Centre-to-limb variations in coronal hole and quiet sun regions obtained with IRIS spectroscopic observations

Abstract

ABSTRACT The centre-to-limb variations (CLV) of Gaussian fit parameters of the transition region Si iv 1402.77 Å spectral line in quiet Sun (QS) and coronal hole (CH) regions are presented. The results are derived from a full-disc mosaic scan obtained by the Interface Region Imaging Spectrograph on 2017 September 24. The CLV for a CH transition region line has not previously been reported, and the parameters are found to show variations consistent with the QS. The intensity increases towards the limb, consistent with an increasing plasma column depth due to line-of-sight effects. The Doppler velocity is normalized to be zero at the limb for both QS and CH and increases to +4.8 km s−1 (redshift) at disc centre for CH and +5.2 km s−1 for QS. Non-thermal broadening in the CH decreases from a maximum of 24 km s−1 at the limb to 10 km s−1 at disc centre. For QS the broadening decreases from 25 km s−1 at the limb to 14 km s−1 at disc centre. Both Doppler velocities and non-thermal velocities vary linearly with $\cos \, \theta$, where θ is the heliocentric angle. The QS results for both parameters are consistent with earlier work.