arXiv subjects

Astrid M. Veronig

Publications and source records attributed to Astrid M. Veronig.

At least 37 records · Page 2Linked to original sources

arXiv2025

Detecting Stellar Coronal Mass Ejections via Coronal Dimming in the Extreme Ultraviolet

Stellar flares and coronal mass ejections (CMEs) can strip planetary atmospheres, reducing the potential habitability of terrestrial planets. While flares have been observed for decades, stellar CMEs remain elusive. Extreme ultraviolet (EUV) emissions are sensitive to both flares and CME-induced coronal dimming. We assess the detectability of stellar CME-induced EUV dimming events by adapting a known "Sun-as-a-star" dimming technique -- validated by the Solar Dynamics Observatory's EUV Variability Experiment (EVE) -- to stellar conditions. We adapt the solar data to reflect a range of stellar intensities, accounting for intrinsic brightness, distance, and interstellar medium (ISM) attenuation. We generate synthetic light curves for two different missions: the legacy EUV Explorer (EUVE) and the proposed ESCAPE mission. Our results indicate that dimming detections are well within reach. EUVE's broadband imager was capable of detecting stellar CMEs -- albeit with limited spectral (temperature) resolution -- but that was not part of the observing plan. EUVE's spectroscopic survey lacked sufficient sensitivity for CME detections. Optimizing modern instrument design for this task would make the observation fully feasible. In this work, we present a tool to explore the stellar-CME detection parameter space. Our tool shows that an instrument with performance similar to ESCAPE, setting a 600-second integration period, and integrating the spectra into bands, any star with an X-ray flux $\geq 2.51 \times 10^{-12}$ergs$^{-1}$~cm$^{-2}$ should have a $\geq 3σ$ detection even for a modest few-percent dimming profile, regardless of ISM attenuation. Such measurements would be crucial for understanding the space weather environments of exoplanet host stars and, ultimately, for evaluating planetary habitability.

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arXiv2025

Deep learning image burst stacking to reconstruct high-resolution ground-based solar observations

Large aperture ground based solar telescopes allow the solar atmosphere to be resolved in unprecedented detail. However, observations are limited by Earths turbulent atmosphere, requiring post image corrections. Current reconstruction methods using short exposure bursts face challenges with strong turbulence and high computational costs. We introduce a deep learning approach that reconstructs 100 short exposure images into one high quality image in real time. Using unpaired image to image translation, our model is trained on degraded bursts with speckle reconstructions as references, improving robustness and generalization. Our method shows an improved robustness in terms of perceptual quality, especially when speckle reconstructions show artifacts. An evaluation with a varying number of images per burst demonstrates that our method makes efficient use of the combined image information and achieves the best reconstructions when provided with the full image burst.

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arXiv2025

Coronal dimmings from active region 13664 during the May 2024 solar energetic events

Coronal dimmings are regions of transiently reduced brightness in extreme ultraviolet (EUV) and soft X-ray (SXR) emissions associated with coronal mass ejections (CMEs), providing key insights into CME initiation and early evolution. During May 2024, AR 13664 was among the most flare-productive regions in recent decades, generating 55 M-class and 12 X-class flares along with multiple Earth-directed CMEs. The rapid succession of these CMEs triggered the most intense geomagnetic storm in two decades. We study coronal dimmings from a single active region (AR 13664) and compare them with statistical dimming properties. We investigate how coronal dimming parameters - such as area, brightness, and magnetic flux - relate to key flare and CME properties. We systematically identified all flares above M1.0, all coronal dimmings and all CMEs (from the CDAW SOHO/LASCO catalogue) produced by AR 13664 during 2024 May 1 - 15, and studied the associations between the different phenomena and their characteristic parameters. We detect coronal dimmings in 22 events, with 16 occurring on-disc and six off-limb. Approximately 83% of X-class flares and 23% of M-class flares are associated with CMEs, with 13 out of 16 on-disc dimmings linked to CME activity. Our results support the strong interplay between coronal dimmings and flares, as we find increased correlations between flare and dimming parameters in this single-AR study compared to the general dimming population. Furthermore, we confirm that coronagraphic observations, unable to observe the lower corona, underestimate correlations between CME velocities and dimming parameters, as they fail to capture the early CME acceleration phase. This highlights the critical role of dimming observations in providing a more comprehensive understanding of CME dynamics.

