Stanislav Gunár

Variations in solar radius (hereafter R _⊙ ) is a key reference for solar magnetic activity over time. The sunlight amount may have varied with R _⊙ and had an effect on the Earth’s climate in the past. Eclipse observations offer a unique opportunity to measure the absolute R _⊙ value before modern direct observations. The scientific community has discussed a possible long-term R _⊙ variability from 1715 onward. Prior to their coverage, Clavius’s eclipse reports had been subjected to qualitative debates regarding the local eclipse visibility and a possible secular R _⊙ trend. This study leverages the recent dramatic developments of lunar topography data and ephemeris data to provide an effective resolution of this debate. Clavius’s eclipse reports described an explicit totality in 1560 at Coimbra and a “slender circle” around the eclipsing Moon in 1567 at Rome. Our study revised the Δ T constraints of −492 s ≤ Δ T ≤ 200 s in 1560 and 140 s ≤ Δ T ≤ 151 s in 1567 to satisfy Clavius’s descriptions, considering the lunar topography data and assuming Auwers’s canonical R _⊙ . This study constrains the R _⊙ margin of 1567, utilizing three scenarios to interpret Clavius’s account. The local totality requires an upper R _⊙ limit of 1567 as R _⊙ ≤ 696200 km in absolute size (959 . ″ 92 in angular size), indicating no linear secular R _⊙ shrinkage but possible R _⊙ modulations on a centennial timescale. Conversely, the annularity scenario is considered unlikely because it requires a R _⊙ decrease of 4 . ″ 7 within three centuries, even beyond the capacity of Eddy's extreme shrinking-Sun hypotheses.

Solar spicules are ubiquitous and strongly overlap in off-limb observations, which complicates the interpretation of individual events. We aim to infer population-level properties of spicules from IRIS Mg II k off-limb spectra using a forward model that explicitly accounts for line-of-sight superposition and instrumental effects. We constructed a stochastic 3D spicule-population model and synthesized Mg II k spectra with radiative transfer, including instrumental degradation and photon noise. We then constrained the hyperparameters that describe the underlying spicule population with likelihood-free Bayesian inference. This approach combines approximate Bayesian computation and an adaptive population Monte Carlo sampler, comparing observed and simulated datasets through principal-component summary statistics at five heights above the limb. The inferred posterior constrains effective population-level properties of the spicules that dominate the observed Mg II k variability and height dependence. These constraints provide a route to connect off-limb spectroscopy with forthcoming spectropolarimetric diagnostics of the chromosphere–corona interface.

Context. Constraining the plasma properties and energetics of coronal mass ejections (CMEs) in the inner corona is essential for understanding their early evolution, yet it remains challenging because of limited observations. Aims. We investigate the mass and density evolution as well as energy partitioning of two CMEs observed on 14 and 16 September 2025, and assess the potential of coordinated Proba-3/ASPIICS and Aditya-L1/VELC observations to constrain CME energetics in the low corona. Methods. We present the first coordinated observations of CMEs obtained simultaneously by Proba-3/ASPIICS and Aditya-L1/VELC. Using ASPIICS white-light observations, we estimated the CME mass, volume, number density, and the evolution of kinetic, thermal, and magnetic energies. Magnetic energies were estimated from observed CME properties using observationally constrained, physically motivated assumptions. Simultaneous VELC Fe XIV 5303 A observations provide independent estimates of the emission measure, electron number density, thermal energy, and CME lateral extent. Results. The electron number density evolves as n _e χ r ^-33 for the 14 September CME and as n _e ∝ r ⁻⁶ for the 16 September CME. The two events exhibit markedly different energy partitioning in the low corona. For the 14 September CME, the kinetic and magnetic energies are comparable, while the thermal energy remains nearly two orders of magnitude smaller, indicating limited plasma heating. In contrast, the 16 September CME exhibits a substantial thermal-energy enhancement, with thermal energy eventually becoming comparable to kinetic energy. For both CMEs, the estimated magnetic energy remains comparable to or exceeds the kinetic energy over the observed height. Conclusions. Our results demonstrate the scientific potential of synergetic ASPIICS and VELC observations for constraining the CME mass, density, and energetics in the inner corona, providing new observational constraints on the early evolution of CMEs.

