An MHD model for solar coronal plumes.

Solar coronal plumes are modelled by solving the steady, ideal, 2-D, magnetohydrodynamic (MHD) equations and assuming azimuthal symmetry around the plume axis. Since magnetic fields are believed to play an essential role in plume formation and structure, a self-consistent method of linearisation of the MHD equations with respect to the magnetic field has been considered here. This consists of three distinct steps: first a potential field is calculated as a deviation from the radial case due to a flux concentration at the plume base, then the other plasma quantities are worked out by solving a Bernoulli-like equation and finally the modifications to the zeroth order field are found. Free functions of the model are the radial field component at the coronal base, the density at the coronal base and the temperature, which is assumed to be constant along the field lines. This method allows one to reproduce basic features of coronal plumes such as the super-radial expansion close to their base. The results are compared with the observations.</p

Dipped Magnetic Field Configurations Associated with Filaments and Barbs

In this paper, three-dimensional linear force-free field configurations that can be associated with filaments are considered. It is assumed that the field configurations are suitable to represent filaments if they contain magnetic dips. With the photospheric Bur distribution chosen to be an arcade with a dextral/sinistral axial component, it is found that dipped configurations exist only for large values of alpha (where, del x B = alpha B). The dips always lie above the polarity inversion line in the centre of the channel between the flux regions. When the dips are viewed from above to a depth of 1 Mm they resemble closely the shape of filaments viewed in absorption on the solar disk. As the magnitude of alpha increases, the horizontal acid vertical extent of the dips also increases, giving active-region filaments for low values of alpha and quiescient filaments for high values of alpha. Dextral filaments only form for negative values of alpha and sinistral filaments for positive values of alpha. The portion of the field line that is dipped is always of inverse polarity and the magnitude of the field in the dipped region increases with height, both of which are consistent with Leroy, Bommier, and Sahal-Brechot (1983). Overlying the region of dips there are arcades of normal polarity which have the correct left-bearing/right-bearing orientation for dextral/sinistral filaments. When the hypothesis of barbs occurring in dipped field lines is used, barbs that branch out of the main axis and to the right/left for dextral/sinistral filaments can be formed around minority polarity elements on either side of the polarity inversion line. No barbs are found around normal polarity elements. The model reproduces many of the observed features of filament channels, filaments and their barbs.</p