Last publications – Cosmology and extragalactic astronomy group

• McKevitt, James; Matthews, Sarah; Brooks, David H.; Shimizu, Toshifumi; Tei, Akiko & Ugarte-Urra, Ignacio [Show all 22 contributors for this article] (2026). Pre-flare and active region plasma flows and structure seen by the short wavelength camera on SOLAR-C/EUVST. Publications of the Astronomical Society of Japan (PASJ). ISSN 0004-6264. 78(4), p. 1524–1545. doi: 10.1093/pasj/psag078. Full text in Research Archive Show summary

  The mechanisms triggering solar flares and driving coronal heating occur across wide temperature ranges on small spatial scales and short timescales, making them difficult to observe with current instrumentation. The upcoming SOLAR-C mission, launching in the late 2020s, will provide unprecedented plasma diagnostic capability with its high-throughput extreme-ultraviolet (EUV) spectroscopic telescope (EUVST), capable of 0.2 arcsec/pix spatial sampling (⁠~0.4 arcsec resolution), continuous temperature coverage from 0.02 to 15 MK, and exposure times down to 0.5 s. We present forward modelling of the spectrograph’s short wavelength camera (170.0–212.3 Å; SOLAR-C/EUVST-SW) and its response to log T ~ 6.2 coronal plasma in a three-dimensional MHD-simulated pre-flare active region. We compare this performance to that of the previous-generation EUV Imaging Spectrometer (EIS) on Hinode (SOLAR-B). Our results demonstrate that SOLAR-C/EUVST can distinguish individual flux tubes in simulated active region loops, which Hinode/EIS cannot resolve. In simulated pre-flare plasma, SOLAR-C/EUVST captures sharp velocity gradients between adjacent upflowing and downflowing plasma which Hinode/EIS is unable to resolve. Doppler velocity measurement accuracy will reach better than 1 km/s in active regions. We show that this next-generation spectrograph can be expected to directly observe processes potentially related to flare triggering, such as plasma flows from low-altitude reconnection linked to emerging flux, and determine whether active region loops consist of a small number of strands or the hundreds predicted by magnetic reconnection-induced nanoflare heating models.

• Dalda, Alberto Sainz; Rodríguez, Jaime de la Cruz; Hansteen, Viggo Haraldson; Pontieu, Bart Walter De & Gošić, Milan (2026). The IRIS^2+ Inversion Tool: Recovering the Radiative Losses and the Thermodynamics in the Lower Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 997(2). doi: 10.3847/1538-4357/ae274c. Full text in Research Archive Show summary

  We introduce an improved and fast inversion tool that is able to provide the thermodynamics of the solar atmosphere from the photosphere to the top of the chromosphere, as well as the integrated radiative losses (IRLs) in the chromosphere for data observed by the Interface Region Imaging Spectrograph (IRIS). This NASA mission has been observing the Sun and providing, among other kinds of data, multiline spectral observations sensitive to changes in the lower solar atmosphere since 2013. In this paper, we explain the new inversion tool IRIS2+ based on the IRIS2+ database, which is based on 135,472 synthetic representative profiles (RPs), each of them consisting of six chromospheric lines and six photospheric lines, their corresponding representative model atmospheres (RMA), and the IRL associated with these atmospheres. A nearest neighbor (k-nn) model algorithm is trained with the synthetic representative profiles to predict the closest RP in the database to the one observed, at which point IRIS2+ assigns the RMA and the IRL to the location of that observed profile. We have compared the results obtained by IRIS2+ with results obtained from the state-of-the-art inversion code STiC, which is also used to build the IRIS2+ database. We find that the thermodynamics and the IRL obtained with both methods are comparable in most cases. Therefore, IRIS2+ is a fast and reliable inversion tool that provides approximate values of the thermodynamic state and the radiative losses in the lower solar atmosphere for a large variety of solar scenes observed with IRIS.

• Bate, W.; Gordovskyy, M.; Prasad, Avijeet; Brun, A. S.; Strugarek, A. & Sieyra, M. V. [Show all 10 contributors for this article] (2026). Energetics and Emission in a Simulated Solar Flare Initialized by a Non-force-free Magnetic Field. The Astrophysical Journal (ApJ). ISSN 0004-637X. 1006(2). doi: 10.3847/1538-4357/ae7c69. Full text in Research Archive Show summary

  Solar flare simulations are commonly initialised using nonlinear force-free field (NLFF) extrapolations derived from photospheric vector magnetograms. However, the force-free assumption neglects plasma forces and may limit the available free magnetic energy. In this work, we perform a controlled comparison of two three-dimensional resistive magnetohydrodynamic simulations of the X2.1-class flare that occurred on 2011 September 6 in NOAA Active Region 11283. The simulations differ only in their initial magnetic configuration: one is based on a conventional NLFF extrapolation, while the other employs a non-force-free extrapolation. Both models are evolved in an identical stratified atmosphere using the same numerical framework, enabling direct assessment of how the initial magnetic assumptions influence flare dynamics and energetics. We find that the non-force-free model undergoes more extensive magnetic restructuring and releases approximately twice as much magnetic energy (≈4.4 × 10^31 erg) as the NLFF case (≈2.3 × 10^31 erg), bringing the energy budget into closer agreement with expectations for X-class flares. Synthetic extreme-ultraviolet emission in the 94 Å channel is computed for both simulations and compared with observations from the Solar Dynamics Observatory. The non-force-free model produces a brighter and more spatially extended emission structure that more closely resembles the observed flare morphology and light curve. These results demonstrate that assumptions made in constructing the preflare coronal magnetic field can significantly affect flare energetics and observable signatures and suggest that non-force-free extrapolations provide a promising pathway toward more realistic data-constrained flare modelling.

• Ornig, Sascha & Carlsson, Mats (2026). Characterization of white-light enhancements under umbral conditions in one-dimensional simulations of solar flares. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202660723. Full text in Research Archive Show summary

  Context. Solar flares with signatures in the optical continuum (white light, WL) pose a challenge not only to the standard flare model, but also to solar flare simulations. In particular, simulations are so far not able to convincingly reproduce observed WL enhancements. Aims. We investigate the effect of different electron beams on an umbral atmosphere and what the differences and similarities to the quiet-Sun response are. Methods. We characterized WL emission in one-dimensional simulations of solar flares using the radiation hydrodynamics code RADYN. We created such simulations with a similar setup as the F-CHROMA grid, but with a starting atmosphere describing umbral conditions. We investigated the influence of different temporal profiles of an electron beam on this umbral atmosphere. Results. Our simulations show maximum WL increases between 40 and 335%, which is comparable to observed values. The reduced umbral background is the main reason for these large increases. We identify hydrogen recombination in an optically thin chromosphere as the dominant process responsible for the increases, with the radiation from the heated photosphere becoming substantial in the later stages (a few seconds after beam heating is turned o ) due to the longer timescale of the cooling of the photosphere compared to hydrogen recombination in the chromosphere. The short timescale of recombination results in very short gradual phases of excess emissions in our simulations. Shorter, more intense beams (i.e., beams with a higher maximum energy flux) lead to a faster and more dramatic atmospheric evolution. Such beams also cause larger WL enhancements due to a higher electron density in the relevant layers. Both the Balmer ratio and the Paschen ratio are substantially higher in our simulations compared to simulations with a quiet-Sun atmosphere. Conclusions. The detectability and amplitude of excess WL emissions depends both on the spectral and temporal structure of the electron beam impacting on the atmosphere as well as the underlying background radiation. The combination of a short, intense beam and an umbral atmospheric structure provides an excellent seed for substantial WL enhancements.

• Ornig, Sascha; Voort, Luc Rouppe van der; Carlsson, Mats; Baso, Carlos José Díaz; Øyre, Eilif Sommer & Poquet, Ignasi Josep Soler [Show all 7 contributors for this article] (2026). Investigating white-light flare mechanisms via the Paschen jump using high-resolution continuum observations from the Swedish 1-m Solar Telescope. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 712. doi: 10.1051/0004-6361/202660421. Full text in Research Archive Show summary

  Context. The continuum is understood to contain a large portion of the energy emitted by a solar flare. The optical continuum, known as white light (WL), is particularly relevant since it may be observed by ground-based instruments. Aims. We measured the WL enhancements short- and longward of the Paschen jump in order to gain insights into the possible mechanism(s) behind the creation of these increases in our two case studies. Methods. We took measurements from the Swedish 1-m Solar Telescope of the pseudo-continuum around the Ca II 8542 Å line as well as the true continuum around the K I 7699 Å and the Fe I 6173 Å line, providing us with observations on both sides of the Paschen jump. Results. We observe WL enhancements of over 40% against the dark (pen-) umbral background in both flares. No WL excess is detectable against the granulation outside the sunspots. The WL excess in flare 1 is co-temporal with the derivative of the GOES soft X-ray and hard X-ray (HXR) measurements from the Advanced Space-based Solar Observatory (ASO-S), and the flare is compatible with the Neupert effect. For the second flare, a preceding smaller flare may be the cause of the temporal discrepancy. Signatures of chromospheric evaporation and condensation are found in the WL area for both flares. The ratio of intensities blueward and redward of the Paschen jump (i.e., the Paschen ratio) in flare 1 is below one for most WL pixels. This is in disagreement with the accepted WL formation mechanisms, which are both of photospheric and chromospheric origin. We believe this is a consequence of the Ca II 8542 Å pseudo-continuum being affected by line wing opacity changes. Conclusions. The co-temporality of WL and HXR enhancements suggests that the WL emission enhancements in flare 1 (and parts of flare 2) are a result of direct electron precipitation. We conclude that more reliable continuum measurements free of any nearby line influence are necessary in order to obtain conclusive evidence for the formation mechanism(s) behind optical continuum enhancements from such analysis as presented in this work.

• Cliver, Ed; Hudson, Hugh & Fletcher, Lyndsay (2026). Tension in the solar energetic proton paradigm for historical cosmogenic nuclide events. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. ISSN 1364-503X. 384(2329). doi: 10.1098/rsta.2025.0291. Full text in Research Archive Show summary

  ‘Miyake events’ are exceptional increases in atmospheric 14C recorded in tree rings that, if confirmed, are accompanied by simultaneous 10Be and 36Cl enhancements in ice cores. During the last 15 000 years, six such events have been confirmed. They are generally thought to originate in solar flares that produce strong high-energy solar proton events. The 774 CE Miyake event is currently estimated to have been produced by a flare with a soft X-ray (SXR) flare classification of X430. A recently constructed size distribution of SXR events from 1975 to 2022 permits an estimate of the ‘return time’ for any given SXR event. We extrapolate the observed distribution to obtain a flare return time of ⋟106 years for all six of the Miyake events. We examine various possibilities (e.g. overestimates of greater than 200 MeV proton fluences, episodes of multiple smaller flares) to reduce the tension between the estimated long return (waiting) times for these extreme flares and the 15 000-year observation interval. We conclude that the most likely explanation for the inferred outsized proton events is a yet unknown Dragon-King type mechanism or scenario that can increase high-energy solar proton fluences in certain events by one to two orders of magnitude.

• Li, Ting; Duan, Xuchun; Hou, Yijun; Aulanier, Guillaume; Zimovets, I. V. & Zhang, Jun [Show all 16 contributors for this article] (2026). Unprecedented Fast Winking of Solar Flares Triggered by Bursty Magnetic Reconnection. The Astrophysical Journal (ApJ). ISSN 0004-637X. 1006(1). doi: 10.3847/1538-4357/ae80ae. Full text in Research Archive Show summary

  Flare ribbons form as a result of energy deposition associated with particles accelerated in low layers of the solar atmosphere. The fine-scale structures of flare ribbons, also called ribbon kernels, offer a potentially powerful diagnostic of the flare reconnection process, however, to date the dynamic evolution of ribbon kernels has not been fully characterized in statistical studies. Here, we checked the state-of-the-art observations (cadence ≤2.5 s) of solar flares in the ultraviolet from space by Interface Region Imaging Spectrograph over the past 12 yr. Our results showed the first statistical study of multiple spatially resolved flare kernel quasiperiodic pulsation events for 31 flares, with the period of 6–24 s. The ribbon kernels have a spatial scale of 480–1200 km and some kernels exhibit an unprecedented fast “winking” process, i.e., quasiperiodic pulsation-like flashing of individual kernels. The shortest heating time reaches about 2–3 s, implying that the energy is deposited only in a small localized region within flare ribbons, persisting for only a few seconds. Meanwhile, some ribbon kernels were observed to slip along the ribbon at speeds of 20–1800 km s−1. These observations strongly imply a joint picture for the dynamics and the bursty nature of ribbon kernels as being due to coupled effects of plasmoid formation and three-dimensional magnetic reconnection in the overlaying coronal current sheet. We suggest that the observed flare behaviors provide strong observational evidences of 3D bursty reconnection.

