Spectroscopic Studies of the Solar Corona. IV. Physical Properties of Coronal Structure

We obtained spectrographic observations of several coronal structures at the limb overlying the sunspot regions, simultaneously in the Fe XIV (5303 Å) and Fe X (6374 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 slit width of 160 μm provided a spatial resolution of 4" and a spectral resolution of 77 mÅ (Fe XIV), 128 mÅ (Fe X), and 291 mÅ (Fe XIII). The width and intensity of all these lines were computed using Gaussian fits to the observed line profiles. The FWHM of the emission lines increases at an average rate of 1.24 mÅ arcsec^-1 for Fe X, 0.29 mÅ arcsec^-1 for Fe XIII, and -0.66 mÅ arcsec^-1 for Fe XIV. These values are inversely correlated with the corresponding ionization temperature for these emission lines. We speculate that the FWHM of emission lines in coronal structures increases with height if the associated ionization temperature is less than 1.6×10⁶ K, with the gradient depending upon the ionization temperature of the line, while it decreases with height for lines whose ionization temperature is greater than 1.6×10⁶ 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 nonthermal velocity.