Siphon flows in coronal loops of rapidly-rotating stars.

J. M. Ferreira, M. Jardine

Abstract
We study the effect of a steady, isothermal siphon flow on the equilibrium of a coronal magnetic loop on a rapidly-rotating star. We assume that the loop is thin and embedded in a potential field located on the stellar equator. The flow properties are then found from simple algebraic equations and the loop shape from an integral. The effect depends principally on the Mach number of the flow. If the flow is of the order of the sound speed, it affects the loop equilibrium by altering the pressure distribution along the loop. Flows subsonic at the upstream footpoint decelerate with height and raise the plasma pressure above its static value. The magnetic pressure must fall to compensate and the maximum attainable height falls. Supersonic flows, on the other hand, accelerate with height and lower the plasma pressure below its static value. As the flow speed is increased, the maximum attainable height increases. This effect is amplified when the flow approaches the Alfven speed and the outwards centrifugal force due to flow along curved field lines can oppose magnetic tension. Beyond some critical flow speed the loop can only remain in equilibrium by reducing its footpoint separation and further increases in the flow speed reduce the loop height. At these high speeds, Coriolis forces also affect the loop height. For loops whose footpoints are tied, the introduction of a flow, or a change in its speed, may lead to a loss of equilibrium if the neighbouring equilibrium has a different footpoint separation. We find that this is most likely for loops whose footpoint separation is greater than that of the ambient field. We estimate this separation to be 60 degrees, based on Doppler images of the surface starspot positions. While the structure of the coronal field is not directly observed, it is traced out by the prominences observed at several stellar radii above the surface. Their existence also demonstrates that at some time flows must have occurred along these loops. Our models show that a loop of this height will be unaffected by flows if the surface field strength is of the order of a few thousand Gauss.

Astronomy and Astrophysics
Volume 305, Page 265
January 1996

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