• Chaurasiya, Ravi; Pereira, Tiago & Bayanna, Ankala Raja (2026). Multi-thermal dynamics and transverse oscillations of solar spicules revealed by coordinated SST, IRIS, and SDO observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 712. doi: 10.1051/0004-6361/202660406. Full text in Research Archive Show summary

  Context. Solar spicules are highly dynamic chromospheric jets that play an important role in the mass and energy balance of the solar atmosphere, even though their connection to the transition region and corona remains unclear. Aims. We investigated the dynamical and multi-thermal properties of spicules, their connection to higher atmospheric layers, and their transverse oscillations and associated energy flux. Methods. We analyzed coordinated high-resolution observations from the Swedish 1-m Solar Telescope in Hα, the Interface Region Imaging Spectrograph in Si IV 1400 Å, and the Solar Dynamics Observatory/Atmospheric Imaging Assembly coronal channels. Space-time diagrams, spectral analysis, and wavelet techniques were used to study temporal evolution, Doppler velocities, and oscillatory properties. Transverse displacements of spicules were tracked to estimate wave properties and energy flux. Results. Space-time analysis reveals a clear correspondence between chromospheric spicules and coronal emission in AIA 171 Å, evolving coherently with spicule extension. Doppler velocities from Hα and Si IV show opposite signs, indicating multi-thermal plasma flows. Wavelet analysis reveals frequently dominant ~3-minute oscillations, along with high-frequency transverse oscillations (65–270 s) with velocity amplitudes of 3.3–9.9 km s−1 and a mean energy flux of (2.14 ± 0.78) × 103 W m−2. Conclusions. These results demonstrate that spicules are multi-thermal dynamic structures connected to the transition region and corona and that transverse waves carry substantial energy, highlighting their role in coronal heating.

• Yang, Dongting; Liu, Hong li; Qin, Sheng li; Liu, Tie; Jiao, Wenyu & Garay, Guido [Show all 31 contributors for this article] (2026). Evolution of Starless Cores in Massive Clumps Seen by the ALMA ASHES and QUARKS Surveys. The Astrophysical Journal (ApJ). ISSN 0004-637X. 1006(2). doi: 10.3847/1538-4357/ae81ab. Full text in Research Archive Show summary

  We present a systematic comparative analysis of 324 starless cores in early-phase infrared-dark clouds (IRDCs; ASHES survey) and evolved-phase infrared-bright clouds (IRBCs; QUARKS survey) using 1.3 mm continuum and line data by the Atacama Large Millimeter/submillimeter Array. Despite having comparable sizes (∼2500 au), starless cores in IRBCs exhibit systematically higher median mass (1.5 M⊙ versus 0.6 M⊙), number density, and surface density—enhancements of approximately a factor of 2 relative to starless cores in IRDCs. Starless cores in IRBCs also display relatively stronger nonthermal motions (σ ∼ 0.5 km s−1 versus 0.3 km s−1), higher total virial parameters (median αvir,tot ∼ 2.3 versus 1.0), and steeper density profiles, indicating more centrally concentrated structures in feedback-driven, turbulence-enhanced environments. These findings support a dual evolutionary origin: (i) new core formation in evolved IRBCs under altered initial conditions and (ii) subsequent dynamical mass growth via accretion from extended reservoirs. The prevalence of low-mass starless cores—even in late-stage IRBC environments—challenges models requiring massive prestellar cores and instead favors competitive-like dynamical mass accretion scenarios for high-mass star formation.

• Chouliaras, Georgios; Wyper, Peter F.; Dahlin, Joel T. & Fletcher, Lyndsay (2026). Automatic Detection of Flare Ribbon Fine Structures as Proxies for Plasmoid Dynamics in Flare Reconnection. The Astrophysical Journal (ApJ). ISSN 0004-637X. 1006(2). doi: 10.3847/1538-4357/ae7c75. Full text in Research Archive Show summary

  Flare ribbons often display fine structures along their fronts that are commonly interpreted as signatures of intermittent reconnection dynamics including plasmoid formation in the flare current sheet. We introduce an automated method that detects and tracks the spiral- and wave-like imprints of these structures and as a proof of concept apply it to maps of magnetic field-line length from a high-resolution 3D eruptive-flare simulation. The workflow applies the correlation-dimension method, density-based clustering, and a minimum-area ellipse fit to summarize each feature. We show that across the simulated flare, the detected spirals remain locked to the ribbon’s outward motion while drifting coherently along the ribbon. The two ribbons show opposite along-ribbon drift and motion away from their hooks in accordance with theoretical expectations, with instantaneous speeds of 10–800 km s^−1, all well below the local surface Alfvén speed. Occurrence, lifetimes, and mean magnetic flux of the features peak during the impulsive phase. The distribution of per-spiral mean unsigned flux shows a scale-free tail above roughly 6 × 10^18 Mx with a power-law exponent near 3.4. Together, these results show that bursty, plasmoid-mediated flare reconnection leaves a clear, measurable signature on the flare ribbons. The method provides a practical surface diagnostic of ribbon fine structure that can potentially be used to inform our understanding of three-dimensional magnetic reconnection in the flare current sheet.

• Bergamini, P.; Meneghetti, M.; Acebron, A.; Clément, B.; Bolzonella, M. & Grillo, C. [Show all 378 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXIII. The first catalogue of strong-lensing galaxy clusters. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554577. Full text in Research Archive
• Bhargava, S.; Benoist, C.; Gonzalez, A. H.; Maturi, M.; Melin, J. B. & Stanford, S. A. [Show all 354 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXIV. First detections from the Euclid galaxy cluster workflow. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554937. Full text in Research Archive Show summary

  The first survey data release by the Euclid mission covers approximately 63 deg2 in the Euclid Deep Fields to the same depth as the Euclid Wide Survey. This paper showcases, for the first time, the performance of cluster finders on Euclid data and presents examples of validated clusters in the Quick Release 1 (Q1) imaging data. We identify clusters using two algorithms (AMICO and PZWav) implemented in the Euclid cluster-detection pipeline. We explore the internal consistency of detections from the two codes, and cross-match detections with known clusters from other surveys using external multi-wavelength and spectroscopic data sets. This enables assessment of the Euclid photometric redshift accuracy and also of systematics such as mis-centring between the optical cluster centre and centres based on X-ray and/or Sunyaev–Zeldovich observations. We report 426 joint PZWav and AMICO-detected clusters with high signal-to-noise ratios over the full Q1 area in the redshift range 0.2 ≤ z ≤ 1.5. The chosen redshift and signal-to-noise thresholds are motivated by the photometric quality of the early Euclid data. We provide richness estimates for each of the Euclid-detected clusters and show its correlation with various external cluster mass proxies. Due to the limited area and evolving data quality, the sample is not intended to serve as a reference for cosmological applications, but to verify and validate the cluster workflow. Out of the full sample, 77 systems are potentially new to the literature. Overall, the Q1 cluster catalogue demonstrates a successful validation of the workflow ahead of the Euclid Data Release 1, based on the consistency of internal and external properties of Euclid-detected clusters.

• Laigle, C.; Gouin, C.; Sarron, F.; Quilley, L.; Pichon, C. & Kraljic, K. [Show all 345 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXVI. Galaxy shapes and alignments in the cosmic web. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554651. Full text in Research Archive Show summary

  Galaxy morphologies and shape orientations are expected to correlate with their large-scale environments, since they grow by accreting matter from the cosmic web and are subject to interactions with other galaxies. We extracted cosmic filaments in projection from the Euclid Quick Data Release 1 (covering 63.1 deg2) at 0.5 < z < 0.9 in tomographic slices of 170 comoving h−1 Mpc using photometric redshifts. Galaxy morphologies are accurately retrieved thanks to the excellent imaging resolution of the visible camera (VIS) of Euclid. The distribution of massive galaxies (M* > 1010 M⊙) in the projected cosmic web is analysed as a function of morphology measured from VIS data. Specifically, the 2D alignment of galaxy shapes with large-scale filaments is quantified as a function of Sérsic indices and masses. We find the known trend that more massive galaxies are closer to filament spines. At fixed stellar masses, morphologies correlate both with densities and distances to large-scale filaments. In addition, the large volume of this data set allows us to detect a signal indicating that there is a preferential alignment of the major axis of massive early-type galaxies along projected cosmic filaments. Overall, these results demonstrate our capabilities to carry out detailed studies of galaxy environments with Euclid, which will be extended to higher redshift and lower stellar masses with the future Euclid Deep Survey.

• Lines, N. E.P.; Collett, T. E.; Holloway, P.; Rojas, K.; Schuldt, S. & Metcalf, R. B. [Show all 304 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXII. From simulations to sky: Advancing machine-learning lens detection with real Euclid data. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202558456. Full text in Research Archive
• Lines, N. E.P.; Collett, T. E.; Walmsley, M.; Rojas, K.; Li, T. & Leuzzi, L. [Show all 353 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXVIII. The Strong Lensing Discovery Engine C – Finding lenses with machine learning. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554542. Full text in Research Archive
• Busillo, V.; Tortora, C.; Metcalf, R. B.; Nightingale, J. W.; Meneghetti, M. & Gentile, F. [Show all 315 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXI. LEMON – LEns MOdelling with Neural networks. Automated and fast modelling of Euclid gravitational lenses with singular isothermal ellipsoid mass profile. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554538. Full text in Research Archive Show summary

  The Euclid mission aims to survey around 14 000 deg2 of extragalactic sky, providing around 105 gravitational lens images. Modelling of gravitational lenses is fundamental to estimate the total mass of the lens galaxy, along with its dark matter content. Traditional modelling of gravitational lenses is computationally intensive and requires manual input. In this paper, we use a Bayesian neural network, LEns MOdelling with Neural networks (LEMON), to model Euclid gravitational lenses with a singular isothermal ellipsoid mass profile. Our method estimates key lens mass profile parameters, such as the Einstein radius, while also predicting the light parameters of foreground galaxies and their uncertainties. We validate LEMON’s performance on both mock Euclid datasets, real lenses observed with Hubble Space Telescope (HST) that have been degraded to match observations with the same depth of the Euclid Wide Survey, and real Euclid lenses, demonstrating the ability of LEMON to predict parameters of both simulated and real lenses. Results show promising accuracy and reliability in predicting the Einstein radius, mass and light ellipticities, effective radius, Sérsic index, lens magnitude, and unlensed source position for simulated lens galaxies. The application to real data, including the latest Quick Release 1 strong lens candidates, provides encouraging results in the recovery of the parameters for real lenses. We also verified that LEMON has the potential to accelerate traditional modelling methods, by giving to the classical optimiser the LEMON predictions as starting points, resulting in a speed-up of up to 26 times the original time needed to model a sample of gravitational lenses, a result that would be impossible with randomly initialised guesses. Moreover, LEMON can be used to cross-validate results from the traditional modelling methods, and thus has the potential to reduce the failure rate of the Euclid modelling pipeline. This work represents a significant step towards efficient, automated gravitational lens modelling, which is crucial for handling the large data volumes expected from Euclid.

• Li, T.; Collett, T. E.; Walmsley, M.; Lines, N. E.P.; Rojas, K. & Nightingale, J. W. [Show all 338 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXIX. The strong-lensing discovery engine D: Double-source-plane lens candidates. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554543. Full text in Research Archive
• Holloway, P.; Verma, A.; Walmsley, M.; Marshall, P. J.; More, A. & Collett, T. E. [Show all 346 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXX. The Strong Lensing Discovery Engine E – Ensemble classification of strong gravitational lenses: lessons for Data Release 1★. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554617. Full text in Research Archive Show summary

  The Euclid Wide Survey (EWS) is expected to identify in the order of 100 000 galaxy-galaxy strong lenses across 14 000deg2. The Euclid Quick Data Release (Q1) of 63.1deg2Euclid images provides an excellent opportunity to test our lens-finding ability, and to verify the anticipated lens frequency in the EWS. Following the Q1 data release, eight machine learning networks from five teams were applied to approximately one million images. This was followed by a citizen science inspection of a subset of around 100 000 images, of which 65% received high network scores, with the remainder randomly selected. The top scoring outputs were inspected by experts to establish confident (grade A), likely (grade B), possible (grade C), and unlikely lenses. In this paper we combine the citizen science and machine learning classifiers into an ensemble, demonstrating that a combined approach can produce a purer and more complete sample than the original individual classifiers. Using the expert-graded subset as ground truth, we find that this ensemble can provide a purity of 52 ± 2% (grade A/B lenses) with 50% completeness (for context, due to the rarity of lenses a random classifier would have a purity of 0.05% and the best machine learning network in this work achieved 7.3% purity for the same completeness). We discuss future lessons for the first major Euclid data release (DR1), where the big-data challenges will become more significant and will require analysing more than ∼300 million galaxies, and thus the time investment of both experts and citizens must be carefully managed.

