Hydrogen recombination continua in stellar flares
An increasing interest in stellar flares stimulated various modelling approaches in order to analyse the observed flare fluxes. Radiation-hydrodynamical simulations, together with a rather rare broad-band spectroscopy, indicate much larger densities in the superflare chromospheres as compared to solar flares. Formation of hydrogen recombination continua under such different densities is governed by physics of optically thin to largely thick plasmas, the continuum optical thickness being within the range of four orders of magnitude. Various authors presented simple approximate methods to analyse the photometric data from Kepler or TESS under such diverse regimes of physical conditions. In this letter, we summarize the general physical approach and compute the hydrogen recombination spectra under the above range of electron densities. We show the theoretical contrast with respect to quiet-star continuum for two characteristic stars of G and dMe type. Based on that we distinguish three regimes of the continuum formation and discuss the applicability of various simple approaches.

P. Heinzel 2024, MNRAS, 532, L56

Onset of penumbra formation
The formation of sunspot penumbrae is still poorly understood. In this paper, we study regions at the edge of the sunspot pore where penumbra forms. Before the formation of the penumbra, we find different properties of the magnetic and velocity fields in the studied regions. However, the mechanism of penumbra formation is the same everywhere. Penumbral filaments with Evershed flow begin to form at the umbra boundary and grow radially primarily outward as the penumbral filaments elongate with time.

M. García-Rivas, et al. 2024, Astronomy & Astrophysics, 686, A112

The relation between magnetic field inclination and the apparent motion of penumbral grains
The bright heads of sunspot penumbral filaments, penumbral grains, show apparent horizontal motions inward, toward the umbra, or outward, away from the umbra. Penumbral grains are locations of rising hot gas from sub-photospheric layers. We used spectropolarimetric observations of five sunspot penumbrae to compare magnetic inclinations inside penumbral grains with those in their surroundings. We found that approximately a half of the inward-moving grains have a magnetic inclination larger than the inclination in their surroundings and a half of the outward-moving grains have an inclination smaller than the surrounding one. The opposite relation of inclinations is observed in only one fifth of the penumbral grains. We conclude that there is a statistical relation between the direction of apparent motions of penumbral grains and the inclination of magnetic field in sunspot penumbra.

M. Sobotka, et al. 2024, Astronomy & Astrophysics, 682, A65

Morphology of Solar Type II Bursts Caused by Shock Propagation through Turbulent and Inhomogeneous Coronal Plasma
We report radio observations of type II burst which has enormously rich and complex spectral morphology. We have exploited its herringbone pattern to study electron density turbulence in the solar corona. For the first time, we obtained properties of the density turbulence in the coronal streamer. This research copes with a relevant task in the physics of solar plasma – probing properties of density turbulence in the corona in a routine way.
A. Koval, et al. 2023, The Astrophysical Journal, 952, id.51

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