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arXiv2025

Spatiotemporal evolution of UV pulsations and their connection to 3D magnetic reconnection and particle acceleration

Quasi-periodic pulsations (QPPs) are a common feature of impulsive solar flare emissions, yet their driving mechanism(s) remain unresolved. We study long-period (3.4 min) QPPs in the M3.7 flare on Feb 24, 2023, using SDO/AIA and Solar Orbiter/STIX to pinpoint their source and driver. We analyzed UV and HXR emissions from four subregions covering the flare ribbons and footpoints of the erupting filament. We find the QPP characteristics varied significantly across these regions. The strongest UV pulsations, which correlated strongly with HXR emissions, originated from a sequence of flare kernels at the expanding front of the southern flare ribbon. In contrast, another expanding ribbon front showed no pulsations. The spatial coincidence of UV and HXR sources points to non-thermal electrons as a common driver, with the HXR spectrum showing a soft-hard-soft evolution matching the pulsation timescale. We also observed UV pulsations from plasma injections into a separate filament channel, distinct from the main flare kernels. Our results suggest that the spatiotemporal evolution of the pulsations is driven by the propagation of magnetic reconnection in an asymmetric geometry. We propose that slipping reconnection along quasi-separatrix layers is key to generating and moving the UV kernels, potentially modulated by another time-varying process.

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arXiv2025

Coronal dimmings and what they tell us about solar and stellar coronal mass ejections

Coronal dimmings associated with coronal mass ejections (CME) from the Sun have gained much attention since the late 1990s when they were first observed in high-cadence imagery of the SOHO/EIT and Yohkoh/SXT instruments. They appear as localized sudden decreases of the coronal emission at extreme ultraviolet (EUV) and soft X-ray (SXR) wavelengths, that evolve impulsively during the lift-off and early expansion phase of a CME. Coronal dimmings have been interpreted as "footprints" of the erupting flux rope and also as indicators of the coronal mass loss by CMEs. However, these are only some aspects of coronal dimmings and how they relate to the overall CME/flare process. The goal of this review is to summarize our current understanding and observational findings on coronal dimmings, how they relate to CME simulations, and to discuss how they can be used to provide us with a deeper insight and diagnostics of the triggering of CMEs, the magnetic connectivities and coronal reconfigurations due to the CME as well as the replenishment of the corona after an eruption. In addition, we go beyond a pure review by introducing a new, physics-driven categorization of coronal dimmings based on the magnetic flux systems involved in the eruption process. Finally, we discuss the recent progress in studying coronal dimmings on solar-like and late-type stars, and to use them as a diagnostics for stellar coronal mass ejections and their properties.

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arXiv2025

Conditioning of the solar corona due to large flares

We aim to better characterize the conditions of the solar corona, especially with respect to the occurrence of confined and eruptive flares. In this work, we model the coronal evolution around 231 large flares observed during solar cycle 24. Using Helioseismic and Magnetic Imager vector magnetic field data around each event, we employed nonlinear force-free field extrapolations to approximate the coronal energy and helicity budgets of the solar source regions. A superposed epoch analysis and dynamical time warping applied to the time series of selected photospheric and coronal quantities were used to pin down the characteristics of the pre- and postflare time evolution, as well as to assess flare-related changes. During the 24 hours leading up to a major flare, the total magnetic energy and unsigned magnetic flux were seen to evolve closely with respect to each other, irrespective of the flare type. Prior to confined flares, the free energy evolves in a way that exhibits more of a similarity with the unsigned flux than the helicity of the current-carrying field, while the opposite trend is seen prior to eruptive flares. Furthermore, the flare type can be predicted correctly in more than 90\% of major flares when combining measures of the active regions nonpotentiality and local stability. The coronal energy and helicity budgets return to preflare levels within approximately six to 12 hours after eruptive major M-class flares, while the impact of eruptive X-flares lasts considerably longer. Finally, the postflare replenishment times of more than 12 hours after eruptive X-class flares may serve as a partial explanation for the rare observation of eruptive X-class flares within a time frame of a few hours.

