Magnetohydrodynamic waves in coronal magnetostatic arcades

The solar corona is structured by the magnetic field, which has the shape o f loops and arcades. These structures remain stable from days to weeks, and there is some evidence, such as sporadic or periodic brightenings detected in UV and soft X-rays, pointing to the existence of periodic oscillations or waves in the corona. Recently Oliver et al. studied, both analytically a nd numerically, the modes of oscillation of potential and nonpotential magn etic arcades; here we investigate the fast mode properties in a fully magne tostatic arcade (Zweibel & Hundhausen) in which the gravity force is presen t. The results show that in this equilibrium the fast and slow modes are es sentially decoupled and that the spatial structure of the perturbed velocit y field is determined by the behavior of the Alfven speed with height. An a rcade in which the Alfven speed increases with height is characterized by v ertically evanescent fast mode solutions. These are the kind of modes that could be more easily detected since their energy is confined within the sol ar corona. On the other hand, an equilibrium in which the Alfven speed decr eases with height has solutions whose energy tends to escape toward large h eights, making difficult the detection of these modes. Also, it is found th at the frequencies of fast modes in a nonpotential equilibrium are consider ably reduced with respect to the potential case.