arXiv subjects

O. Andriienko

Publications and source records attributed to O. Andriienko.

7 recordsLinked to original sources

arXiv2026

The magnetic sensitivity of the Ca II H and K lines

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.

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arXiv2025

Characterization of telecentric dual-etalon Fabry-P\'erot systems from observational data. Properties of the CRISP2 instrument at the Swedish 1-m Solar Telescope

Imaging Fabry-P\'erot Interferometer (FPI) observations are commonly used in solar physics to infer physical parameters in the photosphere and chromosphere through modeling of the observations. Such techniques require detailed knowledge of the spectral instrumental profile in order to produce accurate results. In this study we present a method to characterize the spatial variation of parameters of dual-etalon FPI instruments mounted in telecentric configuration: spatially-resolved cavity separation and reflectivities of both etalons, as well as the prefilter variation across the field-of-view. Here, we aim at characterizing the field-of-view dependence of the parameters of the new CRISP2 FPI. We have implemented a forward model of the FPI instrumental degradation combined with a template average quiet-Sun spectrum at disk center in order to model two sets of observational data. Our method does not require any change in the optical setup or the utilization of external sources of illumination. We assess the validity of several functional forms in the calculation of the FPI transmission profiles. Our results show that (generally) the inclusion of the secondary transmission peaks at 1 FSR and a detailed estimate of the prefilter curve is necessary to obtain accurate values of both etalon reflectivities. Our results show that the cavity separation of CRISP2 is very flat, with an RMS variation below 2 nm over the entire field-of-view for both etalons. Reflectivity RMS variations are 0.4% and 0.3% for the primary and secondary etalons at 617.3 nm. The methods described in this paper are relevant for the characterization of present and future FPI instruments and we have made them publicly available to the solar community.

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arXiv2025

Synthesizing Sun-as-a-star flare spectra from high-resolution solar observations

Spatially resolved observations of the Sun and the astronomical sample size of stellar bodies are the respective key strengths of solar and stellar observations. However, the large difference in object brightness between the Sun and other stars has led to distinctly different instrumentation and methodologies between the two fields. We produce and analyze synthetic full-disk spectra derived from 19 small area field-of-view optical observations of solar flares acquired by the Swedish 1-m Solar Telescope (SST) between 2011 and 2024. These are used to investigate what can and cannot be inferred about physical processes on the Sun from Sun-as-a-star observations. The recently released Numerical Empirical Sun-as-a-Star Integrator (NESSI) code provides synthetic full-disk integrated spectral line emission based on smaller field-of-view input, accounting for center-to-limb variations and differential rotation. We use this code to generate pseudo-Sun-as-a-star spectra from the SST observations. ...

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arXiv2024

A comparative study of two X2.2 and X9.3 solar flares observed with HARPS-N: Reconciling Sun-as-a-star spectroscopy and high-spatial resolution solar observations in the context of the solar-stellar connection

Stellar flares cannot be spatially resolved, which complicates ascertaining the physical processes behind particular spectral signatures. Due to their proximity to Earth, solar flares can serve as a stepping stone for understanding their stellar counterparts, especially when using a Sun-as-a-star instrument and in combination with spatially resolved observations. We aim to understand the disk-integrated spectral behaviors of a confined X2.2 solar flare and its eruptive X9.3 successor as measured by HARPS-N. The behavior of multiple photospheric and chromospheric spectral lines are investigated by means of activity indices and contrast profiles. A number of different photospheric lines were also investigated by means of equivalent widths, and radial velocity measures, which are then related to physical processes directly observed in high-resolution observations made with the Swedish 1-meter Solar Telescope and SDO Our findings suggest a relationship between the evolving shapes of contrast profile time and the flare locations, which assists in constraining flare locations in disk-integrated observations. In addition, an upward bias was found in flare statistics based on activity indices derived from the Ca II H & K lines. In this case, much smaller flares cause a similar increase in the activity index as that produced by larger flares. H$α$-based activity indices do not show this bias and are therefore less susceptible to activity jitter. Sodium line profiles show a strongly asymmetric response during flare activity, which is best captured with a newly defined asymmetrical sodium activity index. A strong flare response was detected in Mn I line profiles, which is unexpected and calls for further exploration. Intensity increases in H$α$, H$β$, and certain spectral windows of AIA before the flare onset suggest their potential use as short-term flare predictors.

