Abstract:
The profiles of forbidden lines in the coronal spectrum contain information on the physical parameters such as temperature, turbulence, mass motion and density of coronal structures. The spectra of Fe x (6374 Å) emission line during the eclipses revealed that non-thermal velocity of the order of 20 - 30kms-1 need to be invoked to explain the observed excess line widths. We have obtained spectrographic observations of several coronal structures at the solar limb overlying sunspot regions with the 25-cm coronagraph in Fe x (6374 Å) and Fe XI (7892 Å) emission lines on several days and simultaneously in three coronal emission lines 6374 Å [Fe x], 10747 Å [Fe XIII] and 10798 Å [Fe XIII] on some other days. The combination of Fe XIV (5303 Å) and Fe x (6374 Å) emission lines was also chosen for making observations. The slit width of 160 microns provided a spatial resolution of 4 and a spectral resolution of 77 mÅ at [Fe XIV], 128 mÅ at [Fe X] and 291 mÅ at the [Fe XIII) lines. The width, intensity and Doppler shift of all these lines were computed using Gaussian fits to the observed line profiles. The FWHM of [Fe x] emission line increases at an average rate of 1.24 mÅ/arcsec, 0.55 mÅ/arcsec for [Fe XI], 0.29 mÅ/ arcsec for [Fe XIII] and -0.66 mÅ/arcsec for [Fe XIV] lines. These values are inversely correlated with the corresponding ionization temperature for these emission lines. We find that the FWHM of emission lines in coronal structures increases with height if the associated ionization temperature is less than 1.6 x 106 K with the gradient depending upon the ionization temperature of the line and decreases for those greater than 1.6 x 106 K. It implies that it may not always be possible to interpret the observed increase in FWHM with height in terms of an increase in non-thermal velocity, and hence rules out the existence of waves in steady coronal structures.
