Early JWST observations of TRAPPIST-1 have revealed an unexpected puzzle: energetic white-light flares (E > 10(30) erg) reach temperatures of only similar to 3500-4000 K, nearly 3 times cooler than typical solar flares, which peak around 9000-10,000 K. Here we explain this difference by identifying the physical mechanism that regulates flare temperatures on late M dwarfs. The key factor is that in the cool, dense atmosphere of TRAPPIST-1, magnetic heating is strongly moderated by the dissociation of molecular hydrogen (H-2) into atomic hydrogen. This "H-2 dissociation thermostat" acts as an efficient energy sink, preventing flare regions from heating above similar to 4000 K. Our chemical equilibrium and heat capacity calculations show that this effect depends sensitively on stellar atmospheric pressure and the local abundance of H-2. In hotter stars, from early M dwarfs to solar-type stars, the scarcity of molecular hydrogen renders this mechanism ineffective; instead, atomic hydrogen ionization limits flare temperatures near similar to 9000 K.

The study of exoplanet atmospheres has brought a renewed interest in stellar astrophysics. Specifically, stellar magnetic activity from the host star contaminates exoplanet atmosphere transmission spectra, as evidenced by observations from the James Webb Space Telescope. The stellar surface magnetic features in the form of dark spots caused by large concentrations of surface magnetic fields and more diffuse and extended faculae caused by small-scale magnetic field concentrations change the apparent size of the star in a wavelength-dependent way. This spot and faculae contribution to the change in transit depth contaminates the planetary signal. Here we study the transition from bright faculae on G- and K-dwarfs to dark faculae on M-dwarfs. This dark appearance of faculae is in significant contrast to the conventional picture that faculae are brighter than the quiet star region as they are on the Sun. We use the 3D radiative magnetohydrodynamics code MURaM to simulate faculae, and calculate the faculae spectra with the MPS-ATLAS radiative transfer code. We present a qualitative explanation for the transition from dark to bright faculae attributing it to shallower flux tubes and reduced vertical temperature gradients at the surfaces of M dwarfs relative to the Sun.

Context. Numerical simulations of the solar chromosphere have progressed towards reproducing spicules, which are transient features observed at the solar limb using spectral lines such as Hα, Ca II H&K, or Mg II h&k. Two types of spicules, referred to as types I and II, have been identified in observations and studied in previous numerical works. The statistics of type II spicules in 3D numerical simulations have not yet been studied. Aims. We aim to compare the statistics of properties such as lengths, lifetimes, widths, heights, inclinations, and maximum velocities of self-consistently formed spicules in a MURaM-ChE simulation with observations. Methods. We employ a Hα proxy to identify fine-scale structures at the solar limb resembling spicules in a simulation of an enhanced network region. We track the evolution of 58 such features found in a 21-minute time sequence, and compare their dynamical and morphological properties with those derived from quiet-Sun observations using the Solar Optical Telescope (SOT) onboard the Hinode mission in the Ca II H spectral line. Previous studies have shown that spicules show very similar properties in Ca II H and Hα. Results. The spicule-like structures found in the simulation have statistical properties which are broadly consistent with those observed with Hinode/SOT. In particular, we find evidence for the self-consistent formation of both type I and type II spicules within the simulation, even in the absence of ambipolar diffusion. We also investigate the properties of rapid blueshifted and redshifted excursions (RBEs and RREs) in the simulation in relation to the spicules.

