Spectroscopic studies of solar corona VI: Trend in line-width variation of coronal emission lines with height independent of the structure of coronal loops

Abstract

We have obtained spectroscopic observations in coronal emission lines by choosing two lines simultaneously, one [Fe x] 6374 A and the other [Fe xi] 7892 A or [Fe xiii] 10747 A or [Fe xiv] 5303 A. We found that in 95 per cent of the coronal loops observed in 6374 A, the FWHM of the emission line increases with height above the limb irrespective of the size, shape and orientation of the loop and that in case of 5303 A line decreases with height in about 89 per cent of the coronal loops. The FWHM of 7892 A and 10747 A emission lines show intermediate behavior. The increase in the FWHM of 6374 A line with height is the steepest among these four lines. We have also studied the intensity ratio and ratio of FWHM of these lines with respect to those of 6374 A as a function height above the limb. We found that the intensity ratio of 7892 A and 10747 A lines with respect to 6374 A line increases with height and that of 5303 A to 6374 A decreases with height above the limb. This implies that temperature in coronal loops will appear to increase with height in the intensity ratio plots of 7892 A and 6374 A; and 10747 A and 6374 A whereas it will appear to decrease with height in intensity ratio of 5303 A to 6374 A lineversus height plot. These findings are up to a height of about 200 arcsec above the limb. The varying ratios with height indicate that relatively hotter and colder plasma in coronal loops interact with each other. Therefore, the observed increase in FWHM with height above the limb of coronal emission lines associated with plasma at about 1 MK may not be due to increase in non-thermal motions caused by coronal waves but due to interaction with the relatively hotter plasma. These findings also do not support the existing coronal loop models, which predict an increase in temperature of the loop with height above the limb.