It is suggested that the flux transfer events (FTE's) observed by ISEE satellites can be the result of multiple X‐line reconnection at the dayside magnetopause, which may be caused by the development of a tearing instability. In the presence of the y‐component of the magnetic field (By) in the transition region of the magnetopause, the tearing instability leads to the interconnection of the geomagnetic field lines and the interplanetary field lines, and hence to the occurrence of FTE's. Twisted field lines and field‐aligned currents are formed as a consequence of the tearing instability. The flow direction of the field‐aligned currents depends on By and the results are found to be consistent with satellite observations.
In an earlier paper we suggested that the multiple X line reconnection process may occur at the dayside magnetopause and lead to the occurrence of flux transfer events. In a series of papers we shall examine in detail various aspects of the multiple X line reconnection processes. In the present paper, the first of the series, we attempt to obtain a criterion for the transition from the classical single X line reconnection to the multiple X line reconnection based on a two‐dimensional simulation model. It is found that the development of tearing instability in the diffusion region of the reconnection configuration may lead to the formation of magnetic islands and hence to the occurrence of multiple X line reconnection. The criterion for the tearing instability is found to be l/δ > 7 coth (8/S0.5), where S is the Lundquist number based on the width of the diffusion region, l is the half‐length, and δ the half‐width of the diffusion region. Furthermore, a formula is obtained for the dependence of the ratio l/δ on the system length Lz, the imposed reconnection rate R0, and the resistivity η. The ratio l/δ is found to increase with an increasing system length Lz, an increasing imposed reconnection rate R0, and a decreasing resistivity η. Therefore the multiple X line reconnection process tends to occur when the system length is long, the reconnection rate is high, or the resistivity is small.
We simulate, on the basis of a two‐dimensional incompressible MHD code, the plasma dynamics of a long magnetotail (∼200 RE) under a constant and time‐varying driving force delivered from the solar wind. It is found that under a constant driving force the magnetic fields in the magnetotail tend to reconnect impulsively and the formation of X line and plasmoids occurs intermittently and repeatedly every 2–4 hours. Under a time‐varying driving force the post‐driven phase of a plasmoid formation and the spontaneous reconnection process are also studied. Our results show that magnetic reconnection in the magnetotail during magnetospheric substorms and storms is basically a driven process. A complete sequence of the event, going from the gradual pile‐up of magnetic flux to the formation and the antiearthward convection of the new X line and to the ejection of plasmoids from the antiearthward end of the magnetotail, requires the presence of the driving force. The simulation results may also be useful in unifying the different interpretations (1) of the plasmoid formation and plasma sheet thinning and (2) of the relative importance of magnetotail processes with respect to the direct solar wind effects during substorms.
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