| UV Spectroscopy (UVCS/SoHO) |
Observations by the UltraViolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO, shown above) are used to calculate ion densities, temperatures and outflow velocity in the extended inner corona. These are important as constraints on models of solar wind heating and acceleration. UVCS is a toric grating spectrometer with a 2D CCD detector, therefore the spatial field of view is a straight line of pixels while the second detector dimension is spectral. More details of the instrument can be found on the official UVCS website or from this chapter from my thesis (PDF). To download a pdf reader, click here. The right figure shown below is an image of the corona made in white light near the maximum of solar activity. The 4 thick black lines shows the position of the UVCS field of view for 4 separate consecutive observations. In this case, a high latitude broad streamer is observed at heights from 1.5 to 2.2 Rs (solar radii). UVCS often observes for a long time (many hours) at the same position to improve signal to noise, particularly at higher heights and/or lower density regions. 
The column of 3 plots on the left show various spectral parameters observed by UVCS across the viewing slit at the height of 2.2 Rs. These values are obtained by fitting the observed spectral lines to Gaussians. The intensities and linewidths of the O VI 1032 and 1038 lines are shown, with shaded areas showing calculated uncertainties. The third plot shows the ratio of 1032 to 1038 intensity. In my thesis, I introduce an iterative semi-empirical modeling technique which uses these observed values to calculate some properties of the coronal ions. In simple terms, we can see from the plots above that in comparison to the surrounding plasma, the streamer (bright area protruding from the Sun at position angle ~160) has a higher intensity (density), lower linewidth (temperature) and higher intensity ratio (lower outflow velocity). When UVCS observes several heights at the same latitude consecutively, we can create maps of observed values across large regions of the corona. We can then apply modeling techniques to map ion parameters across the whole region: 
UVCS has now been observing for ten years. There is still a lot of work to be done, particularly in improving aspects of the semi-empirical modeling. One of the biggest challenges is the line of sight. UVCS observes light emitted by ions along an extended line of sight through the optically thin corona. The spectral line is therefore formed from contributions from all structures which happen to lie along the line of sight (but weighted to the plane of sky due mainly to the sharp drop in density with height). It would be desirable therefore to calculate the 3D density structure of the corona, probably from white light observations, to use as a basis for modeling the spectral line formation. |
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