Flare heating of the chromosphere: Observations of flare continuum from GREGOR and IRIS
In May 2022, astronomers from Ondřejov carried out a coordinated observing campaign using instruments located in Ondřejov (Solar Patrol, SORT and the solar spectrographs FICUS and HSFA-2), the largest European telescope (GREGOR, located in Tenerife, Spain) and space-based solar satellites (IRIS and Hinode). Although flares are very common solar events, it is not possible to predict exactly where and when they will occur. It is therefore not easy to point solar telescopes with very high spatial resolution and a small field of view (smaller than the solar disc) at a region where a flare will occur. In the framework of this campaign, we were successful and we were able to obtain a unique multi-wavelength dataset of an M7.5 flare, including the pre-flare and the impulsive phase. This very rich dataset allowed us to show that flare continuum enhancements are present in faint ribbons as well, and to estimate a lower limit on the mean temperature in the layer where the flare continuum enhancement is formed.

M. García-Rivas, et al. 2024, A&A, 690, A254

Testing the volume integrals of travel-time sensitivity kernels for flows
Helioseismology is the only method that allows us to "see" below the solar surface. Unfortunately, a large portion of helioseismic inferences rely on the accuracy of the sensitivity kernels. The sensitivity kernels constitute the functions that "translate" changes in the parameters of the solar interior into the helioseismic observables on the solar surface. In the study we directly tested a class of sensitivity kernels related to the travel times of the waves propagating through the solar interior. By artificially manipulating the solar observations, we independently determined the spatial integrals of these kernels and compare them with model values. We show that the agreement is acceptable for the near-surface modes having the same radial order, whereas the agreement is less satisfactory for the waves travelling deeper and having a constant phase speed. 

M. Švanda, D. Chmúrny 2024, A&A, 690, A8

Magnetic field diagnostics of prominences with the Mg ii k line 3D Stokes inversions versus traditional methods
The inverse problem, that is, deciphering the physical conditions of observed structures from noisy data and a single point of view, is a fundamental challenge in solar physics.  Spectral line analysis provides a valuable tool for solving this problem, since the thermodynamic and magnetic properties of plasmas often leave significant traces in the intensity and polarization of these lines. However, the solution of the inverse problem is complicated by the non-local and non-linear interactions between different regions of the plasma mediated by radiation. As a consequence, this problem remains unsolved in its generality. In this paper, we present a new method that takes into account previously neglected physical processes and show that the inverse problem is solvable. Specifically, we address the problem of lines of once-ionized magnesium, which are the subject of observations by a proposed NASA satellite project.

J. Štěpán, et al. 2024, A&A, 689, A341

Spectral cleaving in solar type II radio bursts: Observations and interpretation
We have reported radio observations of a previously unrecognized feature, called spectral cleaving, in solar type II bursts, being radio signatures of shock waves in the solar corona. This feature is characterized by the actual branching of a type II radio emission lane in radio spectral data. We found that the spectral cleaving is a new distinct spectral effect indicative of involuted plasma processes that occur within the solar corona. We offered an initial interpretation of the spectral cleaving in type II bursts. The intricate interplay between the shock wave and magnetic field configurations plays a key role here. This discovery enhances our understanding of the mechanisms behind solar radio emissions and emphasizes the need for further observational studies to verify these findings.

A. Koval, et al. 2024, A&A, 689, A345

Physics of solar flares and prominences

The group focuses on research of bright and energetic phenomena, including solar filaments and prominences, flares/CME, their mutual relationships, but also on physics of the solar corona and transition region. The primary goals include understanding of the magnetic flux ropes, and also mechanisms of solar eruptions and coronal heating. To this end, our researchers use a variety of numerical models and/or multiwavelength observations (from X-rays to radio) performed by space-borne and ground-based instruments. Group members also participate in proposing new instrumentation and observing campaigns.

Part of this working group is also the Solar Patrol Service, which provides a daily overview of solar activity in the form of drawings of the solar photosphere and synoptic images. Another task of the Solar Patrol is to issue weekly and daily forecasts of solar activity.

Head: Jaroslav Dudík
Scientists: Arkadiusz Berlicki, Elena Dzifčáková, František Fárník, Vlastislav Feik, Stanislav Gunár, Petr Heinzel, Jana Kašparová, Dieter Nickeler, Martina Pavelková, Maciej Zapiór, Alena Zemanová

Structure and physics of the solar atmosphere

The research goal of the group is to understand the physical conditions and processes in the solar atmosphere. It focuses on both active and quiescent regions of the atmosphere and particularly on sunspots. Using numerical models and analysis of spectroscopic and spectro-polarimetric observations of number of spectral lines that form at different layers of the solar atmosphere, members of the group aim to advance our understanding of the processes that shape the Sun's atmosphere. The group is involved in the development of the large European solar telescopes in the Canary Islands. In particular, it is currently involved in the realisation of the European Solar Telescope (EST). 

Head: Jiří Štěpán
Scientists: Jose Iván Campos Rozo, Marta García Rivas, Jan Jurčák, Michal Sobotka, Michal Švanda, 

Physics of the Heliosphere

The group studies physical processes in the solar wind using in situ spacecraft observations, numerical simulations and theoretical analyses. It concentrates on properties of solar wind particles (electrons & ions) and their interactions with waves, turbulence, and nonlinear structures. Furthermore, it investigates interactions between the solar wind and solar system planets and moons as well analogic interactions between moons and planetary magnetosperes. 

Head: Petr Hellinger
Scientists: Štěpán Štverák, Marek Vandas

Group of solar radioastronomy

The working group studies physical properties of the solar atmosphere and processes there through analysis of solar radio data obtained in the wide range of wavelengths from decimeters to millimeters. At the Ondrejov observatory the group operates several solar radio telescopes running at decimetric wavelengths. The group includes members of the Atacama Large Millimeter/submillimeter Array Regional Centre (ALMA ARC-CZ) Czech node, which runs since 2016 and supports the ALMA user community of Central and Eastern Europe. The Czech ARC node provides scientific and technical support in the field of solar and (extra) galactic research with ALMA.

Head: Artem Koval
Scientists: Miroslav Bárta, Yi Chai, Marian Karlický, Wenjuan Liu