• Gouin, C.; Laigle, C.; Sarron, F.; Bonnaire, T.; Sorce, J. G. & Aghanim, N. [Show all 340 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXV. The role of cosmic connectivity in shaping galaxy clusters. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554655. Full text in Research Archive Show summary

  The matter around galaxy clusters is distributed over several filaments, reflecting their positions as nodes in the large-scale cosmic web. The number of filaments connected to a cluster, i.e. its connectivity, is expected to affect the physical properties of clusters. Using the first Euclid galaxy catalogue from the Euclid Quick Release 1 (Q1), we investigated the connectivity of galaxy clusters and how it correlates with their physical and galaxy member properties. Around 220 clusters located within the three fields of Q1 (covering ∼63 deg2) were analysed in the redshift range 0.2  <  z  <  0.7. Due to the photometric redshift uncertainty, we reconstructed the cosmic web skeleton, and measured the cluster connectivity, in 2D projected slices with a thickness of 170 comoving h−1 Mpc and centred on each cluster redshift, by using two different filament finder algorithms on the most massive galaxies (M★ > 1010.3 M⊙). In agreement with previous measurements, we recovered the mass-connectivity relation independently of the filament detection algorithm, showing that the most massive clusters are, on average, connected to a larger number of cosmic filaments, consistent with hierarchical structure formation models. Furthermore, we explored the possible correlations between connectivities and two cluster properties: the fraction of early-type galaxies and the Sérsic index of galaxy members. Our result suggests that the clusters populated by early-type galaxies exhibit higher connectivity compared to clusters dominated by late-type galaxies. These preliminary investigations highlight our ability to quantify the impact of the cosmic web’s connectivity on cluster properties with Euclid.

• Cleland, C.; Mei, S.; Lucia, G. De; Fontanot, F.; Fu, H. & Lovell, C. C. [Show all 323 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXIX. The evolution of the passive-density and morphology-density relations between z = 0.25 and z = 1. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202554581.
• Fu, Y.; Bouwens, R.; Caputi, K. I.; Vergani, D.; Scialpi, M. & Margalef-Bentabol, B. [Show all 357 contributors for this article] (2026). Euclid Quick Data Release (Q1): XXXVIII. Euclid spectroscopy of quasars. 1. Identification and redshift determination of 3500 bright quasars. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 711. doi: 10.1051/0004-6361/202558490.
• Mamonova, E.; Herbst, K.; Kofman, V.; Ozgurel, Ozge; Kowalski, A. F. & Wedemeyer, S. [Show all 7 contributors for this article] (2026). Stellar-flare-driven evolution of primordial early exo-Earth atmospheres: Insights from the young M dwarf flare model. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 710. doi: 10.1051/0004-6361/202558763. Full text in Research Archive Show summary

  Context . M dwarfs are key targets for terrestrial exoplanet studies, with prospects for atmospheric spectroscopy. However, strong stellar magnetic activity and frequent flaring require modelling efforts to assess their impact on planetary atmospheres. Aims . We investigated one year of atmospheric chemical evolution of a young exo-Earth orbiting an active M dwarf by coupling our young M dwarf flare (YMDF) model of stellar activity with the photochemical kinetic code VULCAN. Methods . The YMDF model provides time-resolved spectral energy distributions for high- and low-energy electron beam–driven flares, which are used as external radiative inputs to VULCAN to compute the time-dependent photochemistry and kinetics for different primordial atmospheric scenarios. Results . We present the impact of stellar flares on atmospheres with a varying water vapour content, ranging from a plausible primordial atmosphere with solar abundances (representative of a planet-forming region in a dissipating protoplanetary disk) to an extreme water-steam atmosphere with other species reduced to trace abundances. This was explored across several configurations: variable flux in the YMDF model, the previous model representing an active but older M dwarf with 10 or 400K additional bottom boundary heat flux, and a constant stellar flux model. Conclusions . Our study suggests that compared to the previous model, the YMDF model produces synthetic flares that exert significantly greater stress on primordial atmospheres, regardless of the water-vapour content. Increased activity and prevalence of mid-size flares has the potential to induce permanent changes in atmospheric mixing ratios, especially in species with low abundances.

• Sand, Mats Ola; Noraz, Quentin; Aulanier, Guillaume; Martinez-Sykora, Juan; Carlsson, Mats & Voort, Luc Rouppe van der (2026). Shock-induced magnetic reconnection driving Ellerman bomb emission and a spicule. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202557602. Full text in Research Archive Show summary

  Context. The formation mechanism for the dynamic type II spicules has remained elusive for many years. Their dynamical behaviour has long been linked to magnetic reconnection, yet no conclusive evidence has been provided. However, one recent observational study found signs of magnetic reconnection, as traced by Ellerman bombs (EBs), at the footpoints of many spicules. The triggering of EBs is generally linked to magnetic reconnection due to flux emergence and convective motions in the photosphere. Aims. We aim to explore whether we can connect EBs to type II spicules, and determine to what extent we can use EBs as an observational proxy to probe magnetic reconnection in this dynamic. We also aim to provide further insight into the mechanisms that trigger EBs. Methods. We used a simulation run with the radiative magnetohydrodynamics code Bifrost to track spicules and study the physical processes underlying their formation. To detect EBs and classify the spicules, we synthesised the chromospheric H α spectral line using the multilevel radiative transfer code RH 1.5D. We also traced shocks and current sheets to decipher the origin of EBs and spicules. We selected one type II spicule with a strong EB near its footpoint and studied their formation in detail. Results. A magnetoacoustic shock advects the magnetic field lines towards an oppositely directed ambient field, creating a current sheet. The current sheet accelerates dense plasma via a whiplash effect generated by magnetic reconnection into the inclined ambient field, launching the spicule. Several EB profiles trace shock- and magnetic-reconnection-induced dynamics during this process at the spicule footpoint. Conclusions. We present a new EB triggering mechanism in which a shock-induced current sheet reconnects, triggering an EB in the lower solar atmosphere. The shock-induced current sheet generates the upwards propagation of a type II spicule via reconnection outflows. These results provide a plausible physical origin for the recently observed connection between EBs and spicules.

• Cozzo, G.; Testa, Paola; Sykora, Juan Martinez; Reale, F.; Pagano, P. & Rappazzo, F. [Show all 16 contributors for this article] (2026). 3D MHD Simulations of Coronal Loops Heated via Magnetic Braiding. I. Continuous Driving. The Astrophysical Journal (ApJ). ISSN 0004-637X. 998(1). doi: 10.3847/1538-4357/ae2d56. Full text in Research Archive Show summary

  The nature and detailed properties of the heating of the million-degree solar corona are important issues that are still largely unresolved. Nanoflare heating might be dominant in active regions and quiet Sun, although direct signatures of such small-scale events are difficult to observe in the highly conducting, faint corona. The aim of this work is to test the theory of coronal heating by nanoflares in braided magnetic field structures. We analyze a 3D MHD model of a multistrand flux tube in a stratified solar atmosphere, driven by twisting motions at the boundaries. We show how the magnetic structure is maintained at high temperature and for an indefinite time, by intermittent episodes of local magnetic energy release due to reconnection. We synthesize the optically thin emission with SDO/AIA and MUSE and compare the synthetic observations with the intrinsic coronal plasma properties, focusing on the response to impulsive coronal heating. Currents’ buildup and their impulsive dissipation into heat are also investigated through different runs. In this first paper, we describe the proliferation of heating from the dissipation of narrow current sheets in realistic simulations of braided coronal flux tubes at unprecedented high spatial resolutions.

• Sen, Samrat; Sukarmadji, A. Ramada C.; Nóbrega-Siverio, D.; Moreno-Insertis, F.; Martinez-Sykora, Juan & Antolin, Patrick (2026). Extreme-ultraviolet Synthesis of Nanojet-like Ejections due to Coalescing Flux Ropes. Astrophysical Journal Letters. ISSN 2041-8205. 997(2). doi: 10.3847/2041-8213/ae35fc. Full text in Research Archive Show summary

  Detection and characterization of small-scale energetic events such as nanoflares and nanojets remain challenging owing to their short lifetimes, small spatial extent, and relatively low energy release, despite their potential role in coronal heating. Recent observations have identified nanojets as small-scale (length ≲6.6 Mm, width ≲1 Mm), fast (∼ few 100 km s−1), and short-lived (≲30 s) ejections associated with nanoflare-scale energies, providing evidence of magnetic reconnection at small spatial scales. However, the lack of synthetic diagnostics has limited the connection between magnetohydrodynamic (MHD) models and observations. In this Letter, we present synthetic observations of the coalescence of two flux ropes, leading to nanojet-like signatures from a numerical model obtained with the MPI-AMRVAC code. We report synthetic observables in extreme-ultraviolet lines compatible with existing instruments such as Solar Dynamics Observatory/Atmospheric Imaging Assembly and the upcoming Multi-slit Solar Explorer mission and compare the synthetic observables with an existing observation of nanojets. The synthetic diagnostics of the emissivity maps, Doppler velocity, and thermal and nonthermal line broadening produce key observational properties, suggesting a plausible 3D scenario for nanojet generation where tiny flux ropes reconnect within loops. Our results provide predictions for the detectability of nanojets with current and future spectroscopic facilities and establish a bridge between MHD modeling and observations.

• Alanazi, Nawal & Fletcher, Lyndsay (2026). Systematic time evolution of the horizontal magnetic field around the polarity inversion line in flares. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 548(1). doi: 10.1093/mnras/stag573. Full text in Research Archive Show summary

  The evolution of the horizontal magnetic field (⁠Bh⁠) at the photosphere during solar flares provides insights into how the photospheric field responds to coronal energy release. In this work, we investigate the spatial and temporal changes in the behaviour of Bh using unsupervised machine learning, namely k-shape clustering, to identify regions exhibiting step-like changes in Bh⁠. We analysed 35 solar flares spanning X-, M-, and C-class events, using 135 s-cadence vector magnetograms obtained from the Solar Dynamics Observatory (SDO) Helioseismic and Magnetic Imager, and 1600 Å images from the SDO Atmospheric Imaging Assembly. Our results show that (i) in 14 out of 35 flares examined, stepwise increases in the horizontal magnetic field are concentrated near the region of polarity inversion and show spatial and temporal association with the onset of flare ribbon emission; (ii) the peak rate of increase of Bh generally lags the onset of UV ribbon brightening by several minutes and this time lag shows no statistically significant dependence on flare class; (iii) the lag increases with increasing distance from the region of polarity inversion; and (iv) decreases in Bh are more scattered across the active region and its penumbra. The increase of Bh progressing outwards from the region of polarity inversion provides strong observational evidence of the photospheric response to the sequential relaxation and downward contraction of coronal magnetic fields during energy release, sometimes identified as a coronal ‘implosion’.

• Hoque, Ariful; Baug, Tapas; Guzman, Estrella; Lopez, Manuel Fernandez; Liu, Tie & Garay, Guido [Show all 28 contributors for this article] (2026). The ALMA-QUARKS Survey: Evidence of an Explosive Molecular Outflow in IRAS 15520-5234. The Astrophysical Journal (ApJ). ISSN 0004-637X. 1001(2). doi: 10.3847/1538-4357/ae5224. Full text in Research Archive Show summary

  We present a study of the massive protocluster IRAS 15520–5234, which displays evidence of an explosive molecular outflow that unleashed a kinetic energy of at least 1048 erg. The protocluster contains 16 dense cores detected in the Atacama Large Millimeter/submillimeter Array (ALMA) band 6 continuum emission maps, having masses in the range from 0.2 to 11.0 M⊙. Our analysis of CO (2−1) emission reveals 28 well-collimated outflow fingers, the majority of which follow a Hubble–Lemaître velocity law. The outflow fingers show no preferred orientation in the plane of sky and emerge from a common center of origin. We estimate the total mass, momentum, and kinetic energy of the outflow fingers and find that the values are at least 1 order of magnitude higher than the typical bipolar outflows associated with massive protostars. The morphology and kinematics of the outflow fingers suggest that the outflow associated with IRAS 15520–5234 is explosive in nature. We calculate the dynamical age of the explosive event to be approximately 6550 yr. Additionally, we estimate the frequency of such explosive outflows in the Galaxy, which is one event every 83 yr. Finally, we speculate that the rearrangement of masses within the massive protocluster and the dynamical interaction among the massive cores may result in the formation of such an energetic event.

