The primary difficulty in interpreting coronal optical data is that the corona is optically thin. Hence the emergent radiation is a summation of the contribution from all the regions along the line of sight. There is evidence that the plasma inside a given region, for example a coronal hole, is highly structured (Koutchmy, 1977). In this case assuming that the coronal hole is homogeneous is not a good approximation. The plasma parameters derived from observed quantities, such as polarization brightness, spectral lines and spectral line intensities, might therefore be influenced by the variation of the plasma along the line of sight inside the observed feature,and in addition by emission from adjacent regions with higher densities (quiet regions and/or streamers) (Esser and Withbroe, 1989).
To estimate the importance of the line of sight effect for a given feature, it is necessary to make measurements along many different radial directions both inside the feature and in the adjacent regions. Following the change of the observed quantities along the different radial directions over at least half a solar rotation will make it possible to construct a model of the solar wind inside the coronal hole, and in the adjacent regions, provided the regions are stable. To gain insight into the stability of the features it is necessary to carry out these measurements over two or more solar rotations. Knowledge about time changes of the plasma distribution along the line of sight will increase the confidence in the modeling of the line of sight effect.
Modeling the Solar Wind in Coronal Holes
