The magnetic sensitivity of the Ca II H and K lines
arXiv:2604.11568 · doi:10.1051/0004-6361/202558065
Abstract
The solar chromosphere is a transition layer between the cool, dense photosphere and the hot, rarefied corona. This boundary region plays a key role in regulating energy transport and structuring the magnetic field throughout the solar atmosphere. Understanding its thermodynamic and magnetic properties is essential to model and interpret solar phenomena. This study investigates the theoretical properties and diagnostic potential of the polarisation signals in the Ca II H and K lines, with particular emphasis on their capability to probe magnetic fields in the upper chromosphere with the CHROMIS instrument at the Swedish 1-m solar telescope. We combine semi-empirical atmospheric models with high-resolution solar observations to model the formation of the Ca II H and K lines using non-local thermodynamic equilibrium radiative transfer calculations. The sensitivity of the lines to the magnetic field is examined through response functions and synthetic inversions, enabling an assessment of their diagnostic performance under realistic chromospheric conditions. For typical chromospheric field strengths, the linear polarisation of the Ca ii H & K lines is less than 1.7%, below the expected detection threshold of CHROMIS. However, their circular polarisation reaches more than 10% in strong-field regions, which is detectable by CHROMIS. Both lines are sensitive to magnetic fields in the upper chromosphere, with the K line forming slightly higher due to its larger opacity, and the H line exhibiting a somewhat stronger Zeeman sensitivity owing to its higher effective Lande factor and longer wavelength. Using the weak-field approximation, the line of sight magnetic field can be reliably inferred. These results confirm that the Ca II H & K lines constitute powerful diagnostics for studying the magnetic structure of the upper solar chromosphere.
References in corpus (29)
- CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
- Spectroscopy Made Easy: Evolution
- STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar observations
- Non-LTE inversions of the Mg II h&k and UV triplet lines
- DELO-Bezier formal solutions of the polarized radiative transfer equation
- Multi-frame blind deconvolution with linear equality constraints
- SSTRED: Data- and metadata-processing pipeline for CHROMIS and CRISP
- Mapping the magnetic field of flare coronal loops
- Spectropolarimetric capabilities of Ca II 8542 A line
- Three-dimensional modeling of the Ca II H&K lines in the solar atmosphere
- Magnetic Diagnostics of the Solar Chromosphere with the Mg II h-k Lines
- Fast approximation of angle-dependent partial redistribution in moving atmospheres
- Mapping Solar Magnetic Fields from the Photosphere to the Base of the Corona
- The Visible Spectro-Polarimeter of the Daniel K. Inouye Solar Telescope
- A method for global inversion of multi-resolution solar data
- Spectropolarimetric observations of the Ca II 8498 A and 8542 A lines in the quiet Sun
- On chromospheric heating during flux emergence in the solar atmosphere
- Frequency Redistribution Function for the Polarized Two-Term Atom
- Spectropolarimetric inversions of the Ca II 8498 A and 8542 A lines in the quiet Sun
- Stratification of physical parameters in a C-class solar flare using multi-line observations
- TIC: A Stokes inversion code for scattering polarization with partial frequency redistribution and arbitrary magnetic fields
- Sunrise III: Overview of Observatory and Instruments
- The impact of quantum interferences between different J-levels on scattering polarization in spectral lines
- Spectropolarimetry of C-class flare footpoints
- Magnetic field inference in active region coronal loops using coronal rain clumps
- Ubiquitous hundred-Gauss magnetic fields in solar spicules
- Improved reconstruction of solar magnetic fields from imaging spectropolarimetry through spatio-temporal regularisation
- The making of robust and highly performing imaging spectropolarimeters for large solar telescopes
- The magnetic sensitivity of the Ca II resonance and subordinate lines in the solar atmosphere