Space weather and its potential negative consequences for life on Earth have received increasing scientific attention in recent decades. In particular, predicting the onset of coronal mass ejections (CMEs) has become important from a security perspective. To predict CMEs, one must first understand the dynamics leading to preeruptive magnetic field configurations, which in many theories include a flux rope. In this study, we investigate the realistic formation of coronal flux ropes above the solar photosphere. The aim is to find out if and how flux ropes can form there, and how the formation is related to flux cancellation at the photosphere. Previously, such formation has been shown in smooth boundary-driven line-tied simulations and in idealized non-convective and symmetric flux-emergence simulations. We ran a convective nonsymmetric 3D radiative magnetohydrodynamic (MHD) simulation with the code . Within the simulation box of $ 24 24 $ horizontal extent, a linear force-free field with sheared coronal arcades was slowly inserted. Following the insertion, the self-consistent stochastic plasma flows of the convection zone drove several small-scale flux cancellations and magnetic reconnection, without external influence. Lagrangian markers called corks were used to track the dynamic evolution of the magnetic field. Over a period of SI 2.5 , a flux rope was generated with photospheric footpoints separated by up to SI 12 . The flux rope was gradually formed through several individual events, such as slipping reconnection, U-loop emergence, and thick-photosphere tether-cutting reconnection. Flux ropes, which can lead to CMEs, can be formed in the solar atmosphere solely driven by convection and flux cancellations at the photosphere. However, not all flux cancellations contribute to the buildup of the flux rope, and some coronal reconnection events that do are not clearly related to flux cancellation. The formation process of flux ropes from coronal sheared arcades driven by convection is therefore more complex than in the original smooth flux cancellation model. However, the end result is qualitatively the same. Flux cancellation works. A flux rope is formed.

The exact location of the solar dynamo remains uncertain-whether it operates primarily in the near-surface shear layer, throughout the entire convection zone, or near the tachocline - a region of sharp transition in the solar rotation, located at the base of the convection zone, approximately 200,000 km beneath the surface. Various studies have supported each of these possibilities. Notably, the solar magnetic 'butterfly' diagram and the pattern of zonal flows ('torsional oscillations') exhibit strikingly similar characteristics, suggesting a link between magnetic field evolution and solar flows. Since magnetic fields cannot be measured directly in the deep solar interior, torsional oscillations and rotation gradients are employed as diagnostic proxies. Our analysis reveals that the gradient of rotation displays 'butterfly'-like behavior near the tachocline, which is similar to the magnetic butterfly diagram at the surface. This result supports the idea that the solar dynamo has a deep-seated origin, likely operating either near the tachocline or throughout the convection zone, thereby disfavoring the recent scenario of a shallow, near-surface dynamo. This finding may also have important implications for understanding how stellar dynamos operate in general.

An important key to studying the impact of solar activity variations on the Earth’s climate is the Maunder Minimum (late 17th century), during which extremely little sunspots were observed. Applying the rare event analysis method to these observations led the researchers to conclude that the appearance of sunspots during the Maunder minimum had a weak amplitude 22-year cycle. The concept of continuity of magnetic cycles at this time is also confirmed by measurements of cosmogenic radionuclides in natural terrestrial archives. Therefore, today it is believed that during the Maunder Minimum, the cyclic magnetic activity of the Sun did not stop, although the amplitude of the cycles was quite low. In the αΩ dynamo model, this may be due to the fact that the magnitude of the magnetic induction of the toroidal field, excited by radial differential rotation in the solar convection zone (SCZ), at this time did not reach the threshold value required for lifting magnetic power tubes to the solar surface (nonlinear dynamo mode). Possible physical mechanisms describing the suppression of the dynamo process at time intervals, when no sunspots were observed, are analysed. A scenario for explaining the north-south asymmetry of magnetic activity during the Maunder Minimum is proposed. A key role in the proposed scenario is played by the special nature of the internal rotation of the Sun, revealed in the helioseismological experiments. According to helioseismology data, the SCZ is naturally divided into polar and equatorial domains with opposite signs of the radial angular velocity gradient. In addition, the radial angular velocity gradient penetrates into the deep layers of the stable radiant zone below the SCZ. It is shown that, taking into account these helioseismology data, the αΩ dynamo excites two harmonics (dipole and quadrupole) of the toroidal magnetic field in the SCZ, which cyclically change their direction. The αΩ dynamo excites two harmonics (dipole and quadrupole) of the toroidal field in the RMS, which cyclically change their direction. At the same time, the deep Ω effect in the radiant zone creates the toroidal field of stationary orientation. The summary toroidal magnetic field (the dynamo-field of the SCZ + the field of the radiant zone) rising to the Sun’s surface due to magnetic buoyancy may contribute to the north-south asymmetry in sunspot activity.