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arXiv2024

Solar flares in the Solar Orbiter era: Short-exposure EUI/FSI observations of STIX flares

Aims: This paper aims to demonstrate the importance of short-exposure extreme ultraviolet (EUV) observations of solar flares in the study of particle acceleration, heating and energy partition in flares. This work highlights the observations now available from the Extreme Ultraviolet Imager (EUI) instrument suite on board Solar Orbiter while operating in short-exposure mode. Methods: A selection of noteworthy flares observed simultaneously by the Spectrometer Telescope for Imaging X-rays (STIX) and the Full Sun Imager of EUI (EUI/FSI) are detailed. New insights are highlighted and potential avenues of investigation are demonstrated, including forward-modelling the atmospheric response to a non-thermal beam of electrons using the RADYN 1D hydrodynamic code, in order to compare the predicted and observed EUV emission. Results: The examples given in this work demonstrate that short-exposure EUI/FSI observations are providing important diagnostics during flares. A dataset of more than 9000 flares observed by STIX (from November 2022 until December 2023) with at least one short-exposure EUI/FSI 174 Å image is currently available. The observations reveal that the brightest parts of short-exposure observations consist of substructure in flaring ribbons that spatially overlap with the hard X-ray emission observed by STIX in the majority of cases. We show that these observations provide an opportunity to further constrain the electron energy flux required for flare modelling, among other potential applications.

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arXiv2024

Three-part structure of solar coronal mass ejection observed in low coronal signatures of Solar Orbiter

This study examines the relationship between early solar coronal mass ejection (CME) propagation, the associated filament eruption, and coronal dimming in the rare event observed on March 28, 2022, which featured a three-part CME in the low corona of active region AR 12975, including a bright core/filament, dark cavity, and bright front edge. We employ 3D filament and CME shock reconstructions using data from SolO, STEREO-A, and SDO to track the filament's path, height, and kinematics. Our analysis across three viewpoints shows the outer front in SolO/EUI 304 Å aligns with shock structures in STEREO-A/EUVI 195 Å, showing a full 3D EUV wave dome, later matching the outer CME front in STEREO-A COR2. We introduce the method ATLAS-3D (Advanced Technique for single Line-of-sight Acquisition of Structures in 3D) and validate it against traditional approaches to reconstruct CME shock using SOLO data exclusively. Additionally, we estimate early CME propagation characteristics based on coronal dimming evolution with the DIRECD method. Results indicate that the filament height increased from 28 to 616 Mm (0.04 to 0.89 Rs) within 30 minutes (11:05 to 11:35 UT), reaching peak velocity of around 648 km/s and acceleration of around 1624 m/s$^2$. At 11:45 UT, the filament deflected by 12° to a height of 841 Mm (1.21 Rs), while the CME shock expanded from 383 to 837 Mm (0.55 to 1.2 Rs) over 10 minutes. Key parameters include a CME direction inclined by 6°, a 21° half-width, and a 1.12 Rs cone height at the dimming's impulsive phase end. This event demonstrates that expanding dimming correlates with early CME development, with the DIRECD method linking 2D dimming to 3D CME evolution. These insights underscore the value of multi-viewpoint observations and advanced reconstructions for improving space weather forecasting.

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arXiv2024

Estimating early coronal mass ejection propagation direction with DIRECD during the severe May 8 and follow-up June 8, 2024 events

On May 8, 2024, solar active region 13664 produced an X-class flare, several M-class flares, and multiple Earth-directed Coronal Mass Ejections (CMEs). The initial CME caused coronal dimmings, characterized by localized reductions in extreme-ultraviolet (EUV) emissions, indicating mass loss and expansion during the eruption. After one solar rotation, on June 8, 2024, the same region produced another M-class flare followed by coronal dimmings observed by the SDO and STEREO spacecraft. We analyzed early CME evolution and direction from coronal dimming expansion at the end of the impulsive phase using the DIRECD (Dimming Inferred Estimation of CME Direction) method. To validate the 3D CME cone, we compared CME properties from the low corona with white-light coronagraph data. The May 8 CME expanded radially, with a 7.7 deg inclination, 70 deg angular width, and 0.81 Rsun cone height, while the June 8 CME had a 15.7 deg inclination, 81 deg width, and 0.89 Rsun height. Our study shows that tracking low coronal signatures, like coronal dimming expansion, can estimate CME direction early, providing crucial lead time for space weather forecasts.

