Working Group 2: Advances in the Magnetic Diagnostics of the Solar Chromosphere

The magnetism of the solar atmosphere drives and shapes all phenomena of space weather (SW). Hence, the capability to model and predict SW events fundamentally hinges upon our understanding of the evolution of solar magnetic fields, from the convection dominated photosphere to the magnetically organized corona. The chromosphere is an intermediate “layer” of the upper solar atmosphere, where the structure and dynamics of the magnetized plasmas undergo very dramatic changes, and where the scientific community is faced with the most challenging physical scenarios, which we must diagnose through the observation and interpretation of the polarized solar spectrum.

Sample template image
Fractional polarization profiles, Q/I (left), U/I (center), and V/I (right), of the Ca II H-K doublet (top row), and the three lines of the IR triplet (second to fourth row). The black and blue curves show the profiles emerging near the limb (μ=0.1) and at disk center, in the presence of a magnetic field of 10 gauss inclined by 30 degrees from the local vertical to the solar surface, pointing away from the observer.

As a consequence, new theoretical advances and modeling techniques about the formation of chromospheric spectral diagnostics become necessary. For example, an increasingly important aspect of chromospheric modeling under realistic solar scenarios is the need to account for the higher temporal coherence between the processes of absorption and re-emission of the solar radiation by the solar plasma – a physical condition that is characteristic of the tenuous chromospheric gas. This condition of “partially coherent” scattering gives rise to a plethora of phenomena (commonly dubbed partial frequency redistribution, or PRD) that critically affect the shape and polarization of chromospheric lines. Hence, PRD effects must be taken into account and correctly quantified in our models in order to confidently infer the strength and topology of the chromospheric magnetic fields from spectro-polarimetric observations.

The difficulties in achieving this goal are significant, because of the need to couple an extremely complex local problem (how polarized light is scattered by the plasma ions in the presence of arbitrary magnetic fields, and under the peculiar conditions of the solar chromosphere) to the implementation of a numerically challenging (multi-dimensional) non-local problem, where the polarized signals produced at each point in the atmosphere must be propagated through a highly inhomogeneous and anisotropic medium, before they can be synthesized in the form of the diagnostic observables captured with our instruments. This constitutes the “forward problem” that ultimately is at the core of any attempt to infer the magnetic, thermal, and dynamic structure of the outer solar atmosphere and the basis of the tools we will develop to support the cohort of students and community scientists.

At HAO we have undertaken the development of new numerical tools for the modeling of magnetic sensitive diagnostics in the solar upper atmosphere. Over the past year, we successfully completed HANLE-RT, a massively parallel and optimized Fortran code for modeling the formation of polarized spectral lines in a 1-D solar atmosphere in the presence of magnetic fields of any strength and topology, which also fully implements the physics of partial coherent scattering of polarized radiation in a self-consistent way. This tool has allowed us to model for the first time the polarization profiles of the Mg II h-k UV doublet at 280 nm and their modification by the weak (order of 10 gauss) magnetic fields of the quiet-Sun upper chromosphere. More recently, thanks to the computational efficiency and optimized memory handling of HANLE-RT, we were able – again for the very first time – to model the formation of the complex spectral system of Ca II encompassing the H-K doublet around 395 nm (a well known marker of the Sun’s magnetic activity cycle) and the IR triplet around 858 nm, which is one of the prime magnetic diagnostics of the chromosphere. An anticipation of the modeling results is given in the attached figure. A paper with a full description of the magnetic modeling of Ca II chromospheric lines including PRD is in preparation (del Pino Aleman, Casini, & Manso Sainz 2018).