Sascha Ornig

• 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.

• 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.

• Ornig, Sascha; Veronig, A. M. & Dissauer, K. (2025). Extreme ultraviolet late-phase flares as observed by EVE and AIA on board the Solar Dynamics Observatory. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202556242. Full text in Research Archive Show summary

  Context. Extreme ultraviolet (EUV) late-phase (ELP) flares exhibit a second peak in warm coronal emissions minutes to hours after the main peak of the flare. This phase is all but negligible, and it is still poorly understood what role it plays across the solar cycle and what governs it. Aims. We present a statistical analysis of ELP flares over four years between May 2010 and May 2014 based on properties such as eruptivity, magnetic configuration, and late-phase duration, delay, and strength in order to understand what influences the likelihood of this class of flares and their behavior on a general scale. Methods. We primarily made use of data from the Solar Dynamics Observatory (SDO) Extreme ultraviolet Variability Experiment (EVE), as well as complementary spatial information provided by the Atmospheric Imaging Assembly (AIA), to assess relationships between the various parameters and to see if ELP flares differ from the general flare population. We quantified the criteria for ELP flare definition and determined the characteristics of the flares. Results. Our analysis shows that about 10% of all flares with a GOES class ≥C3.0 experience an EUV late phase (179 out of 1803). This percentage decreases from solar minimum to solar maximum. C-class flares are considerably less likely to be identified as ELP flares than their higher-energy counterparts, which is in line with previous investigations. The majority of this type of flare are confined (67%), more so than in the general flare population (≥C5.0). There appears to be a (linear) relationship between the late-phase delay and its duration. The ratio of the emission peak of the late and main flare phase lies between 0.3 and 5.9, and exceeds 1 in 71.5% of cases, which is considerably higher than previously reported.

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• Ornig, Sascha (2026). Tackling the Enigma of Solar White-Light Flares in Simulations and Observations. Full text in Research Archive
• Ornig, Sascha (2025). White-light Emission in the F-CHROMA Grid. Full text in Research Archive
• Ornig, Sascha (2025). White-Light Emission in the F-CHROMA Grid. 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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