MAGNETOHYDRODYNAMIC WAVES IN A POTENTIAL CORONAL ARCADE

An important result obtained from solar observations made in coronal e mission lines has been to establish the existence of periodic oscillat ions or waves in the corona. These are detected, although not always s imultaneously, in line intensity, line-width or Doppler velocity with oscillation periods of several minutes. For this reason, we have studi ed the magnetohydrodynamic modes of oscillation of a coronal potential arcade under different boundary conditions, solving the wave equation s for different density profiles. Our results show that only the Alfve n and the fast mode exist in such a structure and that the slow mode i s absent, because of the assumed low-beta limit. The Alfven mode produ ces a continuous spectrum of frequencies with periods which can vary f rom zero to infinity, depending on the assumed density profile. Due to the polarization direction of the plasma motions, these waves could i n principle be detected in front-view observations of coronal arcades at the limb. Concerning the fast mode, we have obtained several diagno stic diagrams using different density profiles and boundary conditions . These diagrams would be useful for comparing the predicted and obser ved periods. However, the observational identification of fast modes c ould be harder since we would need lateral observations of the coronal arcades.