Context. Spicules are ubiquitous, small-scale features in the solar atmosphere, exhibiting a jet-like appearance most clearly identified by their apparent motion in off-limb observations. While they are often interpreted as narrow, thread-like structures, their true 3D structure remains unknown. Aims. We aim to uncover the 3D morphology and dynamics of fast-evolving spicules (type II) using a MURaM-ChE simulation. Methods. We used a Hα proxy that has been developed using non-equilibrium (NE) hydrogen populations in MURaM-ChE. The proxy, modelled as an escape probability, was synthesised to isolate on-disc as well as off-limb Hα wing features. The 3D structure of these features was investigated using the 3D information on opacity in Hα. Results. We identify type II spicules with unique 3D morphologies, the dominant ones being thread-like and slab-like. The appearance of spicules as slabs or threads is a function of time and Doppler velocity. The spicules extending above the spicule-forest (2–3 Mm above the surface) tend to be located at quasi-separatrix layers (QSLs). We find that the spatially resolved contributions to the opacity of spicules are often similar for spicules synthesised in the horizontal direction, and their on-disc rapid blueshifted excursion (RBE) synthesised in the vertical direction at the same Doppler velocity of 37 km/s. This confirms that RBEs are indeed the on-disc counterparts of spicules. Furthermore, our analysis indicates that cross-field motions can significantly contribute to spicule dynamics. Conclusions. Spicules exhibit a range of morphologies, including both slab-like and thread-like structures. Their observed appearance depends strongly on line-of-sight (LoS) projection and Doppler sampling. Spicules are preferentially located at QSLs, highlighting the role of magnetic topology in driving spicular dynamics.

We report on the measurement of the height-dependent time shifts of wave pulses in the lower solar atmosphere from high-resolution spectro-polarimetric observations obtained with the SUSI instrument on board the SUNRISE III balloon-borne solar observatory during its successful science flight in July 2024. The line-of-sight velocities derived from the line-core positions of 19 spectral lines in a 2 nm-wide window around the Ca II H line were used to determine the time shifts of propagating pulses at their respective formation heights. Our analysis reveals that these shifts are roughly ordered according to the computed formation heights of the respective spectral lines. A statistical analysis of the time shifts using sit-and-stare observations with a total duration of one hour reveals that wave pulses propagating upwards from near the solar surface to heights of approximately 500-700 km are most common, with average time lags of 20 s to 30 s between these heights. Also present are pulses with close-to-zero phase shifts, predominantly above intergranular lanes and areas of enhanced magnetic activity. Additionally, downward propagating wave pulses with negative time lags of 10 s to 15 s are seen, mostly above areas of enhanced magnetic activity. A common feature of all the observed pulses is that in the lower 250 km they show small time lags of zero to a few seconds, and only at higher layers do the propagating pulses become more dominant. This study demonstrates the potential of the many-line approach for investigating the height dependence of the physical conditions in the solar atmosphere.

The interaction between the magnetic field and turbulent convection in the Sun's photosphere drives the dynamics, evolution and structuring of its magnetized atmosphere. This interaction often takes place at or below the spatial resolution of modern-day observations. Here we report on high-spatial-resolution observations of the solar photosphere acquired using the world's first 4-m class solar telescope, the US National Science Foundation's Daniel K. Inouye Solar Telescope. Time sequence images reveal a far more complex and dynamic solar scene than previously observed. We identify ubiquitous magnetized Kelvin-Helmholtz instabilities at the edges of magnetic flux concentrations and provide experimental confirmation of a long-standing theoretical prediction1,2. The discovery of small-scale magnetized Kelvin-Helmholtz instabilities in the solar photosphere, which can be reproduced by high-resolution numerical simulations, has far-reaching implications for our understanding of the creation and dissipation of magnetic fields exhibiting vortex motion, which can lead to flux braiding. Our results support the picture of disjoint magnetic field concentrations in layers below the visible solar surface that connect to monolithic flux regions visible as facular concentrations and pores in the solar photosphere. Kelvin-Helmholtz instabilities are an efficient mechanism for transporting mass, energy, momentum and magnetic flux in magnetohydrodynamic systems, and they offer transformative insights into processes in magnetically active regions such as the one observed here.