• Chen, Guoyin; Guo, Yang; Hao, Qi & Ding, Mingde (2026). A Novel Way for Extrapolating the Solar Magnetic Field with PRO-GAN. The Astrophysical Journal (ApJ). ISSN 0004-637X. 1001(1). doi: 10.3847/1538-4357/ae5108. Full text in Research Archive Show summary

  The magnetic field in the solar corona cannot be observed routinely at present. Nonlinear force-free field (NLFFF) extrapolation serves as a crucial method for modeling the coronal magnetic configuration. We present a novel physics- reinforced generative adversarial network (PRO-GAN), an integrated deep learning framework designed to efficiently transform solar potential fields into corresponding NLFFFs while enforcing physical constraints to improve the result. This dual-phase framework synthesizes multifaceted integration, including the data-driven mapping relation acquisition, the constraint of physics equations, and supervision by a numerical solver. On the other hand, by incorporating a gradient penalty reinforced by a numerical method during the physics reinforcement training process, this approach resolves the inherent gradient conflicts in physics-informed neural networks from a novel perspective. The validity and efficiency of PRO-GAN have also been verified on the dataset and two analytic models.

• Wang, Ya; Mulay, Sargam M. & Fletcher, Lyndsay (2026). Extremely Energetic Extreme-ultraviolet Late Phase of a Pair of C-class Flares Caused by a Noneruptive Sigmoid. The Astrophysical Journal (ApJ). ISSN 0004-637X. 997(2). doi: 10.3847/1538-4357/ae2ad7. Full text in Research Archive Show summary

  The extreme-ultraviolet (EUV) late phase (ELP) is the second increase of the irradiance of warm coronal lines during solar flares, and has a crucial impact on the Earth’s ionosphere. In this paper, we report on the extremely energetic ELP of a pair of C-class flares (SOL2012-06-17T17:26:11) observed on 2012 June 17 in NOAA active region 11504 by the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory. The light curves integrated over the flaring region show that a factor of 4.2 more energy is released in the “warm-temperature” (2–3 × 10^6 K) passbands (e.g., AIA 335 Å) during the late phase than during the main peaks. The origin of the emission in this extremely energetic ELP is a noneruptive sigmoid situated in a multipolar magnetic field configuration, which is rapidly energized by C-class flares. The sigmoid plasma appears to reach temperatures in excess of 10^7 K, before cooling to produce the ELP emission. This is seen in high-temperature passbands (e.g., AIA 131 Å) and by using differential emission measure analysis. Magnetic extrapolations indicate that the sigmoid is consistent with formation by magnetic reconnection between previously existing J-shaped loops. The sigmoid experienced fast and slow cooling stages, both of which were dominated by conductive cooling. The estimated total cooling time of the sigmoid is shorter than the observed value. So, we proposed that the noneruptive sigmoid, heated by continuous magnetic reconnection, leads to the extremely energetic ELP.

• Bura, Annu; Siverio, Daniel Elias Nóbrega; Samanta, Tanmoy & Joshi, Jayant (2026). On the Origin of Coronal Picoflare Jets. The Astrophysical Journal (ApJ). ISSN 0004-637X. 1000(1). doi: 10.3847/1538-4357/ae48e6. Full text in Research Archive Show summary

  Small-scale jet-like eruptions, such as picoflare jets and jetlets, are recognized as potential contributors to coronal heating and solar wind acceleration, yet their physical origin is still not fully established. Using ultra-high-resolution extreme ultraviolet imaging datasets from the Extreme Ultraviolet Imager on board the Solar Orbiter mission, we investigate tiny coronal jets observed off-limb in the Sun’s polar regions. Visual inspection reveals that the majority of these jets exhibit distinct morphological features, including a bright spire accompanied by a dark, eruptive jet component. We analyzed 11 of these jets in detail and found that their spatial and temporal scales are comparable to previously reported jetlets, while their kinetic energies are two to three orders of magnitude lower, placing them in the picoflare regime. The bright and dark components show distinct dynamics, with the dark structures generally displaying lower speeds. A comparison with coordinated Interface Region Imaging Spectrograph data and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory data, together with 2.5D radiative-MHD simulations performed with the Bifrost code, reveals a one-to-one morphological correspondence between the dark counterparts and cool chromospheric surges accompanying the bright jet spire. This association suggests that flux emergence and magnetic reconnection at low atmospheric heights may produce coupled bright–dark structures, providing a plausible mechanism for the generation of picoflare jets. Our results demonstrate Solar Orbiter’s ability to resolve the dynamics of small-scale jets and place new constraints on their origin.

• Faber, Jonas Thoen; Joshi, Reetika; Voort, Luc Rouppe van der; Wedemeyer, Sven; Øyre, Eilif Sommer & Poquet, Ignasi J. Soler [Show all 7 contributors for this article] (2026). Fine details in solar flare ribbons: Statistical insights from observations with the Swedish 1-m Solar Telescope. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556738. Full text in Research Archive Show summary

  Context. Flare ribbons serve as chromospheric footprints of energy deposition resulting from particle acceleration during magnetic reconnection. Their fine-scale structure provides a valuable tool for probing the dynamics of the flare reconnection process. Aims. Our goal is to investigate the fine-scale structure of flare ribbons through multiple observations of flares, utilising data obtained from the Atmospheric Imaging Assembly (AIA) and the Swedish 1-m Solar Telescope (SST). Methods. The aligned AIA and SST datasets for the three solar flares were used to examine their overall morphology. The SST datasets were specifically used to identify fine-scale structures within the flare ribbons. For spectroscopic analysis of these fine-scale structures, we applied machine-learning methods ( k -means clustering) and Gaussian fitting. Results. Using k -means, we identified elongated features in the flare ribbons, termed as ‘riblets’, which are short-lived and jet-like small-scale structures that extend as plasma columns from the flare ribbons. Riblets are more prominent near the solar limb and represent the ribbon front. Riblet widths are consistent across observations, ranging from 110−310 km (0.15−0.41), while vertical lengths span 620−1220 km (0.83−1.66), with a potential maximum of 2000 km (2.67), after accounting for projection effects. Detailed H β spectral analysis reveals that riblets exhibit a single, redshifted emission component, with velocities of 16−21 km s −1 , independent of the viewing angle. Conclusions. Our high-resolution observations of the three flare ribbons show that they are not continuous structures, but rather are composed of vertically extended, fine-scale substructures. These irregular features indicate that the reconnection region is not a smooth, laminar current sheet, but rather a fragmented zone filled with magnetic islands (blobs or riblets), consistent with the theory of patchy reconnection within the coronal current sheet.

• Zhang, Fan; Lani, Andrea & Poedts, Stefaan (2026). On energy consistency of intermediate states in HLL-type MHD Riemann solvers. Journal of Computational Physics. ISSN 0021-9991. 553. doi: 10.1016/j.jcp.2026.114724. Full text in Research Archive Show summary

  Approximate Riemann solvers are widely used for solving hyperbolic conservation laws, including those of magnetohydrodynamics (MHD). However, due to the nonlinearity and complexity of MHD, obtaining accurate and robust numerical solutions to MHD equations is non-trivial, and it may be challenging for an approximate MHD Riemann solver to preserve the positivity of scalar variables, particularly when the plasma β is low. As we have identified that the inconsistency between the numerically calculated magnetic field and magnetic energy may be at least partly responsible for the loss of positivity of scalar variables, we propose a consistency condition for calculating the intermediate energies within the Riemann fan and implement it in HLL-type MHD Riemann solvers, thereby alleviating erroneous magnetic field solutions that break scalar positivity. In addition, (I) for the HLLC-type scheme, we have designed a revised two-state approximation, specifically reducing numerical error in magnetic field solutions, although sacrificing the contact-resolving capability, and (II) for the HLLD-type scheme, we replace the constant total pressure assumption by a three-state assumption for the intermediate thermal energy, which is more consistent with our other assumptions. The proposed schemes perform better in numerical examples with low plasma β. Moreover, we explained the energy error introduced during time integration.

• Zbinden, Jonas; Rodríguez, Jaime de la Cruz; Baso, Carlos José Díaz & Kleint, Lucia (2026). Insights into penumbral formation from high-resolution photospheric and chromospheric observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556465. Full text in Research Archive Show summary

  Context. Penumbra formation occurs within hours and remains not fully understood. Two competing theories suggest either a top-down mechanism, whereby the chromospheric magnetic field becomes more horizontal, or a bottom-up process involving flux emergence near the sunspot umbra. Aims. We investigate penumbra formation and test both hypotheses by inferring and analyzing the full magnetic field from the photosphere to the chromosphere above a forming sunspot. Methods. We applied Milne-Eddington, weak field approximation, and the multiatom non-local thermodynamic equilibrium inversion code STiC to obtain the model atmosphere that reproduces the Stokes profiles of Fe I 6302 Å and Ca II 8542 Å observed at the Swedish Solar Telescope, complementing the analysis with images taken in Hβ. Results. A penumbra formed only in the southern sector of a sunspot in AR 13010 on May 15, 2022. We find that a preexisting, strong (> 500 G) and nearly horizontal magnetic field in the chromosphere is a key prerequisite. Penumbral filaments grew southward where localized flux emergence was observed, accompanied by strong redshifts (2–3 km s−1) throughout the atmospheric layers. These redshifts likely represent field-aligned horizontal flows, possibly combining the onset of the Evershed flow and a siphon-like flow driven by field-aligned pressure gradients. Conclusions. Our findings indicate that penumbra formation results from both top-down and bottom-up processes. While strong horizontal chromospheric magnetic fields are a necessary boundary condition, flux emergence initiates filament growth. The fallen flux tube model is ruled out, based on clear bottom-up signatures, though it remains unclear why the chromospheric and photospheric fields become more horizontal concurrently. Extended spectropolarimetric data are needed to get a deeper insight.

• Cherry, George; Gudiksen, Boris Vilhelm; Finley, Adam J. & Noraz, Quentin (2026). Decomposing wave activity in the solar atmosphere: Shocks, jets, and swirls in the quiet Sun. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202557226. Full text in Research Archive Show summary

  Context. There remains much mystery about how wave energy in the photosphere can be transferred sufficiently upward through the solar atmosphere to contribute to coronal heating. In light of a plethora of theoretical and idealised studies, we must complement our understanding with realistic and self-driven simulations in order to confidently quantify such contributions. Aims. In this study we aim to connect wave drivers in the photosphere with their impact on the low corona, transitions, and dissipation mechanisms. We analyse the effects of the presence of twisted magnetic features and vortical flows on the transport of such wave modes as the structures evolve. Methods. We adopted the most significant frequency (MSF) decomposition method to trace wave activity through a 3D realistic quiet Sun simulation. We focused on vertical and temporal evolution, identifying wave sources and shifts in the dominant modes. Results. We identify two frequencies, at 3.5 and 5 mHz, that connect oscillations in the upper convection zone to the dynamics in the solar atmosphere. We see distinct differences in the absence and presence of swirling structures on the upward propagation of these oscillations. Furthermore, we validate the use of the highest MSF bin as a proxy for the location of shocks in the chromosphere and use the results to understand the connection between shocks and the propagation of oscillations in the upper atmosphere. We discuss the relation of energy transfer via shocks, mode conversion, and jets. Finally, we find the contribution of 3.5 and 5 mHz signals to the overall wave power in the domain to be significant, up to 50%.