ABSTRACT A large part of the hot corona consists of magnetically confined, bright plasma loops. These observed loops are in turn structured into bright strands. We investigate the relationship between magnetic field geometry, plasma properties, and bright strands with the help of a three-dimensional resistive magnetohydrodynamic (MHD) simulation of a coronal loop rooted in a self-consistent convection zone layer. We find that it is impossible to identify a loop as a simple coherent magnetic flux tube that coincides with plasma of nearly uniform temperature and density. The location of bright structures is determined by a complex interplay between heating, cooling, and evaporation time-scales. Current sheets form preferentially at the interfaces of magnetic flux from different sources. They may also form within bundles of magnetic field lines since motions within magnetic concentrations drive plasma flows on a range of time-scales that provide further sub-structure and can locally enhance magnetic field gradients and thus facilitate magnetic reconnection. The numerical experiment therefore possesses aspects of both the flux tube tectonics and flux braiding models. While modelling an observed coronal loop as a cylindrical flux tube is useful to understand the physics of specific heating mechanisms in isolation, it does not describe well the structure of a coronal loop rooted in a self-consistently evolving convection zone.

We present a radiation-magnetohydrodynamics (RMHD) simulation of a magnetic cancellation event. The model is calculated with the Bifrost code and spans from the uppermost convection zone to the corona. The cancellation occurs between the positive polarity of an emerged magnetic bipole and a preexisting negative polarity. We try both to understand the RMHD aspects as well as to carry out comparison to observations, in part via spectral synthesis of optically thick photospheric and chromospheric lines using the RH1.5D code, and optically thin coronal ones. The reconnection between the opposite flux systems takes place at chromospheric heights through a quasi-separatrix layer without null points. Sharp V-shaped upward-moving field lines and highly warped downward-moving post-reconnection loops are created. The chromospheric reconnection is in full swing when the colliding magnetic patches are still separated by a granular cell at the photosphere. In a later phase, photospheric cancellation takes place with submergence of the closed magnetic loops linking the opposite polarities. We carry out comparisons with the observations of the photospheric magnetic flux loss rates, as well as of the horizontal magnetic field and vertical velocity at the polarity inversion line. The reconnection outflows cause intensity brightenings, jets and different spectral features in the synthesized chromospheric spectral lines, strongly reminiscent of those found in recent observations. Coherent, twisted magnetic flux ropes are created by the flows associated with the process. Including coronal levels is crucial for proper modeling, even if no major ejection or brightening is produced in the corona in this event.

The paper presents a mean-field model for large-scale flows in convection zones of the Sun and solar-type stars. The model extends former differential rotation models by allowance for variations of the flow with time and its deviation from axial symmetry. The model is realized as a numerical code, which combines the spectral method of decomposition in spherical functions with second-order accurate finite-difference method in time and radius. First computations show close agreement of the axially symmetric part of the computed flow with helioseismological detections of differential rotation and meridional circulation. Patterns of the time-decaying non-axisymmetric flow computed with the model qualitatively agree with the Rossby waves observed on the Sun. The paper also formulates a problem for further development of the large-scale flow theory.

