Center-to-limb variations in coronal hole and quiet Sun regions obtained with IRIS spectroscopic observations

TL;DR

This study investigates the center-to-limb variations of spectral line parameters in quiet Sun and coronal hole regions using IRIS observations, revealing how plasma dynamics and non-thermal broadening change across the solar disk.

Contribution

First report of center-to-limb variations in coronal hole transition region lines, showing consistent behavior with quiet Sun and providing new insights into solar plasma dynamics.

Findings

Intensity increases towards the limb due to line-of-sight effects.

Doppler velocity increases from zero at the limb to about +5 km/s at disk center.

Non-thermal broadening decreases from limb to disk center in both regions.

Abstract

The center-to-limb variations (CLV) of Gaussian fit parameters of the transition region Si~{\sc iv} 1402.77~{\AA} spectral line in quiet Sun (QS) and coronal hole (CH) regions are presented. The results are derived from a full-disk mosaic scan obtained by the Interface Region Imaging Spectrograph on 24 September 2017. 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 $+4.8$~\kms\ (redshift) at disk center for CH and $+5.2$~\kms\ for QS. Non-thermal broadening in the CH decreases from a maximum of 24~\kms\ at the limb to 10~\kms\ at disk center. For QS the broadening decreases from 25~\kms\ at the limb to 14~\kms\ at disk center. Both Doppler velocities and non-thermal velocities vary linearly with $\cos\,\theta$, where $\theta$ is the heliocentric angle. The QS results for both parameters are consistent with earlier work.