Correlation Height of the Source Region of Si II Emission Lines in Coronal Hole Regions

The height of the source region of Si II emission lines characterizes the height of the bottom layer of solar atmosphere's transition region. The correlation analysis of the intensities of ultraviolet spectral lines and the threedimensional structure of magnetic field yielded by force-free extrapolation is a new method for determining the height of ultraviolet emission lines’ source region. It has been found that the height thus obtained is larger than that given by traditional viewpoint. Because the existing numerical analyses with this method are scarce, this result has to be further verified with more observational materials. In this work, this method is applied to the Si II emission lines observed by SOHO/SUMER for the solar surface region beneath the solar coronal hole at southern pole and to the magnetic fields measured by National Solar Observatory/Kitt Peak (NSO/KP) in U.S.A. The correlation height of the source region of Si II emission lines in coronal hole region is approximately 5.0 Mm. This result supports the conclusion that the height of the bottom layer of transition region in coronal hole region is larger than that in quiet regions. Moreover, some new phenomena have been discovered and their causes are probed.