• Ornig, Sascha & Carlsson, Mats (2026). Predicted white-light solar flare emission from the F-CHROMA grid of models. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556302. Full text in Research Archive Show summary

  Context. Much of a solar flare’s energy is thought to be released in the continuum. The optical continuum, white light (WL), is of special interest since it can be observed from the ground. Aims. We aim to investigate the prevalence of WL emissions in simulations of purely electron beam-driven solar flares, what determines the occurrence of these enhancements, and the underlying causes. Methods. We utilized the F-CHROMA grid of flare simulations created using the radiative hydrodynamics code RADYN. We probed the spectral index, total energy, and low-energy cutoff to draw conclusions about their relationships to the WL intensity. Furthermore, we calculated the 6684 Å continuum intensities as well as the Balmer and the Paschen ratios. Finally, we analyzed two particular cases, one with a high 6684 Å intensity and one with a large Balmer ratio, to determine the dominant mechanisms in these simulations. Results. Of the 84 flares included in the F-CHROMA grid, 33 show WL intensity enhancements that exceed 0.1% relative to the pre-flare level. We conclude that with the parameters presented in the F-CHROMA grid, purely electron beam-driven simulations of solar flares are not able to reproduce observed WL enhancements, as the maximum enhancements in the grid are below 4%, which is significantly lower than observational values. The total energy (which is correlated with the maximum beam flux) is the main factor for deciding whether excess WL emissions will be detectable or not. There is a linear relationship between the Balmer (and Paschen) ratio and the relative continuum increase. Both case studies show that during the time of maximum WL excess, hydrogen ionization and subsequent recombination in an optically thin medium is the dominant mechanism for WL continuum emission enhancements in these electron beam-driven atmospheres. Increased H− emission in the photosphere, as a result of radiative backwarming, becomes dominant during the declining phase of WL emissions in both case studies. We confirm the inability to reproduce the characteristics of type II WL flares, namely, the appearance of photospheric continuum enhancements without prior chromospheric emission increases and the lack of a clear Balmer jump, with the F-CHROMA grid.

• Devi, Pooja; Chandra, Ramesh; Miteva, Rositsa; Ibrahim, M. Syed & Joshi, Kamal (2026). Band splitting in m-Type II radio bursts and their role in coronal parameter diagnostics. Advances in Space Research. ISSN 0273-1177. doi: 10.1016/j.asr.2025.10.006. Full text in Research Archive Show summary

  Type II radio bursts are signatures of shock waves generated by solar eruptions, observed at radio wavelengths. While metric (m) type II bursts originate in the lower corona, their longer-wavelength (up to kilometers) counterparts extend into interplanetary space. A rare but valuable feature observed in some type II bursts is band splitting in their dynamic spectra, which provides crucial insights into physical parameters such as shock speed, Alfvén Mach number, Alfvén speed, and coronal magnetic field strength (B). In this study, we investigate band-splitting in 44 m-type II radio bursts observed by the Radio Solar Telescope Network during solar cycle 24 (2009–2019). These events exhibit splitting in both fundamental and harmonic bands and are analyzed under both perpendicular and parallel shocks. All events are associated to solar flares and 41 (93 %) with the coronal mass ejections. Shock speeds, derived using a hybrid coronal density model proposed by Vršnak et al. (2004), range from ~ 350 to 1727 km/s. The relative bandwidth (BDW) of the split bands remains constant with frequency and height. Alfvén Mach numbers indicate moderate shock strength (1.06–3.38), while Alfvén speeds and B vary from 230–1294 km/s and 0.48–7.13 G, respectively. Power-law relationships are established as BDW ∝ f_L^{-0.4} and BDW ∝ R^{~1} , while the coronal magnetic field decreases with height as B ∝ R^{~-3}. These results enhance our understanding of shock dynamics and magnetic field structures in the solar corona.

• Mamonova, Elena; Kowalski, A.F.; Herbst, Konstantin; Wedemeyer, Sven & Werner, Stephanie C. (2026). Young M-dwarfs flare activity model: Towards better exoplanetary atmospheric characterisation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556844. Full text in Research Archive Show summary

  Context. Stellar flares can significantly influence the atmospheres and habitability of orbiting exoplanets, especially around young and active M dwarfs. Understanding the temporally and spectrally resolved activity of such stars is essential for assessing their impact on planetary environments. Aims. We aim to examine in detail state-of-the-art concepts of flare models to identify what is missing in our understanding of energy deposition during the flare event. By comparing synthetic and observed flare spectra, we seek to determine the modelling frameworks best suited for representing flare energetics and spectral far-ultraviolet features while providing a foundation for investigating flare impacts on exoplanet atmospheres. Methods. In this work, we built the Young M dwarf flare (YMDF) model utilising the combination of radiative-hydrodynamic (RHD) stellar atmosphere models with a high- and low-energy electron beam and corresponding synthetic observables. These models are based on physical principles and were validated with solar and stellar observations. Results. The newly developed YMDF model reproduces the observed continuum rise in both the TESS photometric band and the FUV-A spectral range. Furthermore, the flare distributions generated within this framework show consistency with those observed in our sample of stars. Conclusions. We have developed the YMDF model as a tool to reproduce the time-dependent spectra of flaring young M dwarfs, providing a physically motivated description of their spectral and temporal evolution during flare events.

• Reche, Antonio; Yabar, Adur Pastor & Marin, Ana Belen Grinon (2025). Polar faculae and their relationship to the solar cycle. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 699. doi: 10.1051/0004-6361/202554578. Full text in Research Archive Show summary

  Context. The study of magnetic activity in the Sun's polar regions is essential for understanding the solar cycle. However, measuring polar magnetic fields presents challenges due to projection effects and their intrinsically weak magnetic field strength. Faculae, bright regions on the visible solar surface associated with increased magnetic activity, offer a valuable proxy for measuring polar fields. Aims. This research aims to analyze the magnetic activity of the Sun's polar regions through the use of polar faculae. Methods. A neural network model (U-Net) was employed to detect polar faculae in images from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). The model was trained on synthetic data, eliminating the need for manual labeling, and was used to analyze 14 years of data from May 2010 to May 2024. Results. The U-Net model demonstrates superior performance and efficiency over existing methods, enabling automated large-scale studies. We find that polar faculae numbers exhibit cyclical behavior with distinct minima and maxima, showing similar patterns between poles but with notable temporal delays (south pole: minimum early 2014, maximum late 2016; north pole: minimum late 2014, maximum mid-2019). Polar faculae magnetic fields remain consistent in magnitude (∼±75 G) across both poles and throughout the solar cycle. A strong linear correlation was found between the polar faculae count and the overall polar magnetic field strength. The spatio-temporal evolution reveals systematic migration of field polarity reversals from mid-latitudes toward the poles at rates of 3−8 m/s. During solar minimum, we observe a small relative increase in stronger-field faculae compared to solar maximum, suggesting either the coexistence of two magnetic distributions or subtle solar cycle dependence in faculae properties.

• Kumar, A; Howson, T A; Pagano, P & Moortel, Ineke de (2025). Alfvén wave propagation, reflection, and trapping in the solar wind. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 541(4), p. 2843–2852. doi: 10.1093/mnras/staf1189. Full text in Research Archive Show summary

  Alfvén waves are known to be important carriers of magnetic energy that could play a role in coronal heating and/or solar wind acceleration. As these waves are efficient energy carriers, how they are dissipated still remains one of the key challenges. Using a series of 1.5D magnetohydrodynamic (MHD) simulations, we explore wave energy trapping associated with field-aligned density enhancements. We examine the parameters that govern the wave reflection and trapping. The goal of our simulations is to find optimal conditions for wave trapping, which would ultimately promote the energization of the solar atmosphere. In agreement with previous studies, we find that maximum wave reflections happen only for a narrow range of density enhancement widths, namely when it is comparable to the Alfvén wave wavelength. In our paper, we explain this scale-selectivity using a semi-analytical model that demonstrates the importance of wave interference effects. As expected, we find that spatially extended regions of density inhomogeneities favour enhanced wave reflection and trapping. However, wave interference causes saturation of the reflected energy for very extended regions of varying density.

• Lomazzi, Paul; Rouillard, Alexis P.; Lavarra, Michael A.; Poirier, Nicolas Aurelien Corentin; Blelly, Pierre-Louis & Dakeyo, Jean-Baptiste [Show all 10 contributors for this article] (2025). A Parametric Study of Solar Wind Properties and Composition Using Fluid and Kinetic Solar Wind Models. The Astrophysical Journal (ApJ). ISSN 0004-637X. 984(2). doi: 10.3847/1538-4357/adc2f6. Full text in Research Archive Show summary

  The physical processes in the solar corona that shape the solar wind remain an active research topic. Modeling efforts have shown that energy and plasma exchanges near the transition region play a crucial role in modulating solar wind properties. Although these regions cannot be measured in situ, plasma parameters can be inferred from coronal spectroscopy and ionization states of heavy ions, which remain unchanged as they escape the corona. We introduce a new solar wind model extending from the chromosphere to the inner heliosphere, capturing thermodynamic coupling across atmospheric layers. By including neutral and charged particle interactions, we model the transport and ionization processes of the gas through the transition region and corona and into the solar wind. Instead of explicitly modeling coronal heating, we link its spatial distribution to large-scale magnetic field properties. Our results confirm that energy deposition strongly affects wind properties through key mechanisms involving chromospheric evaporation, thermal expansion, and magnetic flux expansion. For sources near active regions, the model predicts significant solar wind acceleration, with plasma outflows comparable to those inferred from coronal spectroscopy. For winds from large coronal holes, the model reproduces the observed anticorrelation between charge state and wind speed. However, the predicted charge state ratios are overall lower than observed. Inclusion of a population of energetic electrons enhances both heavy ion charge states and solar wind acceleration, improving agreement with observations.

• Larsen, J. R.; Lundkvist, M. S.; Davies, G. R.; Nielsen, M. B.; Ludwig, H.-G. & Zhou, Yixiao [Show all 8 contributors for this article] (2025). Granulation signatures in 3D hydrodynamical simulations: Evaluating background model performance using a Bayesian nested sampling framework. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 701. doi: 10.1051/0004-6361/202555661. Full text in Research Archive Show summary

  Context. Understanding the granulation background signal is vital when interpreting the asteroseismic diagnostics of solar-like oscillators. Various descriptions exist throughout literature for modelling the surface manifestation of convection, with the choice of description affecting our interpretations. Aims. We aim to evaluate the performance of and preference for various granulation background models for a suite of 3D hydrodynamical simulations of convection across the Hertzsprung-Russell diagram, thereby expanding the number of simulations and coverage of parameter space for which such investigations have been made. Methods. We took a statistical approach by considering the granulation signatures in power density spectra of 3D hydrodynamical simulations, in which no biases or systematics of observational origin are present. To properly contrast the performance of the background models, we developed a Bayesian nested sampling framework for model inference and comparison. This framework was subsequently extended to real stellar data using the solar analogue KIC 8006161 (Doris) and the Sun. Results. We find that multi-component models are consistently preferred over a single-component model, with each tested multicomponent model demonstrating merit in specific cases. This occurs for simulations with no magnetic activity, ruling out stellar faculae as the sole source of the second granulation component. Similar to a previous study, we find that a hybrid model with a single overall amplitude and two characteristic frequencies performs well for numerous simulations. Additionally, a tentative third granulation component beyond the value of ν max is seen for some simulations, but its potential presence in observations requires further study. Conclusions. Studying the granulation signatures in these simulations paves the way for studying real stars with accurate granulation models. This deeper understanding of the granulation signal may lead to complementary methods to existing algorithms for determining stellar parameters, with the goal of providing an independent radius estimate for stars where oscillations are not observable.

• Howson, T. A.; Moortel, Ineke de & Priest, E. R. (2025). Coronal heating driven by random foot point motions: the effects of magnetic field topology. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 539(2), p. 1820–1833. doi: 10.1093/mnras/staf548. Full text in Research Archive Show summary

  We investigate the effects of magnetic field geometry on energy injection and dissipation, current sheet formation, magnetic reconnection rates and plasma dynamics in the solar corona when energized by random foot point motions. Using a series of 3D magnetohydrodynamics simulations, we compared the effects of high- and low-amplitude random velocity drivers acting on two different initial magnetic fields; (a) a uniform field case and (b) a tectonics case, in which the field is anchored in localized flux patches. In all simulations, the imposed drivers stress the field, generating small scales and leading to energy dissipation through Ohmic and viscous heating. The rates of energy injection, current formation, magnetic reconnection, and the associated energy dissipation are higher when the field is concentrated in flux patches at the simulation boundary. As such, expected heating rates are larger with a tectonics field. However, when high-amplitude driving is imposed on an initially uniform field, flux patches spontaneously form as the field evolves to become a tectonics field. As a result, energy injection and dissipation rates converge to become independent of the initial field state in the high-amplitude cases. As coronal field foot points are typically embedded in concentrated flux patches, we confirm that tectonics is a viable model for coronal heating. Unsurprisingly, we find that the nature of an initial field only influences heating rates as long as this field persists. However, whether solar atmospheric heating is dominated by transverse foot-point motions, as considered here, or by flux emergence events remains an unanswered question.

