Correlation heights of the sources of solar ultraviolet emission lines in a quiet-Sun region

The radiance and Doppler-velocity maps of the emission lines of Si II, C IV, and Ne VIII obtained in a quiet region of the Sun by SUMER ( Solar Ultraviolet Measurements of Emitted Radiation) are correlated with the vertical component, B-z, of the magnetic field vector as extrapolated, by means of a force-free field model, from the photospheric magnetic field measured by MDI (Michelson Doppler Imager). It is found that, with increasing vertical height, each of the correlation coefficients initially increases to a maximum value before it decreases again. The height corresponding to this maximum is called the correlation height. For the data sets selected from a quiet-Sun region, the correlation heights of Si II and C IV are near 2 Mm, and for Ne VIII near 4 Mm. At their correlation heights, the averaged square root of the radiance of the emission lines of Si II and C IV, considered as a proxy of the plasma density, has a linear relationship with \B-z\. This result supports the empirical concept that the solar transition region is very thin and still affected by frozen-in convection. A way for improvement of such studies is also outlined.