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

Peter Brandt, A. S. Gadun+1 more|arXiv (Cornell University)|May 12, 2010

Solar and Space Plasma Dynamics4 citations

TL;DR

This study uses high-resolution Fourier-transform spectroscopy of solar active regions near disk center to measure absolute Doppler shifts of Fe I and Fe II lines. It finds that weak Fe I and all Fe II lines are redshifted, while strong Fe I lines formed above log τ₅ ≈ −3 (≈425 km) are blueshifted, with shift magnitude increasing for lines of lower excitation potential. 2D MHD models qualitatively reproduce deep photospheric redshifts but fail to explain upper photospheric blueshifts observed in data.

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 $\\log \ au_5\\approx-3$ (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.

Motivation & Objective

• To measure absolute Doppler shifts of Fe I and Fe II lines in solar active regions near disk center using high-resolution Fourier-transform spectroscopy.
• To investigate how small-scale magnetic fields affect the vertical velocity structure of the solar photosphere across different atmospheric layers.
• To test whether 2D MHD models can reproduce observed line shifts, particularly the discrepancy between redshifts in deep-photospheric lines and blueshifts in upper-photospheric lines.
• To resolve conflicting prior results on line shifts in active regions by using a large, systematically selected sample of spectral lines with known formation heights.

Proposed method

• Acquired high-resolution Fourier-transform spectra (resolution ~200,000) of solar active regions near disk center using the McMath telescope at Kitt Peak.
• Reduced FTS wavelengths to the absolute solar scale using the Mg I λ517.27 nm line as a stable reference, assuming its shift is negligible in active regions.
• Selected 189 Fe I and 30 Fe II lines from laboratory wavelength catalogs, ensuring coverage of lines formed at various optical depth levels (log τ₅ from −5 to −1).
• Calculated absolute line shifts by comparing observed FTS line positions to laboratory wavelengths, corrected for instrumental and atmospheric effects.
• Applied 2D MHD models (including nonmagnetic HD models for comparison) to simulate line shifts, using LTE approximation and varying excitation potential (EPL) to probe height dependence.
• Used two model sets: [2,3] with overestimated heating and [6] with coarse spatial resolution, to test robustness of simulated shifts.

Experimental results

Research questions

• RQ1 How do absolute Doppler shifts of Fe I and Fe II lines vary across different formation heights in solar active regions?
• RQ2 Do strong Fe I lines formed in the upper photosphere (log τ₅ ≈ −3) exhibit blueshifts relative to quiet-Sun lines, and if so, how do they depend on excitation potential?
• RQ3 Can 2D MHD models reproduce the observed line shifts, particularly the blueshifts of upper-photospheric lines?
• RQ4 Why do some studies report blueshifts for high-formation-height lines while others report redshifts, and what causes this discrepancy?
• RQ5 To what extent do model limitations (e.g., heating overestimation or coarse resolution) affect the simulation of photospheric velocity fields in active regions?

Key findings

• Weak Fe I lines and all Fe II lines are systematically redshifted relative to their quiet-Sun positions, indicating downward motions in their formation layers.
• Strong Fe I lines with low excitation potential (EPL < 2 eV), formed above log τ₅ ≈ −3 (≈425 km), are blueshifted, with the blueshift increasing for lines of lower EPL.
• 2D MHD models successfully reproduce the redshifts of deep-photospheric lines (log τ₅ < −3) due to enhanced descending flows in magnetic flux bundles.
• The models fail to reproduce the blueshifts of upper-photospheric Fe I lines, suggesting that current MHD models lack sufficient physics to explain upward-velocity components in these layers.
• The discrepancy between observations and models for upper-layer lines is attributed to model limitations: [2,3] overestimate heating, while [6] uses coarse spatial resolution (35 km), distorting temperature and velocity structure.
• The observed blueshifts in upper-photospheric lines (e.g., Fe I λ543.45 nm) are not explained by current 2D MHD simulations, indicating a need for improved modeling of magnetic field effects on atmospheric dynamics.