SIPHON FLOWS IN CORONAL LOOPS OF RAPIDLY-ROTATING STARS

We study the effect of a steady, isothermal siphon flow on the equilib rium 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 algeb raic 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 ord er 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 i ts 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 Alfve n speed and the outwards centrifugal force due to flow along curved fi eld lines can oppose magnetic tension. Beyond some critical flow speed the loop can only remain in equilibrium by reducing its footpoint sep aration and further increases in the flow speed reduce the loop height . At these high speeds, Coriolis forces also affect the loop height. F or loops whose footpoints are tied, the introduction of a flow, or a c hange in its speed, may lead to a loss of equilibrium if the neighbour ing equilibrium has a different footpoint separation. We find that thi s is most likely for loops whose footpoint separation is greater than that of the ambient field. We estimate this separation to be 60 degree s, based on Doppler images of the surface starspot positions. While th e structure of the coronal field is not directly observed, it is trace d out by the prominences observed at several stellar radii above the s urface. 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.