Absolute shifts of Fe I and Fe II lines in solar active regions (disk center)

TL;DR

This study measures the absolute shifts of Fe I and Fe II lines in solar active regions, revealing different shift patterns based on line strength and formation height, and interprets these findings using MHD models.

Contribution

It provides new observational data on line shifts in active regions and compares these with two-dimensional MHD model predictions.

Findings

Weak Fe I and all Fe II lines are red-shifted in active regions.

Strong Fe I lines are relatively blue-shifted, with shifts increasing as excitation potential decreases.

MHD models explain red shifts in deeper layers but not the blue shifts in higher layers.

Abstract

We estimated absolute shifts of Fe I and Fe II lines from Fourier-transform spectra observed in solar active regions. Weak Fe I lines and all Fe II lines tend to be red-shifted as compared to their positions in quiet areas, while strong Fe I lines, whose cores are formed above the level $\log \tau_5\approx-3$ (about 425 km), are relatively blue-shifted, the shift growing with decreasing lower excitation potential. We interpret the results through two-dimensional MHD models, which adequately reproduce red shifts of the lines formed deep in the photosphere. Blue shifts of the lines formed in higher layer do not gain substance from the models.