Dr. Natalia Shchukina is a member of the National Academy of Sciences of Ukraine (NASU) and a senior scientist at the Department of Solar Physics of the Main Astronomical Observatory of NASU.
A key challenge in solar physics is to obtain empirical information on the magnetic field of the million-degree plasma of the solar corona. In this extended and rarefied region of the solar atmosphere the explosive events that impact the Earth’s magnetosphere take place, and understanding the physical mechanisms that trigger such events requires “measuring” the strength and geometry of the magnetic field that permeates the coronal plasma structures during the stable and unstable phases of their temporal evolution. This is an extremely difficult unsolved problem. First, we need to find observables of the solar radiation that are sensitive both to the orientation and strength of the magnetic field. Second, we need telescopes and instruments suitable for measuring such observables. Third, we need to develop plasma diagnostic techniques capable of inferring the coronal magnetic field from the observations. This research project aims at achieving a novel contribution to the first and third points just mentioned.
The most well-known observables of the solar electromagnetic radiation sensitive to the coronal magnetic field are the polarization signals of forbidden lines from highly ionized species (e.g., the Fe XIII line at 10747 Å) and certain permitted lines (e.g., the hydrogen Ly-α line at 1215 Å). Unfortunately, the polarization signal in these lines can only be detected for off-limb line of sights during total solar eclipses or using a coronagraph.
The only coronal line radiation that can be observed for both, off-limb and on-disk line of sights is the extreme ultraviolet (EUV) permitted line radiation produced by highly-ionized species in the solar corona. However, for many years this radiation was thought to be unpolarized, and, therefore, unsuitable for measuring the coronal magnetic field. Fortunately, expectations changed in 2009 when Manso Sainz and Trujillo Bueno (2009; ASP Conf. Ser. Vol. 405, p. 423) theoretically discovered a mechanism through which some permitted EUV lines (e.g., the Fe X line at 174.5 Å) can actually become linearly polarized. It has been shown that, despite these lines being primarily collisionally excited, such polarization can occur if their lower level carries atomic alignment.
The research we began in 2024, with a three-month grant from the Fundación Occident, allowed us to demonstrate that there are many other permitted EUV lines from Fe X, Fe XI, Fe XIII, Fe XIV, Si IX and Si X coronal ions for which this mechanism should also induce linear polarization. Moreover, the linear polarization in some of them is sensitive not only to the orientation but also to the strength of the coronal magnetic field. These results, already submitted for publication, were obtained for an idealized case of single scattering in one-dimensional (1D) models of the solar corona (see Shchukina N., Trujillo Bueno, J., Supriya, H. D., Sukhorukov, A. Coronal Magnetometry with EUV Permitted Lines, 2025, Astrophysical Journal).
The study we are carrying out in 2025 with a two-month funding from the Fundación Occident provides us an opportunity to consider realistic three-dimensional (3D) models of the solar corona and to carry out detailed theoretical predictions of the linear polarization in the most intense EUV lines. To take advantage of this opportunity, we use the P-CORONA code (see https://gitlab.com/polmag/P-CORONA)developed within the framework of the POLMAG research project funded by the Advanced Grant H2020-ERC-ADG 742265 of the European Research Council.
In a very recent paper we describe the P-CORONA code and show some results of its application to the forbidden coronal lines (see Supriya, H. D., de Vicente Á., del Pino Alemán, T., Trujillo Bueno, J., Shchukina, N. P-CORONA: A New Tool for Calculating the Intensity and Polarization of Coronal Lines in 3D Models of the Solar Corona, 2025, Astrophysical Journal, submitted).
