Giovanni Montani, Jacopo Petitta
We examine the morphology of magnetic structures in thin plasma accretion discs, generalizing a stationary ideal MHD model to the time-dependent visco-resistive case. Our analysis deals with small scale perturbations to a central dipole-like magnetic field, giving rise to the periodic modulation of magnetic flux surfaces along the radial direction, which corresponds to the formation of a sequence of toroidal current channels. These microstructures exhibit an exponential damping in time because of the non-zero resistivity coefficient, allowing us to define a lifetime of the configuration which mainly depends on the midplane temperature and on the length scale of the structure itself. If we try to identify a suitable phenomenological scenario where this framework could be applied, it turns out that the steady-state assumption adopted in literature is inconsistent with the evolution time-scale of typical stellar accretion disc systems; we can instead aim to describe peculiar transient events whose duration ranges from months or years.
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http://arxiv.org/abs/1209.5227
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