MASS FLOWS IN CORONAL LOOPS

Although static loop models are often used to describe the structure o f coronal loops, it is evident on both observational and theoretical g rounds that mass motions play a crucial role in the physics of the cor ona and transition region. First we review the observations of emissio n-line broadening and wavelength shifts, which imply the presence of r andom motions and systematic downflows in coronal loops. Some discrepa ncies in the observations are discussed. It is argued that velocities due to gas pressure gradients are the most likely explanation for the observed flows. A number of models that have been proposed for these m otions are reviewed. The implications of the various models on observa tions of the corona and transition region by SOHO are discussed.