Context. Flux emergence is ubiquitous in the Sun's lower atmosphere. The emerging flux can reconnect with the pre-existing magnetic field. Aims. We aim to investigate plasmoid formation and the resulting multithermal emissions during the three-dimensional reconnection process in the lower solar atmosphere. Methods. We conducted 3D radiation magnetohydrodynamic (RMHD) simulations using the MURaM code, which incorporates solar convection and radiation. We simulated the emergence of a flat magnetic flux sheet that was introduced into the convection zone. For comparison with results previously reported from observations, we employed the RH1.5D code to synthesize H alpha and Si IV spectral line profiles and we synthesized the ultraviolet images using the optical thin methods. Results. Flux emergence took place as part of the imposed flux tube crossed the photosphere. In the lower solar atmosphere, magnetic reconnection occurred and formed thin, elongated current sheets. Plasmoid-like features appear as part of the reconnection process; this results in many small twisted magnetic flux ropes, which are expelled toward the two ends of the reconnection region. Consequently, hot plasma with a temperature exceeding 20 000 K and much cooler plasmas with a temperature below 10 000 K can coexist in the reconnection region. Synthesized images and spectral line profiles through the reconnection region display typical characteristics of reconnection occuring in the lower solar atmosphere, such as Ellerman bombs (EBs) and UV bursts. The cooler plasmas that show characteristics of EBs can be found above hot plasma and reach altitudes more than 2 Mm above the solar surface. Meanwhile, some hot plasma that features characteristics of UV bursts can extend downward to the lower chromosphere, approximately 0.7 Mm above the solar surface. Conclusions. Our simulation results indicate that the turbulent reconnection mediated with plasmoid instability can occur in small-scale reconnection events such as EBs and UV bursts. The coexistence of hot and much cooler plasmas in such a turbulent reconnection process can well explain the temporal and spatical connection of UV bursts with EBs.

Stellar activity is fundamental to stellar evolution and the formation and habitability of exoplanets. Magnetic surface activity is driven by the interaction between convective motions and rotation in cool stars, resulting in a dynamo process. In single stars, activity increases with rotation rate until it saturates for stars with rotation periods P-rot < 3-10 d. However, the mechanism responsible for saturation remains unclear. Observations indicate that red giants in binary systems that are in spin-orbit resonance exhibit stronger chromospheric activity than single stars with similar rotation rates, suggesting that tidal flows can influence surface activity. Here, we investigate the chromospheric activity of main-sequence binary stars to understand the impact of tidal forces on saturation phenomena. For binaries with 0.5 < P-rot (d) < 1, mainly contact binaries that share a common thermal envelope, we find enhanced activity rather than saturation. This result supports theoretical predictions that a large-scale alpha-omega dynamo during common-envelope evolution can generate strong magnetic fields. We also observe supersaturation in chromospheric activity, a phenomenon tentatively noted previously in coronal activity, where activity levels fall below saturation and decrease with shorter rotation periods. Our findings emphasize the importance of studying stellar activity in stars with extreme properties compared with the Sun's.

The magnetic fields and dynamical processes in the solar polar regions play a crucial role in the solar magnetic cycle and in supplying mass and energy to the fast solar wind, ultimately being vital in controlling solar activities and driving space weather. Despite numerous efforts to explore these regions, to date no imaging observations of the Sun's poles have been achieved from vantage points out of the ecliptic plane, leaving their behavior and evolution poorly understood. This observation gap has left three top-level scientific questions unanswered: How does the solar dynamo work and drive the solar magnetic cycle? What drives the fast solar wind? How do space weather processes globally originate from the Sun and propagate throughout the solar system? The Solar Polarorbit Observatory(SPO) mission, a solar polar exploration spacecraft, is proposed to address these three unanswered scientific questions by imaging the Sun's poles from high heliolatitudes. In order to achieve its scientific goals, SPO will carry six remote-sensing and four in-situ instruments to measure the vector magnetic fields and Doppler velocity fields in the photosphere, to observe the Sun in the extreme ultraviolet, X-ray, and radio wavelengths, to image the corona and the heliosphere up to 45 Rs, and to perform in-situ detection of magnetic fields,and low-and high-energy particles in the solar wind. The SPO mission is capable of providing critical vector magnetic fields and Doppler velocities of the polar regions to advance our understanding of the origin of the solar magnetic cycle, providing unprecedented imaging observations of the solar poles alongside in-situ measurements of charged particles and magnetic fields from high heliolatitudes to unveil the mass and energy supply that drive the fast solar wind, and providing observational constraints for improving our ability to model and predict the three-dimensional(3D) structures and propagation of space weather events.

