The Center-to-Limb Variation of Non-Thermal Velocities using IRIS Si IV

Abstract

We study the non-thermal velocities in the quiet-sun using various high spatial, temporal, and spectral resolution observations from the Interface Region Imaging Spectrograph (IRIS). We focus our analysis on the transition region using the optically thin line (Si IV 1393.7 Å), and select line profiles that are nearly Gaussian. We find evidence of a centre-to-limb variation using different observations having different exposure times, ranging from 5-30 s. The distribution of non-thermal velocities close to the limb are observed to peak around 20 km s${}^{-1}$ while the disc observations show a peak around 15 km s${}^{-1}$. The distributions are also different. The overall variation in the non-thermal velocities are correlated with the intensity of the line, as found previously. The on-disc velocities are smaller than most previous observations. In general, we find that the non-thermal velocities are independent of the selected exposure times. The Si IV lines didn't seem to exhibit any significant opacity effects. We conclude that these Doppler motions are mostly transverse to the radial direction. The possibility of swaying/torsional motions leading to such variations are validated from these IRIS observations.

21 pages, 2 Tables, 7 figures in the main paper, 18 figures in the appendix; Accepted, MNRAS, 2022

References in corpus (4)

• On the prevalence of small-scale twist in the solar chromosphere and transition region
• Why Is Non-thermal Line Broadening of Spectral Lines in The Lower Transition Region of the Sun Independent of Spatial Resolution?
• Reappraising Transition Region Line Widths in light of Recent Alfvén Wave Discoveries
• Modelling low charge ions in the solar atmosphere

Cited by in corpus (4)

• A benchmark study of atomic models for the transition region against quiet Sun observations
• Non-thermal broadening of coronal lines in a 3D MHD loop model
• Asymmetry of the spectral lines of the coronal hole and quiet Sun in the transition region
• On the Nature of Nonthermal Broadening of Spectral Lines Observed by IRIS