Shearing dynamics of magnetic arches and formation of prominences

Abstract

We study the active role played by shear motion of magnetic field, suggested by observations of prominences, in the formation of the prominences. First, we calculate what physical phenomena will result when the foot-points of a two-dimensional dipole potential field undergo shearing. Three cases were calculated. The results show that, as a general rule, a mushroom-shaped flow will be generated, and, under certain conditions, there will be depression of the arch top. To clarify the cause of the mushroom flow, we explored the linear asymptotic solution for the early stage of evolution, and we found the flow amplitude growing exponentially with time. A qualitative explanation for the arch top depression is given: it is due to the interaction of the magnetic buoyance − J x B z and field deformation damping force J z B x . This interaction also tends to open out the magnetic field in the shear region. Depression of the arch top is an important condition for the formation of prominences. The slight diminution of heating of the coronal arch generates thermal instability, and matter from chromospheric evaporation at the foot-points rises along the magnetic tubes and enters the depression on the top, aggravating the thermal instability: matter becomes denser still, and temperature falls further, leading to the eventual formation of a prominence.