The H$\alpha$ line as a probe of chromospheric magnetic fields

TL;DR

This study demonstrates that the Hα line can effectively probe higher chromospheric magnetic fields, providing complementary information to existing diagnostics like Ca II 8542 Å, through detailed radiative transfer modeling and analysis.

Contribution

The paper introduces a method to use the Hα line for diagnosing chromospheric magnetic fields at higher altitudes, validated by 3D radiative MHD simulations and comparison with other spectral lines.

Findings

Hα line core probes higher chromospheric layers than Ca II 8542 Å.

Hα polarization signals remain detectable at realistic noise levels.

Hα provides complementary magnetic field stratification information.

Abstract

We explore the diagnostic potential of the H$\alpha$ line for probing the chromospheric magnetic field using a realistic 3D radiative magnetohydrodynamic (rMHD) model. The Stokes profiles of the H$\alpha$ line are synthesized through full 3D radiative transfer under the field-free approximation, alongside the Ca II 8542 {\AA} and Fe I 6173 {\AA} lines for comparison. The line-of-sight (LOS) magnetic fields are inferred using the weak field approximation (WFA) for theH$\alpha$ and Ca II 8542 {\AA} lines, while the Fe I 6173 {\AA} line is analyzed through Milne-Eddington inversion techniques. The comparison between the inferred LOS magnetic field maps and the magnetic fields in the rMHD model revealed that the H$\alpha$ line core primarily probes the chromospheric magnetic field at log tau_500 = -5.7, which corresponds to higher layers than the Ca II 8542 {\AA} line core, which is most sensitive to conditions at log tau_500 = -5.1. On average, the Stokes V profiles of the H$\alpha$ line core form 500 km higher than those of the Ca II 8542 {\AA} line core. The H$\alpha$ polarization signals persist after adding noise, and with noise at the level of 10^-3 Ic, most simulated magnetic structures remain visible. These findings suggest that spectropolarimetric observations of the H$\alpha$ line can provide complementary insights into the stratification of the magnetic field at higher altitudes, especially when recorded simultaneously with widely used chromospheric diagnostics such as the Ca II 8542 {\AA} line.