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

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.