Accurate calculations of starspot spectra are essential for multiple applications in astronomy. The current standard is to represent starspot spectra by spectra of stars that are cooler than the quiet star regions. This implies approximating a starspot as a nonmagnetic 1D structure in radiative-convective equilibrium, parametrizing convective energy transport by mixing-length theory. It is the inhibition of convection by the starspot magnetic field that is emulated by using a lower spot temperature relative to the quiet stellar regions. Here, we take a different approach, avoiding the approximate treatment of convection and instead self-consistently accounting for the interaction between matter, radiation, and the magnetic field. We simulate spots on G2V, K0V, and M0V stars with the 3D radiative magnetohydrodynamics code MURaM and calculate spectra (R approximate to 500 from 250 to 6000 nm) using ray-by-ray radiative transfer with the MPS-ATLAS code. We find that the 1D models fail to return accurate umbral and penumbral spectra on K0V and M0V stars, where convective and radiative transfer of energy is simultaneously important over a broad range of atmospheric heights, rendering mixing-length theory inaccurate. However, 1D models work well for G2V stars, where both radiation and convection significantly contribute to energy transfer only in a narrow region near the stellar surface. Quantitatively, the 1D approximation leads to errors longward of 500 nm of about 50% for both umbral and penumbral flux contrast relative to quiet star regions on M0V stars and less than 2% (for umbrae) and 10% (for penumbrae) for G2V stars.

The European Space Agency has selected PLATO (PLAnetary Transits and Oscillations of stars) for its M3 launch which is scheduled for 2026. With its extremely large field of view, PLATO is designed to obtain photometric measurements over an extended period for bright stars in order to detect and characterise (primarily) rocky planets in the habitable zones of solar type stars. The PLATO measurements will have sufficient sensitivity to determine the mass, radius and age of the host stars with unprecedented accuracy. The PLATO planet database will provide the first large-scale catalogue of accurately and homogeneously characterised small planets at intermediate orbital periods, which will can be used to severely constraint planet formation theories. This would facilitate large scale comparative exo-planetology. In addition the bright PLATO host stars will be ideal targets for atmospheric study with next generation facilities such as the ELT. The PLATO sensitivity will be sufficient to detect pulsations from stars across the HR diagram allowing a deep understanding of stellar structure and evolution to be developed using parameters determined from asteroseismology.

Context. H alpha observations of the solar chromosphere reveal dynamic small-scale structures known as spicules at the limb and rapid blueshifted and redshifted excursions (RBEs and RREs) on-disc. Aims. We want to understand what drives these dynamic features, their magnetohydrodynamic (MHD) properties, and their role in energy and heat transport to the upper solar atmosphere. To do this, we aim to develop a proxy for synthetic H alpha observations in radiative-MHD simulations to help identify these features. Methods. We used the chromospheric extension to the MURaM code (MURaM-ChE) to simulate an enhanced network region. We developed a proxy for H alpha based on a photon escape probability. This is a Doppler-shifted proxy that we used to identify fine structures in the line wings. We studied on-disc features in 3D, obtaining their 3D structure from the absorption coefficient. Results. We validate the H alpha proxy by comparing it against features detected in the wings of H alpha synthesised using MULTI3D. We detect numerous small-scale structures rooted at the network patches, similar to observations in H alpha. The dynamics of an example feature (RBE) at a Doppler shift of 37 km/s show that flux emergence and consequent reconnection drive the formation of this feature. Pressure gradient forces build up to drive a flow along the field line carrying the feature, making it a jet. There is strong viscous and resistive heating on the first appearance of the feature associated with the flux emergence. At the same time and location, a heating front appears and propagates along the field lines at speeds comparable to the Alfv & eacute;n velocity. The feature shows an oscillatory behaviour as it evolves. Conclusions. We show that a synthetic observable based on an escape probability is able to reliably identify features observed with the H alpha spectral line. We demonstrate its applicability by studying the formation, dynamics and properties of an RBE.