The paper presents a mean-field model for large-scale flows in convection zones of the Sun and solar-type stars. The model extends former differential rotation models by allowance for variations of the flow with time and its deviation from axial symmetry. The model is realized as a numerical code, which combines the spectral method of decomposition in spherical functions with second-order accurate finite-difference method in time and radius. First computations show close agreement of the axially symmetric part of the computed flow with helioseismological detections of differential rotation and meridional circulation. Patterns of the time-decaying non-axisymmetric flow computed with the model qualitatively agree with the Rossby waves observed on the Sun. The paper also formulates a problem for further development of the large-scale flow theory.

We present a new framework for constructing agnostic and yet physical models for planetary interiors and apply it to Uranus and Neptune. Unlike previous research that either impose rigid assumptions or rely on simplified empirical profiles, our approach bridges both paradigms. Starting from randomly generated density profiles, we applied an iterative algorithm that converges towards models that simultaneously satisfy hydrostatic equilibrium, match the observed gravitational moments, and remain thermodynamically and compositionally consistent. The inferred interior models for Uranus and Neptune span a wide range of possible interior structures, in particular encompassing both water-dominated and rock-dominated configurations (rock-to-water mass ratios between 0.04–3.92 for Uranus and 0.20–1.78 for Neptune). All models contain convective regions with ionic water and have temperature–pressure profiles that remain above the demixing curves for hydrogen–helium–water mixtures. This offers both a plausible explanation for the observed non-dipolar magnetic fields and indicates that no hydrogen–helium–water demixing occurs. We find a higher H-He mass fraction in the outer-most convection zones for Uranus (0.62–0.73) compared to Neptune (0.25–0.49) and that Uranus’ magnetic field is likely generated deeper in the interior compared to Neptune. We infer upper limits of 0.69–0.74 (Uranus) versus 0.78–0.92 (Neptune) for the outer edges of the dynamo regions in units of normalised radii. Overall, our findings challenge the conventional classification of Uranus and Neptune as ’ice giants’ and underscore the need for improved observational data or formation constraints to break compositional degeneracy.

Horizontally periodic Boussinesq Rayleigh-Bénard convection (RBC) is a simple model system for studying the formation of large-scale structures in turbulent convective flows. We performed a suite of 2D numerical RBC simulations between no-slip boundaries at different Prandtl (Pr) and Rayleigh (Ra) numbers, such that their product was representative of the upper solar convection zone. When the fluid viscosity was sufficiently low (Pr ≲ 0.1) and turbulence was strong (Ra > 10 6 ), large structures began to couple in time and space. For Pr = 0.01, we observed long-lived swaying oscillations of the upflows and downflows that synchronized over multiple convection cells. This new regime of oscillatory convection might offer an interpretation of the wave-like properties of the dominant convection scale on the Sun (supergranulation).

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. 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. 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. 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 $50%$.

Abstract This study investigates the gravitational waves (GWs) generated by the emergence of magnetic flux tubes in the solar convection zone. We focus on the upward buoyancy of magnetic flux tubes, which leads to significant magnetic activity and the formation of active region sunspots. This study adopts parameters representative of a moderate-sized solar active region to estimate the GWs generated by the emergence of magnetic flux tubes. Our results indicate that the GW strain amplitude, achievable through signal superposition and detection at close proximity (e.g., approximately one solar radius from the solar surface), may reach $\sim$10$^{-29}$. The characteristic GW frequency is estimated at $\sim$10$^{-5}$ Hz, placing it at the high-frequency end of the sensitivity band of Pulsar Timing Array (PTA) methods. \textcolor{RedAdd}{However, the estimated strain amplitudes remain orders of magnitude below the sensitivity thresholds of current and foreseeable gravitational wave detectors. Notably, reducing the cadence $\Delta t$ of Pulsar Timing Array (PTA) observations to approximately 2 hours ($\Delta t = 2\text{hours}$) would raise the maximum detectable frequency to about $5.8 \times 10^{-5} \text{Hz}$, thereby encompassing the dominant spectral component of solar activity-related GWs predicted in this study, offering a potential pathway for future detection.} Successful detection in the future may help to predict the super solar active region emergence in space weather forecasting.

