STATIONARY SUBALFVENIC AND LOW-BETA MHD FLOWS IN SOLAR CORONAL LOOPS AND ARCADES

We present general two-dimensional solutions for low-beta and subalfve nic stationary MHD flow. Our method of solution applies to any type of boundary conditions. It solves for the pertubation of the magnetic co nfiguration brought about by flows and by the development of shock wav es in it. Solutions in cartesian and cylindrical geometries are presen ted to model flows in coronal loops and counter-Evershed flows above s pots. In symmetrical magnetic configurations, when the distribution of pressure at the foot points is symmetrical, the flow is necessarily s ubsonic. Otherwise it can become supersonic at the summit of the magne tic field line and then passes through a shock. Such shocks can be ver y inclined to the magnetic field and the shocked material may form a d ense hot sheet around a cooler core, a situation which seems to be obs erved in cool loops. For asymmetrical magnetic configurations, the flo w accelerates towards the low gas pressure foot point and could be sub sonic or trans-sonic depending on the pressure difference between the foot points. Loops can have a significant density contrast against the ir environment only if their energy flux differs markedly from the bac kground one. In asymmetrical loops one leg can be much less dense than the other and poorly visible. Near spots, the sign of the difference of pressure between the two foot points is such as to drive a reverse Evershed flow towards the spot. Additional effects would be needed to drive a direct Evershed flow.