Coronal and transition region observations of the sun and cool stars

Abstract

This thesis presents observations of dynamic events in the outer atmosphere of the Sun and other cool stars. The effect of opacity on ratios of strong transition region lines is investigated for the flare dMe star YZ CMi, and the RS CVn binary Capella. Opacity is shown to be both an important effect in these stars' atmospheres and an important diagnostic tool for scale distances. Also presented are observations of a class C5.2 flare on the Sun. The flare's energy release is shown to be largely in the form of mass motions of ejected plasma, and is compared to the energy of particles injected into the lower atmosphere. The largest part of the thesis deals with the investigation of short-period oscillations in the solar corona. Theoretical considerations suggest that short period MHD waves make a significant contribution to coronal heating. We have identified a strong 6 sec oscillations in an active region coronal loop. We use near-simultaneous soho observations to determine the physical parameters of the loop. The oscillation is associated with a fast-mode wave travelling with a velocity of 2000 km s^-1. Although the wave travels a distance greater than λ/4π and therefore meets a necessary criterion for slow dissipation, the dissipation length is well in excess of the loop length. The temporal evolution of the intensity is in agreement with the model of an impulsively generated, fast-mode wave.