The Lithium-Dip is a severe lithium depletion observed in mid-F (6200-6650 K) dwarfs, which has puzzled astronomers since it was discovered in 1986. Proposed mechanisms include effects related to rotation, magnetic fields, diffusion, gravity waves, and mass loss. Which, if any, of these is realistic remains unclear. Here we show that mixing due to shear induced by stellar angular momentum loss is the unique mechanism driving the lithium depletion. Each mechanism leaves a different signature in the subsurface lithium distribution. The deepening surface convection zones of subgiants of NGC 188 evolving out of the Lithium-Dip dredge up the subsurface material and thus reveal the signature of the responsible mechanism, rotation. Subgiants can also be used more generally, thereby improving fundamental understanding of stellar evolution. Rotational mixing may be the dominant lithium-depleting mechanism in a wide range of solar-type stars, including in the Sun. Our results may further reconcile the cosmological lithium discrepancy.

Context. Understanding the complex interactions between convection, magnetic fields, and rotation is key to modeling the internal dynamics of the Sun and stars. Under rotational influence, compressible convection forms prograde-propagating convective columns near the equator. The interaction between such rotating columnar convection and the small-scale dynamo (SSD) remains largely unexplored. Aims. We investigate the influence of the SSD on the properties of rotating convection in the equatorial regions of solar and stellar convection zones. Methods. A series of rotating compressible magnetoconvection simulations were performed using a local f -plane box model at the equator. The flux-based Coriolis number, Co * , was varied systematically. To isolate the effects of the SSD, we compared results from hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations. Results. The SSD affects both convective heat and angular momentum transport. In MHD cases, convective velocity decreases more rapidly with increasing Co * than in HD cases. This reduction is compensated for by enhanced entropy fluctuations, maintaining the overall heat transport efficiency. Furthermore, a weakly subadiabatic layer is maintained near the base of the convection zone even under strong rotational influence when the SSD is present. These behaviors reflect a change in the dominant force balance: the SSD introduces a magnetostrophic balance at small scales, while geostrophic balance persists at larger scales. The inclusion of the SSD also reduces the dominant horizontal scale of columnar convective modes by enhancing the effective rotational influence. Regarding angular momentum transport, the SSD generates Maxwell stresses that counteract the Reynolds stresses, thereby quenching the generation of mean shear flows. Conclusions. Small-scale magnetic fields interact nonlinearly with columnar convection and induce substantial modifications in the dynamics of rotating convection. These effects should be accounted for in models of solar and stellar convection.

Abstract Core-collapse supernovae (CCSNe) are among the most energetic and complex astrophysical phenomena, requiring three-dimensional (3D) simulations to capture their intricate explosion mechanisms. One of the key ingredients for such simulations is the 3D pre-collapse structure, which can impact the development and geometry of the subsequent explosion. While stellar convection simulations can provide such 3D initial conditions, these remain too expensive and demanding for widespread use. In this work, we present a method to generate synthetic 3D velocity fields for convective zones from 1D initial conditions, creating initial conditions for CCSN simulations using a vector spherical harmonics expansion without the need for expensive hydrodynamic progenitor simulations. The synthetic velocity field is designed to capture the typical scales and velocities of the convective flow as the most relevant parameters for the subsequent explosions. In addition, it respects relevant physical constraints such as the near-anelasticity of flow, vanishing radial vorticity, and zero net angular momentum in the convective zones. A Python implementation of this method is publicly available, offering the CCSN community a practical tool for generating synthetic velocity fields for multi-dimensional simulations to study the impact of 3D progenitor asymmetries on the CCSN mechanism.