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arXiv2024

The observational evidence that all microflares that accelerate electrons to high energies are rooted in sunspots

In general, large solar flares are more efficient at accelerating high-energy electrons than microflares. Nonetheless, we sometimes observe microflares that accelerate electrons to high energies. We statistically characterize 39 microflares with strikingly hard spectra in the hard X-ray (HXR) range, which means that they are efficient in accelerating high-energy electrons. We refer to these events as "hard microflares." The statistical analysis is built upon spectral and imaging information from STIX, combined with EUV and magnetic field maps from SDO. The key observational result is that all hard microflares in this dataset have one of the footpoints rooted directly within a sunspot (either in the umbra or the penumbra). This clearly indicates that the underlying magnetic flux densities are large. For the events with the classic two-footpoints morphology, the absolute value of the mean line-of-sight magnetic flux density (and vector magnetic field strength) at the footpoint rooted within the sunspot ranges from 600 to 1800 G (1500 to 2500 G), whereas the outer footpoint measures from 10 to 200 G (100 to 400 G), therefore about 10 times weaker. Approximately 78% of hard microflares, which exhibited two HXR footpoints, have similar or even stronger HXR flux from the footpoint rooted within the sunspot. This contradicts the magnetic mirroring scenario. In addition, about 74% of the events could be approximated by a single loop geometry, demonstrating that hard microflares typically have a relatively simple morphology. We conclude that all hard microflares are rooted in sunspots, which implies that the magnetic field strength plays a key role in efficiently accelerating high-energy electrons, with hard HXR spectra associated with strong fields. This key result will allow us to further constrain our understanding of the electron acceleration mechanisms in flares and space plasmas.

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arXiv2024

Coronal dimmings as indicators of early CME propagation direction

Coronal mass ejections (CMEs) are solar eruptions of plasma and magnetic fields that significantly impact Space Weather, causing disruptions in technological systems and potential damage to power grids when directed towards Earth. Traditional coronagraphs along the Sun-Earth line struggle to precisely track the early evolution of Earth-directed CMEs. Coronal dimmings, localized reductions in extreme-ultraviolet (EUV) and soft X-ray emissions, are key indicators of CMEs in the low corona, resulting from mass loss and expansion during the eruption. This study introduces a novel method, DIRECD (Dimming InfeRred Estimate of CME Direction), to estimate the early propagation direction of CMEs based on the expansion of coronal dimmings. The approach involves 3D simulations of CMEs using a geometric cone model, exploring parameters like width, height, source location, and deflection from the radial direction. The dominant direction of dimming evolution is then determined, and an inverse problem is solved to reconstruct an ensemble of CME cones at various heights, widths, and deflections. By comparing the CME orthogonal projections onto the solar sphere with the dimming geometry, the 3D CME direction is derived. Validated through case studies on October 1, 2011, and September 6, 2011, the DIRECD method reveals the early propagation directions of CMEs. The CME on October 1, 2011, predominantly expands towards the South-East, while the CME on September 6, 2011, inclines towards the North-West. These findings align with previous studies using multi-viewpoint coronagraphic observations. The study demonstrates the utility of coronal dimming information for early CME direction estimation, providing valuable data for space weather forecasting and mitigating potential adverse impacts on Earth before observation in coronographs' field-of-view.

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arXiv2024

Recovery of coronal dimmings

Coronal dimmings are regions of reduced emission in the lower corona, observed after coronal mass ejections (CMEs) and representing their footprints. In order to investigate the long-term evolution of coronal dimming and its recovery, we propose two approaches that focus on both the global and the local evolution of dimming regions: the fixed mask approach and the pixel boxes approach. We present four case studies (September 6, 2011; March 7, 2012; June 14, 2012; and March 8, 2019) in which a coronal dimming is associated with a flare/CME eruption. We identified the dimming region by image segmentation, then restricted the analysis to a specific portion of the dimming and tracked the time evolution of the dimming brightness and area. In addition, we studied the behavior of small subregions inside the dimming area, of about 3x3 pixels, to compare the recovery in different regions of the dimming. Three out of the four cases show a complete recovery 24 hours after the flare/CME eruption. The recovery of the brightness follows a two-step trend, with a steeper and quicker segment followed by a slower one. In addition, some parts of the dimming, which may be core dimmings, are still present at the end of the analysis time and do not recover within 3 days, whereas the peripheral regions (secondary dimmings) show a full recovery. We demonstrate that the primary mechanism for recovery identified in the observations is the expansion of coronal loops into the dimming region, which gradually increase their intensity. Our developed approaches enable the analysis of dimmings alongside these bright structures, revealing different timescales of recovery for core and secondary/twin dimming regions. Combined with magnetic field modeling, these methods lay the foundation for further systematic analysis of dimming recovery and enhance the knowledge gained from already-analyzed events.

