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

TL;DR

This study analyzes non-thermal velocities in the solar transition region using IRIS Si IV data, revealing a center-to-limb variation and suggesting transverse Doppler motions like swaying or torsional movements.

Contribution

It provides new observational evidence of center-to-limb variation in non-thermal velocities and discusses their possible transverse nature, using high-resolution IRIS data.

Findings

Non-thermal velocities peak around 20 km/s near the limb.

On-disc velocities are smaller than previous observations.

Velocities are independent of exposure times.

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 \AA), 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.