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arXiv2023

3D non-LTE modeling of the stellar center-to-limb variation for transmission spectroscopy studies

Context. Transmission spectroscopy is one of the most powerful techniques to characterize transiting exoplanets since it allows to measure the abundance of the atomic and molecular species in the planetary atmosphere. However, the stellar lines can bias the determination of such abundances if their center-to-limb variations (CLVs) are not properly accounted for. Aims. This paper aims to show that three-dimensional (3D) radiation hydrodynamic models and non-local thermodynamic equilibrium (non-LTE) line formation are required for an accurate modeling of the stellar CLV of the Na I D$_1$ and K I resonance lines on transmission spectra. Methods. We model the CLV of the Na I D$_1$ and K I resonance lines in the Sun with 3D non-LTE radiative transfer. The synthetic spectra are compared to solar observations with high spatial and spectral resolution, including new data collected with the CRISP instrument at the Swedish 1-m Solar Telescope between $μ=0.1$ and $μ=1.0$. Results. Our 3D non-LTE modeling of the Na I D$_1$ resonance line at 5896 Å and the K I 7699 Å resonance line in the Sun is in good agreement with the observed CLV in the solar spectrum. The simulated CLV curve for a Jupiter-Sun system inferred with a 3D non-LTE analysis shows significant differences from that obtained from a 1D atmosphere. The latter tends to overestimate the amplitude of the transmission curve by a factor that is of the same order of magnitude as a planetary absorption depth (up to 0.2 %). Conclusions. In order to correctly characterize exoplanetary atmospheres, 3D non-LTE synthetic spectra should be used to estimate the stellar CLV effect in transmission spectra of solar-like planet hosts. The work will be extended to other lines and FGK-type stars, allowing synthetic high-resolution spectra to mitigate the stellar contamination of low-resolution planetary spectra, e.g. those from JWST.

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arXiv2023

Center-to-limb variation of spectral lines and continua observed with SST/CRISP and SST/CHROMIS

Observations of center-to-limb variations (CLV) of spectral lines and continua provide a good test for the accuracy of models of solar and stellar atmospheric structure and spectral-line formation. They are also widely used to constrain elemental abundances and are becoming increasingly important in atmospheric studies of exoplanets. However, only a few such data sets exist for chromospheric lines. We aim to create a set of standard profiles by means of mosaics made with the CRISP and CHROMIS instruments of the Swedish 1-m Solar Telescope (SST), as well as to explore the robustness of said profiles obtained using this method. For each spectral line we use a mosaic that ranges from the center to the limb. Each of these mosaics are averaged down to 50 individual spectral profiles, spaced by 0.02 in the $μ$ scale. These profiles are corrected for p-mode oscillations, and their line parameters (equivalent width, line shift, full-width at half-maximum, and line depth) are then compared against literature values where possible. We present a set of 50 average profiles that are spaced equidistantly along the cosine of the heliocentric angle ($μ$) by steps of 0.02 for five continuum points between 4001 and 7772 Å, as well as ten of the most commonly observed spectral lines at the SST (Ca II H & K, H$β$, Mg I 5173 Å, C I 5380 Å, Fe I 6173 Å, Fe I 6301 Å, H$α$, O I 7772 Å, and Ca II 8542 Å). Center-to-limb variation of line profiles and continua are shared in the CDS as machine-readable tables; providing a quantitative constraint on theoretical models that aim to model stellar atmospheres.

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arXiv2021

SSTRED: Data- and metadata-processing pipeline for CHROMIS and CRISP

Context: Data from ground-based, high-resolution solar telescopes can only be used for science with calibrations and processing, which requires detailed knowledge about the instrumentation. [...] Aims: We aim to provide observers with a user-friendly data pipeline for data from the Swedish 1-meter Solar Telescope (SST) that delivers science-ready data together with the metadata needed for proper interpretation and archiving. Methods: We briefly describe the [instrumentation]. We summarize the processing steps from raw data to science-ready data cubes in FITS files. We report calibrations and compensations for data imperfections in detail. Misalignment of \ion{Ca}{ii} data due to wavelength-dependent dispersion is identified, characterized, and compensated for. We describe intensity calibrations that remove or reduce the effects of filter transmission profiles as well as solar elevation changes. We present REDUX, a new version of the MOMFBD image restoration code, with multiple enhancements and new features. [...] We describe how image restoration is used [...]. The science-ready output is delivered in FITS files, with metadata compliant with the SOLARNET recommendations. Data cube coordinates are specified within the World Coordinate System (WCS). Cavity errors are specified as distortions of the WCS wavelength coordinate with an extension of existing WCS notation. We establish notation for specifying the reference system for Stokes vectors [...]. [CRISPEX] has been extended to accept SSTRED output [...]. Results: SSTRED is a mature data-processing pipeline for imaging instruments, developed and used for the SST/CHROMIS imaging spectrometer and the SST/CRISP spectropolarimeter. SSTRED delivers well-characterized, science-ready, archival-quality FITS files with well-defined metadata. The SSTRED code, as well as REDUX and CRISPEX, is freely available through git repositories.

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