| Research Abstract |
A. There are a steady regime and an intermittent regime in the temporal behavior of collisionless reconnection, dependently on the spatial profile of an external driving flow. The steady collisionless reconnection is realized in the case of small input window size, in which the reconnection rate is balanced with the flux input rate at the upstream boundary and the global dynamic process of magnetic reconnection is dominantly controlled by ion dynamics : As the window size increases, the current sheet becomes longer, which is favorable to the excitation of an electron tearing instability. The system evolves into an intermittent regime, in which magnetic islands are frequently generated in the current sheet. Two-scale structure is formed in the current density profile through combined actions of both ion and electron dynamics. Electron pressure tensor associated with electron meandering motion in the central electron current sheet is a main cause of generation of reconnection electric fi
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eld at neutral sheet in two-dimensional steady reconnection, although the evolution of the whole reconnection system is controlled mainly by an external condition.
B. In three-dimensional case plasma instabilities are excited and affect the dynamical behavior of collisionless reconnection. In the presence of an external driving source the lower hybrid drift waves excited in the periphery propagate toward the neutral sheet and thus generate anomalous resistivity in the current sheet. Consequently, the external driving electric field penetrates into the current sheet and triggers magnetic reconnection at the neutral sheet. Magnetic islands are created in the downstream as a result of magnetic reconnection, but they become unstable against the kink instability. After magnetic islands disappear through the downstream boundary, the system relaxes into a quasi-steady state temporarily. An electromagnetic (EM) mode with a low-frequency comparable to ion gyration frequency (drift-kink instability) grows slowly in the current sheet and modifies the spatial structure.
C. Self-generation process of hollow current profile and its influence on tilting instability in field-reversed configurations are investigated by means of two-dimensional and three-dimensional EM particle simulations. The system spontaneously relaxes from a magnetohydrodynamic equilibrium toward a kinetic one, in which the electron current profile becomes hollow. Growth rate of the tilting instability in the case of the hollow current profile and high separatrix beta value is smaller than that in the case of the peaked current profile. When magnetic separatrix shape is much prolate, the unstable region is localized near the edges, and stabilization of the tilt instability becomes effective. Less
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