Context. Joy’s law describes the tilt of bipolar active regions on the Sun away from an east-west orientation, where the flux of the polarity concentrated at the prograde side tends to be closer to the equator than the polarity on the retrograde side. Joy’s law is attributed to the Coriolis force because of the observed increase in the tilt angle at higher latitudes. This tilt plays a crucial role in some solar dynamo models. Aims. Our goal is to model the effects of the Coriolis force on a flux tube as it rises through the near-surface convection zone. Methods. We used a three-dimensional Cartesian magnetohydrodynamic simulation of an untwisted flux tube ascending from a depth of 11 Mm. We modelled the Coriolis effect using the f -plane approximation, which only considers and acts on horizontal flows. On the Sun, Joy’s law is weak and is only evident as an average over many active regions. To achieve a measurable effect in a single simulation, we considered a rotation rate 110 times faster than that of the Sun. Results. The simulation shows that the flux tube emerges at the surface with a tilt angle consistent with Joy’s law when scaled to the Sun’s slower rotation, and the tilt angle does not substantially change after emergence. Conclusions. This shows that the Coriolis force acting on flows horizontal to the surface within the near-surface convection zone is consistent with Joy’s law.

Emerging active regions are associated with convective flows on the spatial scale and lifetimes of supergranules. To understand how these flows are involved in the formation of active regions, we aim to identify where active regions emerge in the supergranulation flow pattern. We computed supergranulation scale flow maps at the surface for all active regions in the Solar Dynamics Observatory Helioseismic Emerging Active Region Survey. We classified each of the active regions into four bins based on the amplitude of their average surface flow divergence at emergence. We then averaged the flow divergence over the active regions in each bin as a function of time. We also considered a corresponding set of control regions. We found that, on average, the flow divergence increases during the day prior to emergence at a rate independent of the amount of flux that emerges. By subtracting the averaged flow divergence of the control regions, we found that active region emergence is associated with a remaining converging flow at 0.5-1 d prior to emergence. This remnant flow, Delta divv(h)=(-4.9 +/- 1.7)x10(-6)s(-1), corresponds to a flow speed of 10-20 m s-1 (an order of magnitude less than supergranulation flows) out to a radius of about 10 Mm. We show that these observational results are qualitatively supported by simulations of a small bipole emerging through the near-surface convective layers of the Sun. The question remains whether these flows are driving the emergence, or are caused by the emergence.

The difference between individual solar cycles in the magnetic butterfly diagram can mostly be ascribed to the stochasticity of the emergence process. We aim to obtain the expectation value of the butterfly diagram from observations of four cycles. This allows us to further determine the generation rate of the surface radial magnetic field. We use data from Wilcox Solar Observatory to generate time-latitude diagrams spanning cycles 21 to 24 of the surface radial and toroidal magnetic fields, symmetrize them across the equator and cycle-average them. From the mean butterfly diagram and surface toroidal field we then infer the mean poloidal field generation rate at the surface of the Sun. The averaging procedure removes realization noise from individual cycles. The amount of emerging flux required to account for the evolution of the surface radial field is found to match that provided by the observed surface toroidal field and Joy's law. Cycle-averaging butterfly diagrams removes realization noise and artefacts due to imperfect scale separation, and corresponds to an ensemble average that can be interpreted in the mean-field framework. The result can then be directly compared to $\alpha\Omega$-type dynamo models. The Babcock-Leighton $\alpha$-effect is consistent with observations, a result that can be appreciated only if the observational data is averaged in some way.

