Ultraviolet emission from the sun and stars: a comparison of IUE and skylab spectra

We present a comparison of IUE low-resolution SWP spectra of late-type stars and of high-resolution Skylab spectra of spatially-resolved solar regions of various degrees of magnetic activity (quiet areas, plages, flares). We degrade the high-resolution solar spectra to the same resolution of IUE and we analyze the solar and stellar data in exactly the same way. We show that the different levels of chromospheric and transition region emission observed in stars of similar spectral types are paralleled by a similar behaviour displayed by solar regions of different magnetic activity. We show that the spatially-resolved solar data obey the same flux-flux relationships as the stellar data over more than three orders of magnitude, with virtually the same slope and similar scatter. We argue that the similar behaviour of the solar and stellar UV line fluxes, together with the dependence of the stellar fluxes on rotation, are indirect evidence that stellar activity is of magnetic origin and likely results from a dynamo process. Finally, we investigate the contribution of blends, under different activity conditions, to line fluxes derived from IUE low-resolution spectra. We show that this contribution varies depending on the activity level, and may be substantial for the lines of 0 I at 1304 A and He II at 1640 A