drift with a velocity v sol = 0 . 5 c in the + x direction, representing the solar wind without an IMF. The electron and ion thermal velocities are v et = ( T e /m e ) 1 / 2 = 0 . 2 c , and
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