• Wang, H. P.; Poedts, S.; Lani, A.; Linan, L.; Baratashvili, T. & Zhang, F. [Show all 11 contributors for this article] (2025). Time-evolving coronal modelling of the solar maximum around the solar storms in May 2024 by COCONUT. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202555760. Full text in Research Archive Show summary

  Context. Time-evolving magnetohydrodynamic (MHD) coronal models driven by a sequence of time-evolving photospheric magnetograms deliver more realistic results than traditional quasi-steady-state models constrained by a static magnetogram. The fully implicit time-evolving coronal model COCONUT performs efficiently enough for real-time coronal simulations during solar minimum. Significant challenges persist in modelling the more complex coronal evolutions of solar maximum scenarios, however. Aims. During solar maxima, the coronal magnetic field is more complex and stronger, and coronal structures evolve more rapidly than during solar minima. Consequently, time-evolving MHD coronal modelling of solar maxima often struggles with poor numerical stability and low computational efficiency. We enhanced the numerical stability of the time-evolving coronal model COCONUT to mitigate these issues with the aim to evaluate the differences between the time-evolving and quasi-steady-state coronal simulation results, and to assess the impact of the spatial resolution on global MHD coronal modelling of solar maxima. Methods. After enhancing the positivity-preserving property of the time-evolving coronal model COCONUT, we employed it to simulate the evolution of coronal structures from the solar surface to 0.1 AU in an inertial coordinate system over two Carrington rotations around the solar storms in May 2024. These simulations were performed on unstructured geodesic meshes containing 6.06, 1.52, and 0.38 million (M) cells to assess the impact of grid resolution. We also conducted a quasi-steady-state coronal simulation that treated the solar surface as a rigidly rotating spherical shell to demonstrate the impact of the emergence and cancellation of the magnetic flux in global coronal simulations. A comparison with observations further validated the reliability of the efficient time-evolving coronal modelling technique. Results. We demonstrate that incorporating the evolution of the magnetic field in the inner boundary conditions can significantly improve the fidelity of global MHD coronal simulations around a solar maximum. A simulated magnetic field strength using a refined mesh with 6.06 M cells can be stronger by more than 40% than that in a coarser mesh with 0.38 M cells. A time step of 5 minutes and a mesh containing 1.5 M cells can effectively capture the evolution of large-scale coronal structures and small-sized dipoles. Thus, the fully implicit time-evolving model COCONUT shows promise for accurately conducting real-time global coronal simulations of solar maxima. This makes it suitable for practical applications such as daily space-weather forecasting.

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• Mylius-Kroken, Johan; Wally, Youssef; Mota, David & Wickstrøm, Kristoffer (2026). Hyperbolic Representation Learning for Astrophysical Photometric Survey Data. doi: https:/openreview.net/forum?id=hGV1cIU2N1&referrer=%5BAuthor%20Console%5D%28%2Fgroup%3Fid%3Dthecvf.com%2FECCV%2F2026%2FWorkshop%2FBEW%2FAuthors%23your-submissions%29. Full text in Research Archive Show summary

  We compare unsupervised representation learning on the miniJPAS narrow-band photometric survey using three architecturally identical autoencoders whose latent spaces differ only in geometry: Euclidean $\mathbb{R}^{32}$ and two variants on the Lorentz hyperboloid $\mathbb{H}^{32}$. The hyperbolic choice is motivated by Gromov $\delta$-hyperbolicity analysis, which confirms that the miniJPAS feature space has intrinsically tree-like geometry. Training on a reconstruction objective without label supervision, the Lorentzian models produce qualitatively different embeddings: stars concentrate in compact regions near the Poincar\'{e} disc boundary, and an emergent photometric redshift gradient is visible across all three latent spaces, though the embedding does not yet exhibit the radial, tree-like organisation that hyperbolic geometry in principle affords. Classification accuracy is comparable across all three models ($\approx$78\%), confirming that these structural differences emerge at no discriminative cost. These results motivate a more systematic investigation of geometry-aware training objectives for astrophysical survey data.

• Liakh, Valeriia (2026). Prominence Modeling in the Era of High-Resolution Solar Observations. Full text in Research Archive Show summary

  Prominences are complex but fascinating phenomena observed in the outer region of the solar atmosphere, the solar corona. Interest in these structures is driven by space weather research in the context of the initiation of coronal mass ejections. Prominences demonstrate a wide variety of morphologies and dynamics, and despite decades of observational, theoretical, and modeling studies, many aspects of their evolution remain not entirely understood. In this talk, we will discuss examples of the different types of evolution observed in solar prominences, from formation to eruption or, alternatively, gradual disappearance, and how we use numerical modeling to explain the physical processes behind them. We will also highlight the perspectives for synergistic studies in the era of high-resolution solar observations and sophisticated simulations performed with modern numerical codes.

• Liakh, Valeriia; Druett, Malcolm & Dubart, Maxime (2026). 2.5D flare simulations with MPI-AMRVAC. Full text in Research Archive
• Liakh, Valeriia (2026). Toward Realistic Modeling of Solar Prominences Across Their Life Cycle. Full text in Research Archive Show summary

  Despite decades of both observational and modeling efforts, a self-consistent picture of the formation and evolution of solar prominences remains unclear. In this talk, we review progress in prominence modeling, beginning with current ideas on their formation and the physical processes responsible for mass supply. We then discuss their subsequent dynamical evolution, including counter-streaming flows, mass drainage, and oscillatory phenomena. Additionally, the modeling of the prominence dynamics will be discussed in the context of the prominence stability. Recent numerical studies highlight the need for increasingly realistic simulations to capture the prominence complexity. Therefore, in the final part of the talk, we outline the major open questions and promising directions for future research needed to strengthen the link between simulations and observations.

• Liakh, Valeriia (2026). Numerical Modeling of Prominence Dynamics induced by Coronal Waves. Full text in Research Archive Show summary

  Energetic events, such as solar flares and eruptions, are frequently accompanied by coronal waves that propagate far from their source region and interact with various structures, including coronal holes, prominences, and coronal loops. Interpreting such interactions with prominences directly from observations remains challenging and requires realistic magnetohydrodynamic (MHD) modeling to understand the underlying physical mechanisms and types of induced oscillation. In this talk, we present recent advances in MHD simulations of the interaction between large-scale coronal waves and solar prominences. Particular emphasis will be placed on the propagation of the coronal waves in the non-uniformly magnetized solar corona, the formation of the secondary fronts, the development of thermal instability within the prominence-hosting flux rope, and the excitation of prominence oscillations.

• Kirkaune, Mats; Wedemeyer, Sven; Carlsson, Mats & Brunvoll, Aline Rangøy (2026). Exploring synthetic nanoflare signatures at millimetre wavelengths. Full text in Research Archive Show summary

  Nanoflares have long been considered to be promising candidates for heating the solar corona to millions of degrees. However, their assumed small sizes, short durations, and subtle signals in accessible observables have made direct detection difficult. In the millimetre regime, the Atacama Large (sub-)Millimeter Array (ALMA) offers the capability of measuring chromospheric temperatures at very high temporal and comparatively high spatial resolution. Yet, it is not clear if the currently offered solar observing mode of ALMA is capable of detecting nanoflares. To explore possible millimetre signatures of nanoflares, the F-CHROMA grid of 1D RADYN flare models has been extended to lower total flare energies, into the nanoflare domain. These models simulate the response of a coronal loop to an electron beam inserted at its top. The new simulations were run as a combined effort to characterise nanoflare signatures at different wavelengths. The radiative transfer code RH1.5 was used to compute the millimetre signatures of the modelled nanoflares. These signatures were then degraded to the temporal and spatial resolution achievable with ALMA. The synthetic ALMA light curves indicate that, in principle, ALMA should be able to detect nanoflare signatures, especially when using wider array configurations in combination with a higher frequency band to maximise spatial resolution. This suggests that dedicated ALMA observations could, in principle, place important constraints on atmospheric heating from nanoflares.

• Kirkaune, Mats & Wedemeyer, Sven (2026). Observing the Sun with AtLAST. Full text in Research Archive Show summary

  The Atacama Large Aperture Submillimeter Telescope (AtLAST) has great potential for solar physics. AtLAST’s unique combination of a large aperture, a wide field-of-view, fast mapping speed, and multi-pixel instrumentation could enable it to map the full solar disk across multiple frequencies at high temporal resolution, effectively mapping the thermal and magnetic structure across the chromosphere, and how these change in time. This would help address several important solar science cases. AtLAST’s capabilities would be complementary to the Atacama Large (sub-)Millimeter Array (ALMA) and to other major solar(-capable) observatories of the future, such as the Square Kilometer Array Observatory (SKAO) in South Africa and Australia, and the European Solar Telescope in Spain.

• Ornig, Sascha (2026). Tackling the Enigma of Solar White-Light Flares in Simulations and Observations. Full text in Research Archive
• Kirkaune, Mats; Wedemeyer, Sven; Marrewijk, Joshiwa van; Mroczkowski, Tony & Morris, Thomas (2026). Simulated solar observations with the Atacama Large Aperture Submillimetre Telescope (AtLAST). Full text in Research Archive Show summary

  Millimetre observations offer a highly complementary view of the Sun, providing insight into the thermal and magnetic structure of the chromosphere. The field of solar millimetre astronomy has seen considerable progress since the advent of solar observations with the Atacama Large (sub-)Millimeter Array (ALMA), offering a view of the solar chromosphere at high spatial and temporal resolution. However, some highly dynamic phenomena, such as solar flares, are hard to predict, and therefore challenging to capture with ALMA’s small field of view. The proposed Atacama Large Aperture Submillimetre Telescope (AtLAST), a next-generation 50-m aperture multi-purpose facility, could greatly complement ALMA, offering diagnostic capabilities not currently provided by existing facilities. AtLAST’s large field of view, fast scan speed, and multi-band instruments could allow for mapping the solar disk at adequate spatial resolution, at several frequencies simultaneously, and at a comparatively high cadence. The utilisation of multi-pixel instruments would make AtLAST capable of completing such scans on short timescales, enabling the creation of three-dimensional maps of the solar chromosphere at high temporal resolution. This would allow for monitoring the temperature and magnetic field of the active chromosphere on a global scale, and at an unprecedentedly high cadence, going beyond the capabilities of current facilities in the (sub-)millimetre regime. It is evident that AtLAST has the potential to produce data that would prove highly useful for solar physics. In the presented study, we have therefore explored its potential capabilities by using the multi-purpose (sub-)mm telescope simulator maria. By testing different instrumental setups and scan strategies, we find that 1st generation instrumentation at AtLAST should be able to produce full-disk solar maps on sub-minute timescales across its frequency range. With a more ambitious 2nd generation instrument, it is likely that the temporal resolution could be pushed to a few seconds. The foreseen capabilities would facilitate the detection of all solar flares above AtLAST’s sensitivity threshold, as continuously scanning the solar disk is likely to capture any fast-evolving dynamic phenomena at the covered spatial and temporal scales.

• Popovas, Andrius (2026). Whole Sun Simulations: Riding Exascale to the Core. Full text in Research Archive Show summary

  In this talk I highlight our global simulations of the entire convection zone of the Sun and point to the outstanding bottleneck that exascale simulations will encounter - data and analysis.

• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj & Zhou, Yixiao (2026). Towards Resolving the Solar Convective Conundrum: A staged approach in progress. doi: 10.5281/ZENODO.20642947. Full text in Research Archive Show summary

  The solar convective conundrum — the order-of-magnitude discrepancy between helioseismically inferred deep convective velocities and those predicted by global simulations — has resisted resolution for over a decade. Three suspects have been proposed: missing physics, insufficient resolution, and the way simulations are configured. ReCon² addresses all three together in a single controlled programme, with the third element implemented through a novel staged column-relaxation strategy: solar convection is first developed in a narrow horizontal column, where large-scale unstable modes are geometrically excluded, then progressively expanded to production widths while carrying the equilibrated small-scale physics forward. This poster, presented at Cool Stars 23 (Tokyo, 15–19 June 2026), reports first-stage results from the cz_f25_d100 run: a 25 Mm wide column at Δx = 100 km finest resolution, with fully realistic physics (OPAL/FreeEOS equation of state with ionization, OP opacities and radiative diffusion in the interior, multi-bin short-characteristics radiative transfer at the surface, fully compressible MHD, entropy-based HLLS solver), initialized from rest. Horizontally-averaged entropy perturbation statistics over 300 hours of simulated time show a coherent thermal mixing layer descending ~140 Mm into the convection zone. The active layer is strongly negative-skewed (≤ −2) — the statistical signature of entropy rain (Käpylä 2025), indicating distributions dominated by narrow cold extremes. A compensating positive-skew layer at the leading edge marks warm material displaced by penetrating cold plumes, and discrete late-time arcs at z ≈ 480–540 Mm correspond to individual extreme plumes reaching ~200 Mm depth, exhibiting Käpylä's predicted "tree" morphology. A complementary photospheric kinetic-energy spectrum from an exploratory wider-box run shows that the surface convective physics matches established surface-tracking diagnostics within the box-resolved range.

• Popovas, Andrius; Nordlund, Åke & Szydlarski, Mikolaj (2026). Recent updates on global Sun modeling with DISPATCH. Full text in Research Archive Show summary

  In this talk i present an update on our global simulations of the solar convection. I demonstrate that we can simulate the entire convection zone from below the tachocline all the way to lower chromosphere with smallest cell size near the surface of 98km. Then I discuss "entropy rain" and it's effects on the convection morphology.

• Kohutova, Petra (2026). Studying the solar atmosphere. Full text in Research Archive
• Kohutova, Petra (2026). Spectropolarimetric Observations of the Solar Corona with DKIST/CryoNIRSP. Full text in Research Archive
• Aakre, Gina & Brunvoll, Aline Rangøy (2026). I kveld kan vi se månen dekke nesten hele sola. [Internet]. Forskning.no. Full text in Research Archive Show summary

  Mens vi ser på formørkelsen, prøver solforsker Aline Rangøy Brunvoll å nøste i et av solas største mysterier. Hvorfor er den ytterste atmosfæren varmere enn overflaten?

• Aakre, Gina & Brunvoll, Aline Rangøy (2026). I kveld kan vi se månen dekke nesten hele sola - Titan.uio.no. Full text in Research Archive
• Pereira, Tiago (2026). 22. juli - Reiseradioen - NRK Radio. [Radio]. NRK. Full text in Research Archive Show summary

  Reiseradioen gir deg sommerfølelsen. I dag skal vi slå av en prat med Harald Stanghelle, som for tiden er på Orknøyene. Reporter Even Lusæter er i Honningsvåg og vi får en stemningsrapport derfra i dag.

• Pereira, Tiago (2026). Dagsnytt 18 - TV - NRK TV. [TV]. NRK. Full text in Research Archive Show summary

  Dagsnytt 18 er NRK nyheters daglige aktualitets- og debattprogram. Programledere er Sigrid Sollund og Espen Aas.

• Glans-Doré, Marie & Wedemeyer, Sven (2026). Steven Spielberg er overbevist om at vi ikke er alene i universet. Full text in Research Archive Show summary

  Hva er oddsen for at stjerneregissøren har rett? Økende, skal vi tro forskerne.

• Hestvik, Ingebjørg & Wedemeyer, Sven (2026). Nå vet vi mer om hvilke planeter som kan ha liv - Titan.uio.no. Full text in Research Archive
• Brunvoll, Aline Rangøy (2026). Arbeidsliv, Nyheter | Aline (26) tar doktorgrad i solforskning. Full text in Research Archive Show summary

  26 år gamle Aline Rangøy Brunvoll fra Jeløy er såpass fascinert av sola at hun nå er midt i en doktorgrad innen solforskning.

• Åkervall, Bente Storsveen & Brunvoll, Aline Rangøy (2026). Mossejenta Aline gjør doktorgrad i solfysikk på La Palma. Full text in Research Archive Show summary

  26 år gamle Aline Rangøy Brunvoll fra Moss forsker på solen ved Roque de los Muchachos-observatoriet på La Palma. Møt den norske doktorgradsstudenten som studerer nanoflares og solens mysterier.

• Brønn, Kathrine; Stenbek, Finn Børge & Brunvoll, Aline Rangøy (2026). Eftan: Et aldri så lite sol-mysterium - Distriktsprogram - Østfold - NRK Radio. [Radio]. NRK. Full text in Research Archive Show summary

  Goood mandag! Finn Børge og Kathrine er stemmene i radioen din denne uka. Vi forsker litt på sola, sammen med mossingen Aline. Hun tar doktorgard i solfysikk ved UiO.

• Brønn, Kathrine; Stenbek, Finn Børge & Brunvoll, Aline Rangøy (2026). Mossing forsker på sola: Bruker digert teleskop – NRK. [Internet]. NRK. Full text in Research Archive Show summary

  Aline Rangøy Brunvoll elsker sola - ikke for å bli brun, men for å lære mer om alle mysterier på - og rundt den. Da bruker hun et digert teleskop som står på kanariøya La Palma.

• Grøndahl, Beate & Brunvoll, Aline Rangøy (2026). Morran: Vi teller geitekillinger! - Distriktsprogram - Østfold - NRK Radio. [Radio]. NRK. Full text in Research Archive Show summary

  God morgen Østfold! Beate og Kathrine våkner opp med deg på en dag hvor vi drømmer oss til La Palma hvor mossingen Aline forsker på sola. Møt henne i Morran i Østfold

• Poirier, Nicolas Aurelien Corentin (2026). Et strålende samarbeid. Full text in Research Archive
• Vogt, Yngve & Carlsson, Mats (2026). Astrofysikere avslører stadig nye mysterier på solen | Tu.no. [Internet]. Tu.no. Full text in Research Archive Show summary

  Solen er ikke så stabil som du tror. Hver dag eksploderer det flere hundre tusen bomber på soloverflaten.

• Kanestrøm, Jorunn & Wedemeyer, Sven (2026). Derfor trenger vi et nasjonalt romværvarsel - Forskningsnytt. Full text in Research Archive
• Kanestrøm, Jorunn & Wedemeyer, Sven (2026). Kraftige soleksplosjoner kan frigjøre energi tilsvarende Norges strømforbruk i millioner av år - Titan.uio.no. Full text in Research Archive
• Øyvann, Stig & Carlsson, Mats (2026). Kraftig solstorm ga GPS-forstyrrelser i Nord-Norge. [Internet]. Telecomrevy.no. Full text in Research Archive Show summary

  En kraftig solstorm i januar har ført til betydelige forstyrrelser i satellittnavigasjon og radiosamband, særlig i Nord-Norge. Forskere advarer om at solaktiviteten er inne i en fase der både GPS-tjenester og deler av den digitale infrastrukturen er mer sårbar enn normalt.

• Vogt, Yngve & Wedemeyer, Sven (2026). Drømmen er å varsle soleksplosjoner før de skjer - Titan.uio.no. Full text in Research Archive
• Carlsson, Mats & Tveito, Ole Jon (2026). Nordlys, Naturfenomen | Massivt nordlys over Tinn. Full text in Research Archive Show summary

  Kraftige solstormer som har funnet sted på sola fører nå til massivt nordlys over store deler av Norge. Over Gausta var det tirsdag kveld flott nordlys.

• Vogt, Yngve & Wedemeyer, Sven (2026). Bedre romværvarsler kan redde satellitter og strømnettet - Forskningsnytt. Full text in Research Archive
• Carlsson, Mats (2026). Solstorm, Astronomi | Professor varsler solstormer som kan føre til store skader på jorda. [Internet]. Hamar Arbeiderblad. Full text in Research Archive Show summary

  Aktiviteten på sola er nå på sitt høyeste. Det kan medføre en så voldsom eksplosjon at den kan ramme satellitter, telekommunikasjon og strømforsyning på jorda.

• Vogt, Yngve & Carlsson, Mats (2026). Eksplosjoner på solen kan lamme det moderne livet på jorden | Tu.no. [Internet]. TU.no. Full text in Research Archive Show summary

  Solaktiviteten er nå på sitt høyeste. Faren er til stede for at en voldsom eksplosjon på solen, i år eller neste år, kan skade alt fra satellitter og telekommunikasjon til strømforsyningen på jorden.

• Carlsson, Mats (2026). Professor varsler solstormer som kan føre til store skader på jorda. [Internet]. MSN. Full text in Research Archive
• Carlsson, Mats (2026). Kan skape kaos på jorda: - – Ville ha vært katastrofalt. [Internet]. Sol.no. Full text in Research Archive Show summary

  Aktiviteten på sola er nå på sitt høyeste. Det kan medføre en så voldsom eksplosjon at den kan ramme satellitter, telekommunikasjon og strømforsyning på jorda.

• Carlsson, Mats (2026). Solstormer kan føre til store skader på jorda. Full text in Research Archive Show summary

  Aktiviteten på sola er nå på sitt høyeste. Det kan medføre en så voldsom eksplosjon at den kan ramme satellitter, telekommunikasjon og strømforsyning på jorda. – Det er ikke et spørsmål om hvorvidt det skjer igjen, men om når, sier professor Mats Carlsson på Astrofysisk institutt ved UiO.

• Carlsson, Mats (2026). Solstormer, Astronomi | Professor varsler solstormer som kan føre til store skader på jorda. Full text in Research Archive Show summary

  Aktiviteten på sola er nå på sitt høyeste. Det kan medføre en så voldsom eksplosjon at den kan ramme satellitter, telekommunikasjon og strømforsyning på jorda.

• Carlsson, Mats (2026). Professor varsler solstormer som kan føre til store skader på jorda. [Internet]. Energiwatch. Full text in Research Archive
• Carlsson, Mats & Vogt, Yngve (2026). Eksplosjoner på solen kan lamme det moderne livet på jorden - Forskningsnytt. Full text in Research Archive
• Kohutova, Petra (2026). Spectropolarimetric observations of coronal oscillations with DKIST/Cryo-NIRSP. Full text in Research Archive
• Wedemeyer, Sven (2026). Radio and Millimeter Observations of the Sun - Probing the Solar Origins of Space Weather. Full text in Research Archive
• Wedemeyer, Sven (2026). Fra solstorm til nordlys: Solens påvirkning på jorden. Full text in Research Archive
• Wedemeyer, Sven (2026). The Heliosphere and Its Protective Role in Planetary Systems. Full text in Research Archive
• Wedemeyer, Sven (2026). Radio and (Sub-)Millimeter Observations of the Sun: Current Status, Future Perspectives, and the Path Toward AtLAST. Full text in Research Archive
• Wedemeyer, Sven (2026). 3D CO + MHD simulations with CO5BOLD. Full text in Research Archive
• Wedemeyer, Sven (2026). The Sun observed at long wavelengths. Full text in Research Archive
• Baso, Carlos José Díaz (2026). Information content of solar spectra. Full text in Research Archive Show summary

  High-resolution observations and advanced data analysis techniques are essential for uncovering fine-scale physical processes in the solar atmosphere. In this presentation, we explore three recent methodological and observational lessons learned from high-cadence solar telescope data. First, we examine the evolution of an eruptive flare observed with the TRIPPEL instrument at the Swedish 1-m Solar Telescope, demonstrating how combining magnetic extrapolations with non-local thermodynamic equilibrium inversions bridges observational and theoretical domains to quantify released magnetic energy and atmospheric heating. Second, we address the challenge of disentangling complex, multi-component spectral line profiles during mini-filament eruptions, showing how linear optimization techniques efficiently resolve overlapping atmospheric components. Finally, we introduce the application of normalizing flows as powerful density estimators to model multi-dimensional spectral probability distributions, enabling the automated detection of statistically rare and unusual phenomena in quiet Sun time-series datasets. Together, these approaches highlight how advanced modeling and machine learning techniques can optimize our analysis of massive observational data rates and reveal hidden solar dynamics.

