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内容記述 |
Magnetic reconnection process has been investigated over half a century. Still, some<br />problems remain unsolved. The typical one is the onset problem, namely, how fast<br />reconnection is initiated. Another important issue is how huge magnetic energy is<br />converted to plasma energies in a short time scale. <br /> Collisionless reconnection sets in when the current sheet is compressed as thin as<br />ion kinetic scale by the external driving sources. After experiencing some transient<br />phase the system can relax to a steady state in which reconnection rate balances the<br />inflow rate of magnetic flux from the external region, and then a new equilibrium<br />state is realized there through kinetic processes. Accordingly, driven reconnection<br />model is applied to study physical processes of collisionless reconnection in a steady<br />state in this thesis. <br /> The research focus of this thesis is on the electron force balance in the electron<br />dissipation region (EDR) where electron frozen-in condition is broken due to micro-<br />scopic electron kinetic effect, and how energy conversion process takes place there, <br />especially from magnetic energy to that of electrons, in steady collisionless driven<br />reconnection. <br /> The simulation domain is implemented on 2D and 3D rectangular open systems, <br />and a one-dimensional Harris equilibrium is employed as an initial condition. Plasma<br />inflow enters the simulation domain in y direction and the outflow leaves it in x<br />direction in our coordinate system. In order to supply plasma inflow into the system<br />and initiate reconnection, a driving electric field, which is stronger within the input<br />window around x = 0 at initial stage, is imposed in z direction at the upstream<br />boundary. Plasma outflow goes through the downstream boundary at x = ±x<small>b</small> , <br />which is open for plasma particles and electromagnetic fields. In the 3D simulation<br />case, the boundary conditions at z= ± z<small>b</small> along the z axis are periodic. The following<br />summary focuses mainly on the physical processes of steady reconnection based on<br />the 3D simulation results. <br /> It is found that a dissipation region with an enhanced electron current density<br />extends from the reconnection point toward the downstream region, and a long thin<br />EDR with dual structure is formed in a steady state. We show that the force balance<br />in the direction of the reconnection electric field (the z direction) is quite different<br />from that in the upstream direction (the y direction) in the EDR<br /> In the z direction, the Lorentz force balances the electric force just outside the<br />EDR, and then decreases rapidly inside the EDR and vanishes at the reconnection<br />point. The electron inertia term has a local peak around the electron skin depth, but<br />it is canceled out by the electron pressure tensor term. Only pressure tensor term<br />sustains the electric field at the reconnection point, i.e., reconnection electric field.<br /> Because the driving electric field works mainly on magnetized electrons inside<br />ion dissipation region and pushes them into the EDR, strong electrostatic field is<br />generated through the charge separation. Thus, a new force balance state is realized<br />in the upstream direction in the steady state. Although the pressure gradient force<br />balances the Lorentz force in the Harris equilibrium, the electrostatic force balances<br />the Lorentz force in the y direction in the new equilibrium state. <br /> The energy conversion process inside the EDR is also studied intensively. Ac-<br />cording to the force balance in the upstream direction it is expected that magnetic<br />energy is effectively converted to electron kinetic or thermal energy in a short time<br />scale since only electron dynamics is dominant there. <br /> We show that spatial variation of electron kinetic energies in the downstream<br />direction is quite different from that in the upstream direction, which is deeply related<br />to the dual structure of the EDR along the downstream direction. Electron inflow<br />kinetic energy drops quickly as electrons come into the EDR in the upstream direction, <br />while only z component of electron kinetic energy increases there through the electron<br />acceleration by strong reconnection electric field in the EDR. <br /> In the downstream direction, z component of electron kinetic energy decreases as<br />they move away from the reconnection point, and it finally drops to a constant at<br />the edge of the inner structure of the EDR. On the other hand, outflow component<br />of electron kinetic energy increases and reaches its maximum around the edge of<br />the inner structure of the EDR. It is also found that total electron kinetic energy<br />is almost conserved in the inner structure region of the EDR, and thus the electron<br />kinetic enerry is converted from the z component to its outflow component by the<br />Lorentz force. <br /> We have clarified two important physical processes controlled by electron dynamics<br />in steady collisionless driven reconnection, i.e., the relaxation into a new equilibrium<br />state in upstream direction and effective conversion from magnetic energy to electron<br />one in the EDR. Electron dynamics manifests itself in two aspects; the first is that<br />continuous in-place electron inflow enhances strong electrostatic field, and the second<br />is that current density is mainly sustained by z directed electron motion accelerated by<br />reconnection electric field. Thus, the new force balance between electrostatic force and<br />Lorentz force associated with enhanced current density is realized in the steady state. <br />Furthermore, strong electron current density provides a path to convert magnetic<br />energy to electron energy in the EDR through the interaction between particles and<br />electric field. <br /> |
要旨・審査要旨 (315.2 kB)
