Probing the Acceleration Region of the Solar Wind Using UltraViolet Spectroscopy

Abstract

This thesis is concerned with observations of the inner corona by UVCS/SOHO. An iterative scheme is introduced which exploits observations of the O VI doublet at 1031.9 and 1037.6Å to give density, temperature and outflow velocity of the O5+ ions. The technique is applied to UVCS observations of the solar maximum corona. The results show less contrast between plasma para- meters within a streamer and a neighboring region than that seen in the solar minimum corona. The non-streamer region has different conditions from the streamer, but not conditions typical of a coronal hole, suggesting that some regions of the corona are best defined as quiet regions.
A contribution from sunspots in the incident radiation which excites the coronal O5+ ions has a large impact on the intensity ratio of O VI. The study of a large solar maximum active region streamer shows that a contribution from sunspots in the quiet disk spectrum allows the model to converge to non-zero outflow velocities at low heights.
A parameter study shows that observed differences between O VI 1032 and 1037 linewidths is due to the different balance of radiative and collisional components in each line, a balance sensitive to outflow velocity and temperature anisotropy. The linewidth ratio serves as an additional constraint on O VI modelling.
Time analysis shows that significant oscillations exist in the intensity of the Ly-α line. A prefer- ence toward 7-8 minute oscillations is found. These are interpreted as compressional density waves, the detection of which provides observational impetus for models of coronal heating.
The Solar Probe mission will reach heights as low as 4R⊙. The spectral profiles from a UV spectrometer aboard Solar Probe are modelled. The instrument will observe away from the Sun along a radial direction, and the modelled O VI profiles have separated collisional and radiative components, promising excellent solar wind diagnostics.

Date of Award	2005
Original language	English
Awarding Institution	Aberystwyth University
Supervisor	Shadia Rifai Habbal (Supervisor)