• Baso, Carlos José Díaz (2026). AI in Solar Physics. Full text in Research Archive Show summary

  Solar physics is entering an unprecedented data-rich era driven by advanced solar observatories generating massive volumes of high-resolution spatiotemporal and spectropolarimetric observations. Analyzing these extensive datasets presents severe computational bottlenecks, as traditional numerical modeling and radiative transfer inversions are extremely time-consuming. In this work, we explore how artificial intelligence and modern machine learning frameworks can revolutionize solar data analysis across three critical domains. First, we implement probabilistic generative models, including diffusion models, to perform efficient unsupervised anomaly detection, allowing researchers to automatically isolate rare and physically interesting phenomena within terabytes of observations by evaluating data probability distributions. Second, we integrate large language model agents as automated optimizers to accelerate complex mathematical inversions, solving systems with hundreds of thousands of unknowns orders of magnitude faster. Finally, we examine the application of large-scale foundation models in heliophysics, such as spatiotemporal transformers trained on extensive Solar Dynamics Observatory archives, demonstrating their potential for zero-shot forecasting of solar conditions and the prediction of explosive events. Together, these approaches demonstrate how advanced AI methodologies help overcome the challenges of big data, opening new avenues for automated discovery and physical understanding in solar physics.

• Baso, Carlos José Díaz (2026). Inspectorch: Efficient rare event exploration. Full text in Research Archive Show summary

  Next-generation facilities like the European Solar Telescope (EST) will observe the Sun with unprecedented spatial and temporal resolution, providing simultaneous multi-wavelength and multi-height diagnostics. However, the sheer volume of high-resolution spectropolarimetric data will make it impossible to fully analyze using conventional methods. While standard machine learning identifies general trends, it routinely overlooks the highly localized, transient, or weak signals crucial for understanding chromospheric heating and flux emergence. Our aim is to introduce an unsupervised probabilistic method to efficiently identify these rare, extreme events in multidimensional solar observations, optimizing our computational resources for the phenomena EST is designed to uncover. To achieve this, we introduce Inspectorch, an open-source framework utilizing flow-based models. These flexible density estimators learn the multidimensional distribution of spectropolarimetric data and assign a probability to each sample, immediately isolating unusual events. We validate this approach using datasets from the Hinode Spectro-Polarimeter, the Interface Region Imaging Spectrograph (IRIS), the Microlensed Hyperspectral Imager (MiHI) at the Swedish 1-m Solar Telescope (SST), and space-based imagers such as the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. Across these datasets, the algorithm successfully isolates profiles with uncommon broadening, strong Doppler shifts associated with small-scale reconnection, and complex profiles exhibiting height-dependent magnetic polarity reversals. Its probabilistic nature makes it highly effective at identifying those cases that occupy a tiny fraction of the total number of samples and are overlooked by traditional methods. Inspectorch demonstrates that flow-based density estimation is robust, requires minimal parameter tuning, and can be applied to large datasets with minimal effort. By providing probabilistic anomaly scores, this publicly available framework prepares the solar physics community to rapidly identify and analyze the most physically relevant and elusive phenomena in the massive datasets soon to be produced by EST.

• Baso, Carlos José Díaz (2026). Thermodynamic and magnetic evolution of an eruptive C-class solar flare: Linking small-scale triggers to eruptive potential. Full text in Research Archive Show summary

  Solar flares and filament eruptions are complex phenomena driven by the release of magnetic energy, and they act as primary drivers of space weather. However, a large energy reservoir alone is insufficient to determine their eruptive potential; the localized magnetic topology and small-scale plasma dynamics play a critical role. We investigated the thermodynamic and magnetic properties of the solar atmosphere during the rise, peak, and decay phases of an eruptive C5.1-class flare in active region NOAA 12561 to understand how fine-scale atmospheric responses dictate broader eruptive behavior. High spatial and spectral resolution spectropolarimetric observations of the chromospheric Ca II 8542 Å line and nearby photospheric lines were obtained with the TRIPPEL-SP spectropolarimeter at the Swedish 1-m Solar Telescope. By combining nonlocal thermodynamic equilibrium (NLTE) inversions with non-force-free field (NFFF) magnetic extrapolations, we bridged the observational and theoretical domains to follow the event's evolution. Before the flare, our analysis revealed a complex, sheared magnetic topology with a high free energy content (~2 x 10^30 erg). In this precursor phase, we detected persistent localized heating (temperature increase of ~2000 K) with strong downflows (10-20 km/s) deep in the atmosphere. This heating was co-spatial with a bald-patch region, demonstrating how low-altitude magnetic reconnection can destabilize a larger filament structure. The flare's rise phase was marked by the filament's eruption with speeds exceeding 70 km/s. Following the eruption, free energy decreased by ~30% as post-flare loops channeled the released energy into the lower atmosphere, driving ribbon heating up to 8500 K and significant downflows. These results highlight how combining high-resolution ground-based spectropolarimetry with advanced modeling is essential for understanding how small-scale localized magnetic phenomena scale up to trigger eruptive events with space weather implications.

• Baso, Carlos José Díaz (2026). Inspectorch: a probabilistic framework for efficient rare event exploration in the EST era. Full text in Research Archive Show summary

  Next-generation facilities like the European Solar Telescope (EST) will observe the Sun with unprecedented spatial and temporal resolution, providing simultaneous multi-wavelength and multi-height diagnostics. However, the sheer volume of high-resolution spectropolarimetric data will make it impossible to fully analyze using conventional methods. While standard machine learning identifies general trends, it routinely overlooks the highly localized, transient, or weak signals crucial for understanding chromospheric heating and flux emergence. Our aim is to introduce an unsupervised probabilistic method to efficiently identify these rare, extreme events in multidimensional solar observations, optimizing our computational resources for the phenomena EST is designed to uncover. To achieve this, we introduce Inspectorch, an open-source framework utilizing flow-based models. These flexible density estimators learn the multidimensional distribution of spectropolarimetric data and assign a probability to each sample, immediately isolating unusual events. We validate this approach using datasets from the Hinode Spectro-Polarimeter, the Interface Region Imaging Spectrograph (IRIS), the Microlensed Hyperspectral Imager (MiHI) at the Swedish 1-m Solar Telescope (SST), and space-based imagers such as the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory and the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter. Across these datasets, the algorithm successfully isolates profiles with uncommon broadening, strong Doppler shifts associated with small-scale reconnection, and complex profiles exhibiting height-dependent magnetic polarity reversals. Its probabilistic nature makes it highly effective at identifying those cases that occupy a tiny fraction of the total number of samples and are overlooked by traditional methods. Inspectorch demonstrates that flow-based density estimation is robust, requires minimal parameter tuning, and can be applied to large datasets with minimal effort. By providing probabilistic anomaly scores, this publicly available framework prepares the solar physics community to rapidly identify and analyze the most physically relevant and elusive phenomena in the massive datasets soon to be produced by EST.

• Baso, Carlos José Díaz (2026). Physics-informed machine learning and probabilistic methods for spectropolarimetry. Full text in Research Archive Show summary

  In the last decade, machine learning and neural networks have emerged as powerful tools for analyzing huge collections of solar data, demonstrating versatility in tasks ranging from image reconstruction to the acceleration of spectropolarimetric inversions. However, while these algorithms offer unprecedented speed, they often lack the precision required for detailed physical interpretation and fail to exploit the inherent spatial coherence of the solar atmosphere to increase fidelity. Moreover, the straightforward application of these tools risks merely shifting the bottleneck—from managing huge raw datasets to sifting through massive volumes of inversion results—without effectively isolating the poorly understood phenomena that drive scientific discovery. In this contribution, we address these limitations by reviewing novel applications that combine physical constraints with advanced statistical methods. We specifically highlight Neural Fields, which leverage continuous parameterization to impose spatio-temporal constraints and improve magnetic field reconstruction, and Normalizing Flows, a probabilistic technique optimized for efficiently detecting rare events and extreme spectra that traditional methods overlook. We demonstrate the potential of these approaches using observations from telescopes such as the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). Finally, we discuss how integrating these tools is essential to maximize the scientific return from current and future missions.

• Poquet, Ignasi Josep Soler; Baso, Carlos José Díaz & Voort, Luc Rouppe van der (2026). Multi-wavelength observations of Ellerman bombs. Full text in Research Archive Show summary

  Context. Ellerman bombs (EBs) are observational signatures of small-scale magnetic reconnection, key to understanding the lower solar atmosphere. While their role in active regions has been widely studied using the Hα line, near-ultraviolet (NUV) spectra routinely observed by the Interface Region Imaging Spectrograph (IRIS) offer a promising alternative for EB identification, enabling large-scale studies. Aims. We aim to identify the most important spectral signatures of EBs in the IRIS NUV spectra. With this, we seek to develop a robust criterion for their detection solely using the IRIS NUV spectra. In parallel, we determine the typical atmospheric stratification associated with EBs. Methods. We used four coordinated observations between the Swedish 1-m Solar Telescope (SST) and IRIS. Using the Hα line as a reference, we detected 18 different EBs and studied their associated IRIS NUV spectra. In addition, we used the IRIS2+ inversion tool to infer the temperature, line-of-sight velocity, and non-thermal broadening from the EB spectra. Results. The defining feature of EBs in the IRIS NUV is the enhancement of the wings of the subordinated Mg ii triplet between the Mg ii h&k lines. Inversions reveal that these signatures are produced by a localized temperature increase of ∆T ∼ 1670 K around log(τ)= −3.8. Using only the Mg ii triplet signatures, we found a detection criterion that successfully recovered 14 of 18 Hα-detected EBs. In addition, the shape of the Mg ii h&k lines in relation to the Mg ii triplet can serve as a proxy for the EB formation height. Conclusions. The NUV spectrum observed by IRIS is a good candidate for detecting EBs, opening the doors to large-scale studies across the extensive IRIS database, removing the dependence on Hα observations. This capability bridges the gap between small- and large-scale event studies, which was not possible before due to the lack of coordinated observations.

• Gudiksen, Boris Vilhelm (2026). Large effect of small scales in the solar corona. Full text in Research Archive
• Ornig, Sascha; Carlsson, Mats & Voort, Luc Rouppe van der (2026). Characterization of White-Light Enhancements in Solar Flares: A Combined Observational and Modeling Approach. Universitetet i Oslo. Full text in Research Archive Show summary

  Solar flares are among the most energetic events in the solar system. These sudden brightenings of localized regions in the solar atmosphere drive particle acceleration and heat the ambient plasma to several millions of degrees. The enormous amounts of radiation released in these events, combined with associated phenomena such as coronal mass ejections, have a major influence on space weather around our planet Earth, and it is therefore of vital importance for us to understand the mechanisms behind them. The continuum contains a large portion of the energy released in solar flares, yet its formation mechanisms pose a challenge to the standard flare model. The optical part of the continuum, termed white light (WL), allows us to utilize ground-based observatories for our investigations. With complementary output from numerical simulations, we can gain insights into the dominant processes. However, to date several problems remain. This work investigates solar WL flares (WLFs) using a symbiosis of simulations and observations. It is comprised of three papers, each aimed at answering one of our outstanding questions. Paper I makes use of an ensemble of electron beam-driven solar flare simulations — the F-CHROMA grid — created with the radiation hydrodynamics code RADYN. We investigate how WL enhancements appear in these simulations, what the underlying mechanisms are, and whether we can identify a useful proxy for deducing the dominant mechanism from observations. Our results show intensity enhancements caused primarily by enhanced hydrogen recombination radiation in the optically thin chromosphere, with minor contributions from the radiatively backheated photosphere. The simulated WL enhancements are at least one order of magnitude below observed values. We identify the Paschen jump and its associated intensity ratio as an appropriate proxy for increased optically thin hydrogen recombination radiation. For Paper II, we utilize observations from the Swedish 1-m Solar Telescope (SST) to probe the Paschen jump via continuum measurements redward and blueward of the jump. We find WL enhancements in the dark (pen)umbral regions in sunspots, without any flare-related quiet-Sun intensity increases. The WL increases in one of the flares are co-temporal and (partially) co-spatial with hard X-ray increases, suggesting a common electron-beam cause. Our results indicate that the filter used for observations redward of the jump is unsuitable for measurements of the true continuum intensity due to influences from the nearby spectral line. In Paper III, we attempt to close the gap in amplitude between WL enhancements in simulations and observations by using a background atmosphere modeled after the umbra of a sunspot. Our results show WL increases comparable to observations, and we identify the reduced background intensity as the main reason for this outcome. The mechanisms are identical to those identified in the simulations with a quiet-Sun starting atmosphere used for Paper I. We further find that the electron beam flux is strongly correlated with the resulting WL increases. We conclude that one-dimensional simulations of solar flares can satisfactorily model WLFs of type I. Observationally, we lack adequate measurements of the true continuum longward of the Paschen jump in order to utilize it for the deduction of the formation mechanisms behind WLFs

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