The centre-to-limb variation of non-thermal velocities using IRIS Si IV

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 to 30 s. The distribution of non-thermal velocities close to the limb is 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 did not 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.