Limb Shift of the Fe I 6569 {\AA} line on the Sun

TL;DR

This paper investigates the limb shift effect of the Fe I 6569 Å line on the Sun by combining observational data and simulations, revealing how different analysis methods and resolutions influence the measured convective blueshift and limb-shift profiles.

Contribution

It introduces two independent methods for analyzing the limb shift and compares their results with simulations, highlighting the impact of spatial resolution and the underlying physical components.

Findings

Method 1 aligns with classical limb-shift behavior at all resolutions.

Method 2 deviates at resolutions below 1 arcsec, showing significant differences.

The contrast and Dopplergram contributions are identified as key components of the limb shift.

Abstract

The convective motions of solar granules generate a center-to-limb variation of Doppler velocity in the photospheric lines, known as the limb shift effect. This study presents a comprehensive analysis of this effect for the Fe I 6569 {\AA} line using both observational data from the CHASE mission and numerical simulations from the Bifrost code. We employ two independent methods to derive the limb shift curve: a spectral-averaging method (Method 1) and a velocity-averaging method (Method 2). By comparing synthetic and observed data, we determine the convective blueshift, which is not accounted for in the CHASE observations. The simulations reproduce the observed trends for both methods at the instrument's spatial resolution of 1.2 arcsec. However, at resolutions below 1 arcsec, Method 2 produces limb-shift curves that depart significantly from both Method 1 results and traditional limb-shift profiles, whereas Method 1 remains in agreement with classical behavior. Further analysis finds that the results from Method 1 comprise two distinct components: a contrast contribution caused by the correlation between velocity and line depth, and a Dopplergram contribution caused by density inhomogeneities and corrugation effects.