Synoptic magnetic field data usually serves as the boundary condition for simulations of the global magnetic field; however, it has been shown that these data suffer from an “aging effects” as the longitudinal 360° information can only be obtained over the course of one solar rotation. To avoid this, we use advanced magnetograms produced by feeding near-side HMI/SDO magnetograms and far-side helioseismic active regions into a modified surface flux transport model to improve the modeling of the far-side magnetic configuration. This allows for a more accurate description of the state of the global magnetic field and thus for an improved forecasting of solar wind parameters. We use potential field source surface (PFSS) and WSA modeling as well as the EUropean Heliospheric FORecasting Information Asset (EUHFORIA) to derive the coronal magnetic field configuration and the heliospheric solar wind structure as well as discuss the changes caused by the implementation of far-side active regions into magnetic field maps. Modeled solar wind results are found to be in good agreement with far-side in-situ measurements taken by various instruments. We can show the importance of considering not only the solar near-side but also the far-side to accurately model the heliosphere in which solar transients are propagated.

Context. Three-dimensional (3D) radiative magnetohydrodynamic (MHD) simulations are the only way to model stellar atmospheres without any ad hoc parameterisations. Several 3D radiative MHD codes have achieved good quantitative agreement with observables for our Sun. Aims. We aim to validate the most recent version of the MURaM code by comparing MURaM results to well-established quiet-Sun measurements, in particular spatially averaged measurements that are relevant for stellar studies. This extends the number of solar observables that MURaM can reproduce with high precision. Our validation is an essential condition to ensure that MURaM can be used to accurately calculate the spectra of other cool stars. Methods. We simulated the solar photosphere and upper convection zone, which harbours a small-scale-dynamo. Using time series of 3D snapshots, we calculated the spectral irradiance, limb darkening, and selected spectral lines, which we compared to observations. Results. The computed observables agree well with the observations; in particular, the limb darkening of the quiet Sun is reproduced remarkably well.

Stars appear darker at their limbs than at their disk centres because at the limb we are viewing the higher and cooler layers of stellar photospheres. Yet, limb darkening derived from state-of-the-art stellar atmosphere models systematically fails to reproduce recent transiting exoplanet light curves from the Kepler, TESS and JWST telescopes-stellar brightness obtained from measurements drops less steeply towards the limb than predicted by models. Previous models assumed stellar atmospheres devoid of magnetic fields. Here we use stellar atmosphere models computed with the three-dimensional radiative magnetohydrodynamic code MURaM to show that a small-scale concentration of magnetic fields on the stellar surface affects limb darkening at a level that allows us to explain the observations. Our findings provide a way forward to improve the determination of exoplanet radii and especially the transmission spectroscopy analysis for transiting planets, which relies on a very accurate description of stellar limb darkening from the visible to the infrared. Furthermore, our findings imply that limb darkening allows estimates of the small-scale magnetic field strength on stars with transiting planets. An outstanding discrepancy between observations and models of stellar limb darkening is resolved here by the inclusion of stellar surface magnetism in models. This will enable an improved characterization of transiting exoplanets.

Stars appear darker at their limbs than at their disk centers because at the limb we are viewing the higher and cooler layers of stellar photospheres. Limb darkening derived from state-of-the-art stellar atmosphere models systematically fails to reproduce recent transiting exoplanet light curves from the Kepler, TESS, and JWST telescopes – stellar brightness obtained from measurements drops less steeply towards the limb than predicted by models. All previous models assumed atmosphere devoid of magnetic fields. Here we use our new stellar atmosphere models computed with the 3D radiative magneto-hydrodynamic code MURaM to show that small-scale concentration of magnetic fields on the stellar surface affect limb darkening at a level that allows us to explain the observations. Our findings provide a way forward to improve the determination of exoplanet radii and especially the transmission spectroscopy analysis for transiting planets, which relies on a very accurate description of stellar limb darkening from the visible through the infrared. Furthermore, our findings imply that limb darkening allows measuring the small-scale magnetic field on stars with transiting planets.