We describe the scientific objectives and instrument design of the ASPIICS coronagraph aboard the Proba-3 mission of the European Space Agency launched on 5 December 2024. Proba-3 consists of two spacecraft in a highly elliptical orbit around the Earth. One spacecraft carries the telescope, and the external occulter is mounted on the second spacecraft. The two spacecraft fly in a precise formation during 6 hours of a 19.63 hour orbit, together forming a giant solar coronagraph called ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun). The very long distance between the external occulter and the telescope (around 144 m) represents an increase of two orders of magnitude compared to classical externally occulted solar coronagraphs. This allows the inner corona to be observed in eclipse-like conditions, i.e., close to the solar limb (down to 1.099 R _⊙ ) and with very little stray light. ASPIICS provides a new perspective on the inner solar corona that will help solve several outstanding problems in solar physics, such as the origin of the slow solar wind and the physical mechanism of coronal mass ejections.

Total solar eclipses (TSEs) offer a unique opportunity to observe the solar atmosphere, detect limb phenomena, and accurately measure the solar radius. Following the TSE in 1733, Wassenius first reported the existence of prominences to the scientific community. Wassenius’s original manuscript is held in the Royal Academy Archives of Sweden; this study translates his report and documents the associated source materials and local eclipse visibility. The solar radius ( R _⊙ ) during the TSE in 1733 is measured 696250 ± 170 km and 959 . ″ 99 ± 0 . ″ 24 in the absolute and apparent scales, respectively. This result contrasts with the modern standard (helioseismic) R _⊙ of 695780 ± 160 km and 959 . ″ 34 ± 0 . ″ 22; however, it is consistent with the R _⊙ values recorded in 1715 and 2017. The observed prominences are located at +23 . ° 5 ± 22 . ° 5, +66 . ° 5 ± 22 . ° 5, and −68 . ° 5 ± 22 . ° 5 in the heliographic latitude. The appearance of prominences at such high latitudes contrasts with the sunspot butterfly diagram for 1725–1750, confirming 1733 as a solar minimum. These high-latitude prominences can potentially be attributed to the so-called “polar rush” prominences that appear a few years after a solar minimum. If they are categorized as “polar rush” prominences, the solar minimum may be redated to before 1733 May. Furthermore, the latitudes of at least two of the prominences reported by Wassenius enable their classification as quiescent prominences, suggesting the presence of a polarity inversion line in the polar regions in early 1733.

Context . ASPIICS is a giant-baseline visible light solar coronagraph, which relies on the millimetric positioning performance of the precision formation flying Proba-3 mission of the European Space Agency. Proba-3 was launched on 5 December 2024, and since then ASPIICS has observed the solar corona with the field of view (1.1–3) R _⊙ . Aims . Diffraction, in particular diffraction of solar disk light on the external occulter, is known to provide a major source of stray light in coronagraphs. Our aims were to analyze diffracted light visible in ASPIICS images, to compare it with the analytical-numerical diffraction model reported earlier, and to fine-tune the model. Methods . We compared the diffraction effects visible in the ASPIICS data with simulated diffraction images; in particular, we compared the geometrical properties and the radiometric signal. The properties of the diffraction described in previous works suggest how to fine-tune the model in order to achieve a better correspondence with the observations. Results . Early ASPIICS observations, where diffraction is pronounced, fully confirm all the qualitative properties of diffracted light suggested by the model. After fine-tuning the model we found a quantitative correspondence of the level of 30–50%, depending on the configuration. Conclusions . The performed analysis allows us to validate our analytical-numerical model and justify the assumptions, and to estimate the contribution of the diffracted light in the ASPIICS images. Over most of the field of view the diffracted light is two orders of magnitude below the coronal signal.