Abstract Magnetic reconnection is an important driving mechanism of many chromospheric phenomena, e.g., UV bursts and chromospheric jets. Information about magnetic fields is indispensable for analyzing chromospheric magnetic reconnection, which is mainly encoded in polarization signals. The purpose of this work is to predict possible Stokes features related to chromospheric reconnection events, from realistic two-dimensional magnetohydrodynamic simulation and Stokes profile synthesis. An emerging magnetic flux sheet is imposed at the bottom boundary of a well-relaxed unipolar atmosphere that spans from the upper convection zone to the corona. The reconnection region is heated to ∼7 kK and the outflow velocity reaches up to ∼35 km s −1 . Through Stokes profile synthesis, several Stokes features related to reconnections and plasmoids are reproduced. We found sign reversal features on circular polarization and amplitude reduction features on linear polarization at reconnection sites. Also, we report strong linear and circular polarization signals corresponding to huge (∼300 km) and tiny (∼40 km) plasmoids, respectively. We conclude that both linear and circular polarization signals may reveal the distinctive physical mechanisms in reconnections and enhance our understanding of magnetic reconnection in observations.

Accurate determination of opacity is critical for understanding radiation transport in both astrophysical and laboratory plasmas. We employ atomic data from R-Matrix calculations to investigate radiative properties in high-energy-density (HED) plasma sources, focusing on opacity variations under extreme plasma conditions. Specifically, we analyze environments such as the base of the convective zone (BCZ) of the Sun (2×106 K, Ne=1023/cc), and radiative opacity data collected using the inertial confinement fusion (ICF) devices at the Sandia Z facility (2.11×106 K, Ne=3.16×1022/cc) and the Lawrence Livermore National Laboratory National Ignition Facility. We calculate Rosseland Mean Opacities (RMO) within a range of temperatures and densities and analyze how they vary under different plasma conditions. A significant factor influencing opacity in these environments is line and resonance broadening due to plasma effects. Both radiative and collisional broadening modify line shapes, impacting the absorption and emission profiles that determine the RMO. In this study, we specifically focus on electron collisional and Stark ion microfield broadening effects, which play a dominant role in HED plasmas. We assume a Lorentzian profile factor to model combined broadening and investigate its impact on spectral line shapes, resonance behavior, and overall opacity values. Our results are relevant to astrophysical models, particularly in the context of the solar opacity problem, and provide insights into discrepancies between theoretical calculations and experimental measurements. In addition, we investigate the equation-of-state (EOS) and its impact on opacities. In particular, we examine the “chemical picture” Mihalas–Hummer–Däppen EOS with respect to level populations of excited levels included in the extensive R-matrix calculations. This study should contribute to improving opacity models of HED sources such as stellar interiors and laboratory plasma experiments.

ABSTRACT About one in five white dwarfs undergoes spectral evolution from a helium atmosphere to hydrogen and then back to helium. These short-lived hydrogen envelopes – the result of residual hydrogen diffusion – are eventually destroyed by either hydrogen or helium convection. An emerging class of double-faced white dwarfs seems to catch this process in the act, with varying amounts of hydrogen across regions of the stellar surface. Here, we quantitatively test the hypothesis that these inhomogeneities are the result of the magnetic inhibition of convection. We compute the critical magnetic field $B_{\rm crit}(M,T_{\rm eff})$ required to inhibit convection in both hydrogen and helium for 0.6–1.2 ${\rm M}_{\odot }$ white dwarfs using two methods. Initially, we estimated $B_{\rm crit}\sim \sqrt{8\pi P}$ where P is the pressure at the base of the convection zone, finding that most (three out of four) of the observed magnetic double-faced white dwarfs could potentially be explained by the magnetic inhibition of hydrogen convective energy transfer, with measured $B\gtrsim B_{\rm crit}^{\rm H}$. Then, we incorporated the magnetic field consistently into the stellar structure and directly computed the boundary of convective mixing. With this more appropriate method, we find that only half (two out of four) of the stars could be explained by the magnetic inhibition of helium convection, with $B\gtrsim B_{\rm crit}^{\rm He}$. Specifically, order of unity variations in the magnetic field’s strength or orientation across the surface could account for the double-faced nature of these stars. Given our mixed results, other – including non-magnetic – scenarios should be considered as well.

