The solar chromosphere is a key layer in understanding how magnetic energy is transported and released into the Sun’s upper atmosphere. Yet, despite decades of effort, direct measurements of magnetic fields in the chromosphere remain challenging. Traditional diagnostics such as Ca II 8542 Å and He I 10830 Å provide valuable constraints, but during periods of enhanced heating these lines become partially ionized and sample deeper, cooler layers, limiting their ability to probe the regions where the most interesting physics occurs.
In this talk, I revisit a widely used but long-neglected diagnostic: the Hα line. Although Hα intensity images are a classical tracer of the chromosphere, its polarization signals—especially Stokes V—have historically been dismissed as predominantly photospheric. I will show that this conclusion arose largely from limitations in early radiative transfer modeling rather than from the line itself.
Using simultaneous spectropolarimetric observations of Ca II 8542 Å and Hα over a magnetic pore, I demonstrate that Hα carries signatures consistent with upper-chromospheric magnetic fields, including cases where the photosphere and chromosphere exhibit opposite polarity due to canopy fields. I then combine these results with forward modeling of synthetic Hα Stokes V profiles in 3D simulations to show that Hα systematically forms higher than Ca II 8542 Å and is sensitive to magnetic variations with height.
Together, these observational and theoretical results point toward Hα as a viable probe of chromospheric magnetism, particularly in heated or dynamic regions where traditional diagnostics fail. The talk will highlight open questions and future directions for Hα-based magnetic field inference.
The Solar atmosphere permeated by the magnetic fields. Credits: "Coupling from the Photosphere to the Chromosphere and the Corona", Wedemeyer-Böhm, S et al .