Stellar eruptive prominences are one of the key components shaping stellar space weather environments. Moreover, due to the association of eruptive prominences with coronal mass ejections and their degrading influence on planetary atmospheres, they play a crucial role in the habitability of exoplanets. In this article, we used the insights from solar extreme-ultraviolet (EUV) observations to develop and test a technique for the detection of stellar eruptive prominences. We focused on the EUV imaging observations of the entire Sun provided by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) in the 304 Å channel. For this proof-of-concept study, we selected only a few examples of solar eruptive prominences and minor solar flares and analysed them in the Sun-as-a-star mode that mimics the stellar EUV photospheric observations. To validate the results obtained from the SDO/AIA Sun-as-a-star analysis, we used the SDO/Extreme Ultraviolet Variability Experiment (EVE) irradiance measurements. Our analysis showed markable intensity enhancements in the SDO/AIA 304 Å light curves during the prominence eruptions, with intensity increasing gradually for tens of minutes to several hours with enhancements of up to 1.5%. Flares exhibited a significantly faster intensity increase phase (a few minutes long) leading to large enhancements in the 304 Å channel of up to 8%. The large difference in the duration of the 304 Å intensity rise phases suggests that EUV stellar photometry in the He ii 304 Å line can provide signatures clearly attributable to stellar prominence eruptions or flares. However, development of robust techniques for detection of stellar eruptive prominences using the insights from EUV solar observations will require significantly broader statistical analyses that are beyond the scope of this work.

We review the development of the non-LTE (i.e. departures from Local Thermodynamic Equilibrium) radiative-transfer modeling of cool coronal condensations, namely solar prominences. The period considered covers five decades, but we particularly focus on current trends and advancements. Our main goal is to critically discuss various issues of the model geometries and how the assumed geometry couples to the specification of the incident illumination from the surrounding atmosphere. We start with initial one-dimensional (1D) models and continue with the discussion of 2D models and the current 3D approaches. A special attention is devoted to highly heterogeneous prominence structures and to fast-moving eruptive prominences currently well observed by the Metis and EUI instruments onboard Solar Orbiter and by the ASPIICS large coronagraph onboard the Proba-3 formation-flight mission.

Increasing interest in understanding the formation and dynamics of cool coronal condensations like solar prominences leads to complex magneto-hydrodynamical (MHD) simulations which assume a variety of physical processes responsible for energy balance. Formation of cool structures and their maintenance over the observed periods requires detailed treatment of heating/cooling processes of which the radiative ones are critically important. Most of up-to-date models use the so-called optically-thin radiative losses to account for radiative cooling. In this article, we present radiative-transfer simulations which demonstrate the importance of optically-thick line and continuum transitions. We model the process of free relaxation of prominence kinetic temperature towards the radiative equilibrium which demonstrates the formation of condensations in case where the radiative processes dominate the energy balance. We show a grid of isobaric models and how they relax to radiative equilibrium where the radiative losses are balanced by radiative gains. We also compare our results with previous works. Finally we stress the importance of realistic net radiative cooling rates for MHD modeling of cool coronal condensations.

Aims. We provide tabulated net radiative cooling rates (NRCRs) in the plasma of cool coronal condensations, together with the electron densities and ionization degrees describing its partial ionization. These readily applicable rates result from combined effects of the radiative cooling and radiative heating in the dominant atomic transitions in hydrogen, Mg II , and Ca II . Methods. These NRCRs represent realistic estimates based on 1D non-LTE (i.e. departures from the local thermodynamic equilibrium) radiative transfer modelling that uses 1D isothermal and isobaric prominence models. To construct easy-to-use NRCR tables, we employed the concept of voxels (volume pixels), which allowed us to incorporate the essential information about the location of the modelled plasma with respect to the source of illumination. We provide tabulated values of NRCRs, electron densities, and ionization degrees for a broad range of plasma parameters representing a wide variety of cool coronal condensations, such as prominences, cool coronal loops, spicules, jets, and coronal rain. Results. The accuracy of the provided voxel-based NRCRs, when tested against non-LTE calculations, is high, often showing a difference of only a few per cent. However, in some cases, the differences increase by up to a factor of two, which is the consequence of the assumptions and simplifications adopted here. Despite such differences, the voxel-based NRCRs that incorporate both the optically thick and thin radiative processes are a significant improvement, compared to the optically thin radiative loss formulas, when dealing with the cool plasmas at temperatures below 30 000 K. Conclusions. The provided NRCRs are tabulated for three different scenarios of the orientation of the modelled structure with respect to the source of illumination: (i) a vertical orientation where a surface receives illumination from one half of the solar disc, (ii) a horizontal orientation where the bottom surface receives illumination from the entire solar disc, and (iii) a horizontal orientation where the top surface does not receive any illumination from the solar disc. This allows the voxel-based NRCRs to be implemented even in complex multi-dimensional simulations of cool coronal condensations.