Context. Radial differential rotation is an important factor in stellar dynamo theory. In the Sun, helioseismology has revealed a near-surface shear layer in the upper 5–10 percent of the convection zone. At low to midlatitudes, the rotation velocity gradient decreases sharply near the surface. A depth gradient in rotational velocity was recently detected in the low photosphere using a differential interferometric method on spectroscopic data. Granular structures at different depths in the Fe I 630.15 nm line showed a systematic retrograde shift compared to continuum structures, which suggests a height-related decrease in angular velocity. This estimate depends on the assumed granulation coherence time. Aims. We use a more direct approach to measure the differential rotational velocity at different photospheric heights. Methods. We performed spectroscopic scans of the same granular region in Fe I 630.15 nm and Ca I 616.2 nm lines, and measured displacements of images at different line chords between consecutive scans. These observations require excellent seeing, stable adaptive optics correction, and scanning times shorter than the granulation lifetime. Adaptive optics stabilizes continuum images but not higher-altitude rotation differences. We used both THEMIS and HINODE Solar Optical Telescope Fe I 630.15 nm data to measure formation height differences via perspective shifts observed away from the disk center with the slit radially oriented. Results. Measurements at disk center and ±25° latitude along the central meridian show a parabolic decrease in rotational velocity with height that reaches about 16% slower rotation at 80 km above the continuum. No significant difference is found between equator and ±25° latitudes. Conclusions. The low photosphere is a transition zone between the convective and radiative layers. Our measurements provide new constraints on its dynamical behavior and valuable boundary conditions for numerical simulations of the Sun’s upper convection zone.

Abstract The convective kissing instability (CKI) is postulated to occur in low mass stars around the fully convective transition. Non-equilibrium 3He burning leads to the merging of core and envelope convective zones, which causes abrupt decreases in the stellar radius. It has been suggested by van Saders & Pinsonneault that these effects may be relevant for cataclysmic variables (CVs). We have performed stellar evolution modeling to study the role of the CKI in CV evolution. We find that the CKI has no effect on normal CVs which evolve via magnetic braking and gravitational radiation above the period gap. CKI cycles either do not occur or are abruptly halted once mass transfer begins. If only gravitational radiation is considered, the CKI does occur. The abrupt radius changes can cause detachment phases which produce small period gaps with widths of a few minutes. We describe how the size of the period gaps is controlled by the 3He profiles of the secondaries. We also discuss how the results of this study apply to the evolution of strong field polars, where the magnetic field of the white dwarf is strong enough to suppress magnetic braking.

Context. The Kepler Legacy sample is, to this day, the sample of solar-like oscillators with the most exquisite asteroseismic data. In this work, we carry out detailed modelling of a sub-sample of these stars for which the surface lithium abundance has also been observed by the LAMOST survey and a photometric surface rotation has been measured. Aims. We aim to study the impact of additional mixing processes on the asteroseismic modelling of Kepler Legacy G and F-type stars. We also investigate whether a single process can be invoked to reproduce the lithium depletion and asteroseismic constraints at the same time Methods. We used detailed asteroseismic modelling techniques combining global and local minimisation techniques. We started by using standard models and then aimed to improve this solution with the addition of extra mixing at the border of convective regions using either convective penetration or turbulence in radiative layers. Results. We find that lower-mass models (≈ 1 M⊙) have no problem in reproducing the observed lithium depletion using only turbulence in the radiative zone, similarly to solar models. F-type stars, which have a shallower convective envelope, are unaffected by additional turbulence at the base of the convective zone, but require significant convective penetration values to actually reproduce the observed lithium depletion. The extent of this penetration is, however, incompatible with the frequency separation ratios. Conclusions. We conclude that the impact of extra mixing is moderate for solar-type stars of the Kepler Legacy sample and well within the requirements of the PLATO mission. For more massive stars (≈ 1.5 M⊙), we conclude that the behaviour of the frequency separation ratios must be further investigated, as even models with large convective penetration at the base of their convective envelope are unable to reproduce them.