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D. A. Uzdensky (Inst. for Theoretical Physics, UCSB)
In the present paper I analyze the process of formation of thin current structures in the magnetosphere of an accretion disk in response to the field-line twisting. I present a simple analytical model illustrating the asymptotic behavior of a force-free axisymmetric magnetic field above a non-uniformly rotating conducting disk. As has been discovered in previous studies, the differential rotation between the star and the disk leads to the inflation of poloidal field lines. In the present study, I demonstrate the existence of a finite (of order one radian) critical twist angle, beyond which the poloidal field starts inflating very rapidly. If the relative star--disk twist is enhanced only in some localized part of the disk (which may be the case for a Keplerian disk that extends inward significantly closer to the central star than the corotation radius), then, as the twist is increased, the field approaches a partially-open configuration, with some field lines going out to infinity in finite time. Simultaneous with this partial field opening, a thin, radially-extended current layer forms, which, as the opening time is approached, asymptotically becomes infinitesimally thin. Thus I demonstrate how the current sheet formation process may take place in these systems, laying out a way towards reconnection in the disk magnetosphere. This, in turn, presents a very interesting scenario for a quasi-periodic behavior of magnetically-linked star--disk systems with successive cycles of field inflation, opening, and reconnection. Finally, I discuss the similarities between this process and solar coronal mass ejections.
The author(s) of this abstract have provided an email address for comments about the abstract: uzdensky@itp.ucsb.edu