MAGNETOHYDRODYNAMIC WAVES IN A BOUNDED INHOMOGENEOUS-MEDIUM WITH PROMINENCE-CORONA PROPERTIES

Short- and long-period oscillations in quiescent solar prominences hav e been abundantly reported during recent years. In this paper, we inve stigate the magnetoacoustic-gravity modes of vibration of an inhomogen eous medium in which the temperature and density vary smoothly from pr ominence to coronal values. The differential equations of slow, fast, and Alfven modes have been solved numerically, and the main properties of these modes have been studied. The perturbations to the plasma pre ssure, magnetic pressure, magnetic tension, and gravitational forces h ave been computed because these forces are responsible for the differe nt nature of fast and slow MHD modes. It has been found that motions p roduced by slow modes are driven by the horizontal component of the pr essure gradient, while the main driver of fast modes is the magnetic t ension which is practically vertical in most, but not all, cases. As f or the association of different modes with different polarizations of the velocity of oscillation, we have found that slow and Alfven modes present a dominant component in the horizontal plane, in a direction t ransverse to and along the prominence axis, respectively. However, and contrary to what was found in previous works, some fast modes produce both vertical and horizontal motions. When the Doppler shift of a spe ctral line is used, this implies that all three MHD modes could be det ected in a limb prominence and that only the fast mode could be detect ed in a filament located in the disk center. No evidence for hybrid, e xternal, and internal modes has been found, which suggests that this s ubdivision, previously reported in Oliver et al. (1993) and Joarder an d Roberts (1992b), disappears when the temperature suffers a smooth tr ansition from prominence to coronal conditions.