Effects of spectral resolution on simple magnetic field diagnostics of the Mg II h & k lines

TL;DR

This study investigates how finite spectral resolution impacts magnetic field measurements from Mg II h & k lines using the weak field approximation, revealing biases introduced by spectral smearing and PRD effects.

Contribution

It provides a detailed analysis of spectral resolution effects on magnetic field diagnostics in Mg II lines, including biases and accuracy limits, using synthetic and sunspot simulation spectra.

Findings

Accuracy in B_LOS retrieval is maintained up to ~1500 G.

Spectral smearing degrades WFA accuracy, especially for outer lobes of Stokes V.

Limited resolution introduces a bias toward weaker magnetic field estimates.

Abstract

We study the effects of finite spectral resolution on the magnetic field values retrieved through the weak field approximation (WFA) from the cores of the Mg II h & k lines. The retrieval of the line-of-sight (LOS) component of the magnetic field, $B_{\rm LOS}$, from synthetic spectra generated in a uniformly magnetized FAL-C atmosphere are accurate when restricted to the inner lobes of Stokes V. As we degrade the spectral resolution, partial redistribution (PRD) effects, that more prominently affect the outer lobes of Stokes V, are brought into the line core through spectral smearing, degrading the accuracy of the WFA and resulting in an inference bias, which is more pronounced the poorer the resolution. When applied to a diverse set of spectra emerging from a sunspot simulation, we find a good accuracy in the retrieved $B_{\rm LOS}$ when comparing it to the model value at the height where the optical depth in the line core is unity. The accuracy is preserved up to field strengths of B~1500 G. Limited spectral resolution results in a small bias toward weaker retrieved fields. The WFA for the transverse component of the magnetic field is also evaluated. Reduced spectral resolution degrades the accuracy of the inferences because spectral mixing results in the line effectively probing deeper layers of the atmosphere.