Magnetically Linked Star‐Disk Systems. II. Effects of Plasma Inertia and Reconnection in the Magnetosphere

Following on the results of the companion paper, we study the processes occurring in a force-free magnetosphere of a magnetically linked star-disk system as it expands owing to the rotation of the disk and, in finite time, approaches an open state. In particular, we use the self-similar solution for the magnetic field evolution to calculate the time-dependent plasma density and velocity profiles. We also perform an asymptotic analysis of the plasma velocity and density at the apex of an expanding field line as it approaches the open state. We use the resulting expressions to investigate how and when the inertial effects in the magnetosphere will intervene to slow down the expansion and to invalidate the force-free assumption. We find that the field opening drastically reduces the density near the apex of the expanding field lines (typically elongated in a direction making a ~60° angle with respect to the rotation axis) while at the same time creating a pronounced density enhancement near the rotation axis. The former effect is conducive to the triggering of microinstabilities in the current layer that forms along the direction of elongation, whereas the latter appears to be related to a mechanism for the formation of an axially outflowing condensation previously identified in axisymmetric numerical simulations. The appearance of a second outflow component along the disk surface in these simulations can also be reproduced by our model. Finally, we discuss the possibility that the expanding field lines reconnect across the current layer before they open up, leading to a quasi-periodic process of field-line expansion and reconnection.

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