Letter to the Editor

Solar magnetic field stratification from Ca I 4227 Å spectropolarimetric inversions

¹ Istituto ricerche solari Aldo e Cele Daccò (IRSOL), Faculty of Informatics, Università della Svizzera italiana 6605 Locarno, Switzerland
² Euler Institute, Faculty of Informatics, Università della Svizzera italiana 6900 Lugano, Switzerland
³ Institut für Sonnenphysik (KIS) Georges-Köhler-Allee 401a 79110 Freiburg, Germany
⁴ Faculty of Mathematics, University of Belgrade Studentski Trg 12-16 11000 Belgrade, Serbia
⁵ Astronomical Observatory Volgina 7 11000 Belgrade, Serbia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 21 January 2026
Accepted: 14 February 2026

Abstract

Spectropolarimetric data for the solar Ca I line at 4227 Å can be acquired from ground-based facilities and theoretical modeling studies have established their sensitivity to magnetic fields across a broad range of atmospheric heights; however, a routinely usable inference tool has been lacking until now. Here, we present the first successful inversion of the spectropolarimetric observations in Ca I 4227 acquired with the ZIMPOL polarimeter at the IRSOL observatory in Locarno. The inversion process incorporates the physical ingredients required for an accurate modeling of these observations, including non-local thermodynamic equilibrium effects, partial frequency redistribution in the general angle-dependent formulation, and the magnetic sensitivity arising from the joint action of the Hanle, Zeeman, and magneto-optical effects. By simultaneously fitting the height-dependent plasma bulk velocity and magnetic field that best reproduce the observed polarization signals, we inferred a physically meaningful stratification of the magnetic field from the photosphere to the low chromosphere. Such inversions demonstrate the feasibility of developing a pipeline to provide information on the magnetism of the low chromosphere and the underlying photosphere from Ca I 4227 spectropolarimetric observations.