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arXiv2024

Revisiting Empirical Solar Energetic Particle Scaling Relations II. Coronal Mass Ejections

Aims. The space radiation environment conditions and the maximum expected coronal mass ejection (CME) speed are being assessed through the investigation of scaling laws between the peak proton flux and fluence of Solar Energetic Particle (SEP) events with the speed of the CMEs. Methods. We utilize a complete catalog of SEP events, covering the last ~25 years of CME observations (i.e. 1997 to 2017). We calculate the peak proton fluxes and integrated event fluences for those events reaching an integral energy of up to E> 100 MeV, covering the period of the last ~25 years of CME observations. For a sample of 38 strong SEP events, we first investigate the statistical relations between the recorded peak proton fluxes (IP) and fluences (FP) at a set of integral energies of E >10 MeV, E>30 MeV, E>60 MeV, and E>100 MeV versus the projected CME speed near the Sun (VCME) obtained by the Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph (SOHO/LASCO). Based on the inferred relations, we further calculate the integrated energy dependence of both IP and FP, assuming that they follow an inverse power-law with respect to energy. By making use of simple physical assumptions, we combine our derived scaling laws to estimate the upper limits for VCME, IP, and FP focusing on two cases of known extreme SEP events that occurred on February 23, 1956 (GLE05) and in AD774/775, respectively. Given physical constraints and assumptions, several options for the upper limit VCME, associated with these events, are investigated. Results. A scaling law relating IP and FP to the CME speed as V^{5}CME for CMEs ranging between ~3400-5400 km/s is consistent with values of FP inferred for the cosmogenic nuclide event of AD774/775. At the same time, the upper CME speed that the current Sun can provide possibly falls within an upper limit of VCME <= 5500 km/s.

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arXiv2024

Multipoint study of the rapid filament evolution during a confined C2 flare on 28 March 2022, leading to eruption

We studied the rapid filament evolution in AR 12975 during a confined C2 flare on 28 March 2022, which led to an eruptive M4 flare 1.5 h later. It is characterized by the breakup of the filament, the disappearance of its southern half, and the flow of the remaining plasma into a longer channel with a topology similar to an EUV hot channel during the flare. Our multipoint study takes advantage of Solar Orbiter's position at 0.33 AU and 83. 5° west of the Sun-Earth line. STIX and EUI onboard Solar Orbiter observed the event at the limb. AIA and HMI onboard SDO provided on-disk observations from which we derived DEM maps and NLFF magnetic field extrapolations. We find that both filament channels likely existed in close proximity before the flare. Based on field structures associated with AIA 1600 Å flare ribbons and kernels, we propose a loop-loop reconnection scenario between field lines that surround and pass beneath the shorter filament channel, and field lines following a portion of the longer channel. Reconnection occurs in an essentially vertical current sheet at a PIL below the breakup region, leading to the formation of the flare loop arcade and the EUV hot channel. The scenario is supported by concentrated currents and free magnetic energy built up by antiparallel flows along the PIL before the flare. The reconnection probably propagated to involve the original filament itself, leading to its breakup and reformation. The reconnection geometry provides a general mechanism for the formation of the long filament channel and realizes the concept of tether cutting, which was active throughout the filament's rise phase, lasting from at least 30 min before the C2 flare until the eruption. The C2 flare represents a period of fast reconnection during this otherwise more steady process, during which most of the original filament was reconnected and joined the longer channel.

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arXiv2024

High-resolution Observation of Blowout Jets Regulated by Sunspot Rotation

Coronal jets are believed to be the miniature version of large-scale solar eruptions. In particular, the eruption of a mini-filament inside the base arch is suggested to be the trigger and even driver of blowout jets. Here we propose an alternative triggering mechanism, based on high-resolution H-alpha observations of a blowout jet associated with a mini-filament and an M1.2-class flare. The mini-filament remains largely stationary during the blowout jet, except that it is straddled by flare loops connecting two flare ribbons, indicating that the magnetic arcade embedding the mini-filament has been torn into two parts, with the upper part escaping with the blowout jet. In the wake of the flare, the southern end of the mini-filament fans out like neighboring fibrils, indicative of mass and field exchanges between the mini-filament and the fibrils. The blowout jet is preceded by a standard jet. With H-alpha fibrils moving toward the single-strand spire in a sweeping fashion, the standard jet transitions to the blowout jet. The similar pattern of standard-to-blowout jet transition occurs in an earlier C-class flare before the mini-filament forms. The spiraling morphology and sweeping direction of these fibrils are suggestive of their footpoints being dragged by the leading sunspot that undergoes clockwise rotation for over two days. Soon after the sunspot rotation reaches a peak angular speed as fast as 10 deg/hr, the dormant active region becomes flare-productive, and the mini-filament forms through the interaction of moving magnetic features from the rotating sunspot with satellite spots/pores. Hence, we suggest that the sunspot rotation plays a key role in building up free energy for flares and jets and in triggering blowout jets by inducing sweeping motions of fibrils.

