Anisotropic nonthermal motions in the transition region of solar active regions

Abstract

Context. We study the nonthermal motions in the transition region of active regions (ARs) using center-to-limb observations of Si IV 1394 Å and 1403 Å lines. Aims. The aim of this study is to investigate the characteristics of nonthermal motions in the transition region of ARs using the center-to-limb measurement. Methods. We used Interface Region Imaging Spectrograph (IRIS) observations of the Si IV doublet lines 1393.78 Å and 1402.77 Å from the transition region of ARs to analyze nonthermal motions. We compared our analyzed data to similar data for the quiet Sun (QS) regions. We derive the nonthermal width by performing a single Gaussian fitting on the average spectral line profiles of Si IV doublet lines. We carried out a statistical analysis of the nonthermal width using the center-to-limb measurement. Results. We find that the nonthermal motions are clearly decreasing on the disk with increasing distance from disk center. Only close to the limb, clearly beyond μ = 0.4, does the nonthermal broadening increase again towards the limb. The decrease in the nonthermal broadening in ARs away from disk center is markedly different from what is seen in the QS. Conclusions. We conclude that nonthermal motions are anisotropic in ARs, with the vertical component being greater than the horizontal one. In contrast, we find the situation in the QS to be consistent with the findings of previous studies; only opacity effects are responsible for the increase in the nonthermal widths towards the limb, and nonthermal motions are isotropic in the QS.