Complex Variations in the Line-Intensity Ratio of Coronal Emission Lines with Height above the Limb

Abstract

We obtained spectroscopic observations simultaneously in pairs of coronal emission lines, one line being [Fe X] λ6374 and the other line being [Fe XI] λ7892, [Fe XIII] λ10747, or [Fe XIV] λ5303, and we studied the variations in the intensity and FWHM ratios of these lines with respect to those of 6374 Å as a function of height above the limb. We find that the intensity ratio of the 7892 and 10747 Å line with respect to the 6374 Å line increases with height and that the intensity ratio of 5303 Å to 6374 Å decreases with height above the limb. This implies that the temperature in coronal loops will appear to increase with height if we consider the intensity ratio of 7892 Å to 6374 Å a negligible variation in temperature in the case of the 10747 and 6374 Å line pair, while the temperature will appear to decrease with height if we consider the intensity ratio of 5303 Å to 6374 Å. The normalized FWHM (with respect to wavelength) ratio of 6374 Å to all the other coronal lines observed increases with height. The FWHM ratio at the limb depends on the pair of emission lines chosen; it is about 1 in the case of the 6374 and 7892 Å emission lines, indicating a common temperature and nonthermal velocity in the coronal loops near the limb, and it is about 0.7 at the limb in the case of the 6374 and 5303 Å lines and becomes about 1 at a height of 120". The varying FWHM ratios with height indicate that hotter and colder plasmas in coronal loops mix with each other. Therefore, the observed increase in the FWHM of coronal emission lines, which are associated with plasma at about 1 MK with height, may not be due to an increase in nonthermal motions caused by coronal waves but may be due to an interaction with relatively hotter plasma.