Dissertation Talk: The Impact of Sunspots on Modelling Coronal UV Observations

Existing model studies of O VI 1032 and 1037 A spectral lines from UVCS/SOHO observations often conclude that O5+ ions in coronal streamers have no significant outflow velocity at heights below 3 Rs, and large increases above this height. These observations were modelled with a typical O VI quiet Sun disk spectrum, and the subsequent results were interpreted as an indication of different flows arising from closed and open magnetic field regions within streamers. Given that the O VI solar disk spectrum emitted from sunspots is very different from the spectrum emitted from the quiet Sun or coronal holes, we show how the inclusion of a contribution from sunspots in the incident disk radiation, which excites the coronal O5+ ions, has a significant impact on the intensity and intensity ratio of the coronal O VI spectral lines. Such a result has important implications for the calculation of the outflow velocity of O5+ ions in streamers. Through the analysis of UVCS observations of a solar maximum active region streamer with a large sunspot cluster at its base, we show how the inclusion of a 3% contribution from sunspots in the modelled quiet disk spectrum allows agreement between the observed and modeled intensity ratios at lower heights with non-zero outflow velocities. Such a result cannot be achieved with a standard quiet Sun disk spectrum. Taking the sunspot contribution into account yields O5+ ions with an outflow velocity of about 80 km/s at a height of 3 Rs, increasing linearly to 260 km/s at a height of 8 Rs. These results imply that the presence of sunspots on the solar disk concurrent with streamer observations requires a revision of published results.