Rugged magnetohydrodynamic invariants in weakly collisional plasma turbulence
We investigated properties of ideal second-order magneto-hydrodynamic (MHD) and Hall MHD invariants (kinetic+magnetic energy and different helicities) in a two-dimensional hybrid simulation of decaying plasma turbulence. The combined (kinetic+magnetic) energy decays at large scales, cascades (from large to small scales) via the MHD non-linearity at intermediate scales. This cascade partly continues via the Hall coupling to sub-ion scales. The cascading energy is transferred (dissipated) to the internal energy at small scales via the resistive dissipation and the pressure-strain effect. The mixed (X) helicity, an ideal invariant of Hall MHD, exhibit a strange behaviour whereas the cross helicity (the ideal invariant in MHD but not in Hall MHD), in analogy to the energy, decays at large scales, cascades from large to small scales via the MHD+Hall non-linearities, and is dissipated at small scales via the resistive dissipation and an equivalent of the pressure-strain effect. In contrast, the magnetic helicity is very weakly generated through the resistive term and does not exhibit any cascade.

P. Hellinger, et al. 2024, A&A, 690, A174

Anisotropy of plasma turbulence at ion scales
We investigated properties of plasma turbulence at ion scales in the context of the solar wind. The ambient magnetic field induces a strong spectral anisotropy, the turbulent fluctuations exhibit larger spatial scales along the magnetic field compared to the perpendicular directions. An analysis using the Kármán-Howarth-Monin equation shows the corresponding anisotropy of turbulent processes (decay, cascade, resistive dissipation, and pression-strain interaction): their characteristic scales shifts to larger scales in the quasi-parallel direction with respect to the ambient magnetic field compared to the quasi-perpendicular ones. This anisotropy is weak at large scales owing to the initial isotropic spectrum and becomes progressively stronger at small scales.

P. Hellinger, et al. 2024, A&A, 684, A120

Magnetic properties of the umbral boundary during sunspot decay: Comparative study of multiple datasets
The vertical magnetic field is the stabilising factor responsible for the different magnetoconvective modes observed in sunspots. That is, there is a critical vertical magnetic field that separating the umbral and penumbral modes of convection. This criterion has been demonstrated by several authors using different datasets and instruments, leading to different values of the critical vertical magnetic field. In this work, we have retrieved records from these instruments (SP/Hinode and HMI/SDO) and datasets (HMI and HMI corrected for scattered light) of a dissipating sunspot. We derived the intrinsic dissimilarities of the inverted vector magnetic fields caused by the different instrumental setups and inversion strategies, and showed that the dissimilarities of the obtained critical vertical field are consistent with such intrinsic dissimilarities. Moreover, the decaying sunspot allowed us to observe how the penumbra provides stability to the umbral region, where the magnetic properties do not vary much during the decay, while the magnetic structure of the naked spots behaves similarly to solar pores, where their magnetic properties vary more dramatically with small changes in the umbral region.

M. García-Rivas, et al. 2024, A&A, 689, A160

Slowly positively drifting bursts generated by large-scale magnetic reconnection
In this work, we studied a flare rich in unique types of radio bursts with slow positive drift at frequencies around 1 GHz. We have shown that these rarely observed radio bursts are related to a magnetic reconnection process that can suddenly release large amounts of energy of a complicated magnetic field. We also found that these radio bursts are likely generated by a beam of accelerated particles moving along the newly formed magnetic lines. When these new interconnections have large scales, some of the bursts can occur quite far from the center of the flare itself. We also discuss the possibilities of why the frequency drift in these isolated radio bursts is so slow, even when they are generated by a beam of accelerated particles.

A. Zemanová, et al. 2024, A&A, 690, A241

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

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