Diagnostic of Spectral Lines in Magnetized Solar Atmosphere: Formation of the H\b{eta} Line in Sunspots

TL;DR

This paper investigates the formation of the Hβ spectral line in sunspot umbrae, revealing that magnetic field signals originate from the deep photosphere rather than the chromosphere, using non-LTE modeling.

Contribution

It provides a detailed non-LTE model of Hβ line formation in sunspot umbrae, clarifying the origin of magnetic signals in solar observations.

Findings

Hβ line signals originate from deep photospheric layers in sunspots.

The umbral chromosphere is transparent to Hβ Stokes parameters.

Magnetic field diagnostics from Hβ differ between sunspot and non-sunspot regions.

Abstract

Formation of the H$\beta$ $\lambda$4861.34 \AA{} line is an important topic related to the diagnosis of the basic configuration of magnetic fields in the solar and stellar chromospheres. Specifically, broadening of the H$\beta$ $\lambda$4861.34 \AA{} line occurs due to the magnetic and micro-electric fields in the solar atmosphere. The formation of H$\beta$ in the model umbral atmosphere is presented based on the assumption of non-local thermodynamic equilibrium. It is found that the model umbral chromosphere is transparent to the Stokes parameters of the H$\beta$ line, which implies that the observed signals of magnetic fields at sunspot umbrae via the H$\beta$ line originate from the deep solar atmosphere, where $\lg \tau_c\approx-1$ (about 300 km in the photospheric layer for our calculations). This is in contrast to the observed Stokes signals from non-sunspot areas, which are thought to primarily form in the solar chromosphere.