Spectroscopic studies of the solar corona - V. Physical properties of coronal structures

Spectra around the 6374 Angstrom [Fe X] and 7892 Angstrom [Fe XI] emission lines were obtained simultaneously with the 25-cm coronagraph at Norikura Observatory covering an area of 200 x 500 of the solar corona. The line width, peak intensity and line-of-sight velocity for both the lines were computed using Gaussian fits to the observed line profiles at each location (4 x 4) of the observed coronal region. The line-width measurements show that in steady coronal structures the FWHM of the 6374 Angstrom emission line increases with height above the limb with an average value of 1.02 mAngstrom arc sec(-1). The FWHM of the 7892 Angstrom line also increases with height but at a smaller average value of 0.55 mAngstrom arc sec(-1). These observations agree well with our earlier results obtained from observations of the red, green, and infrared emission lines that variation of the FWHM of the coronal emission lines with height in steady coronal structures depends on plasma temperatures they represent. The FWHM gradient is negative for high-temperature emission lines, positive for relatively low-temperature lines and smaller for emission lines in the intermediate temperature range. Such a behaviour in the variation of the FWHM of coronal emission lines with height above the limb suggests that it may not always be possible to interpret an increase in the FWHM of emission line with height as an increase in the nonthermal velocity, and hence rules out the existence of waves in steady coronal structures.