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arXiv2024

Sun-as-a-star observations of obscuration dimmings caused by filament eruptions

Filament eruptions often lead to coronal mass ejections (CMEs) on the Sun and are one of the most energetic eruptive phenomena in the atmospheres of other late-type stars. However, the detection of filament eruptions and CMEs on stars beyond the solar system is challenging. Here we present six filament eruption cases on the Sun and show that filament material obscuring part of the solar disk can cause detectable dimming signatures in sun-as-a-star flux curves of He II 304 A. Those filament eruptions have similar morphological features, originating from small filaments inside active regions and subsequently strongly expanding to obscure large areas of the solar disk or the bright flare regions. We have tracked the detailed evolution of six obscuration dimmings and estimated the dimming properties, such as dimming depths, dimming areas, and duration. The largest dimming depth among the six events under study is 6.2% accompanied by the largest dimming area of 5.6\% of the solar disk area. Other events have maximum dimming depths in a range of around 1% to 3% with maximum areas varying between about 3% to 4% of the solar disk area. The duration of the dimming spans from around 0.4 hours to 7.0 hours for the six events under study. A positive correlation was found between the dimming depth and area, which may help to set constraint on the filament sizes in stellar observations.

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arXiv2024

Coupling between magnetic reconnection, energy release, and particle acceleration in the X17.2 2003 October 28 solar flare

The 2003 October 28 (X17.2) eruptive flare was a unique event. The coronal electric field and the π-decay γ-ray emission flux had the highest values ever inferred in solar flares. This study reveals physical links between the magnetic reconnection process, the energy release, and the acceleration of electrons and ions to high energies in the chain of the magnetic energy transformations in the impulsive phase of the solar flare. The global reconnection rate and the local reconnection rate are calculated from flare ribbon separation in Hα filtergrams and photospheric magnetic field maps. Available results of INTEGRAL and CORONAS-F/SONG observations are combined with Konus-Wind data to quantify time behavior of electron and proton acceleration. Prompt γ-ray lines and delayed 2.2 MeV line temporal profiles observed with Konus-Wind and INTEGRAL/SPI used to detect and quantify the nuclei with energies of 10-70 MeV. The global and local reconnection rates reach their peaks at the end of the main rise phase of the flare. The spectral analysis of the high-energy γ-ray emission revealed a close association between the acceleration process efficiency and the reconnection rates. High-energy bremsstrahlung continuum and narrow γ-ray lines were observed in the main rise phase. In the main energy release phase, the upper energy of the bremsstrahlung spectrum was significantly reduced and the pion-decay γ-ray emission appeared abruptly. We discuss the reasons why the change of the acceleration regime occurred along with the large-scale magnetic field restructuration of this flare. We argue that the main energy release and proton acceleration up to subrelativistic energies began just when the reconnection rate was going through the maximum, i.e., after a major change of the flare topology.

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arXiv2024

Efficiency of solar microflares in accelerating electrons when rooted in a sunspot

We investigate two microflares of GOES classes A9 and C1 (after background subtraction) observed by STIX onboard Solar Orbiter with exceptionally strong nonthermal emission. We complement the hard X-ray imaging and spectral analysis by STIX with co-temporal observations in the (E)UV and visual range by AIA and HMI, in order to investigate what makes these microflares so sufficient in high-energy particle acceleration. We performed case studies of two microflares observed by STIX on October 11, 2021 and November 10, 2022 that showed unusually hard microflare X-ray spectra with power-law indices of the electron flux distributions delta = 2.98 and delta = 4.08 during their nonthermal peaks and photon energies up to 76 keV and 50 keV respectively. For both events under study, we found that one footpoint is located within a sunspot covering areas with mean magnetic flux densities in excess of 1500 G, suggesting that the hard electron spectra are caused by the strong magnetic fields in which the flare loops are rooted. In addition, we revisited the unusually hard RHESSI microflare initially published by Hannah et al. (2008b) and found that in this event also one flare kernel was located within a sunspot, which corroborates the result from the two hard STIX microflares under study. We conclude that the characteristics of the strong photospheric magnetic fields inside sunspot umbrae and penumbrae where the flare loops are rooted play an important role in the generation of exceptionally hard X-ray spectra in these microflares.

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