2026 PDG seminars

Dynamic evolution of the solar chromosphere using non-LTE nonequilibrium radiative transfer

To understand the structuring and dynamics of the upper photosphere and chromosphere of the Sun, it is essential to improve and extend existing numerical radiation–magnetohydrodynamical (MHD) simulations. In the solar chromosphere, the assumption of local thermodynamic equilibrium (LTE) for the radiation field is well known to be invalid. A full non-LTE treatment of the radiation field, together with time-dependent nonequilibrium population evolution, is therefore required in a dynamically evolving chromosphere.

The radiation field couples to the hydrodynamic equations through radiative flux divergence and gas pressure, making a self-consistent treatment essential. Based on these considerations, we have developed a non-LTE, nonequilibrium radiative transfer module for the well-known radiation--MHD code MURaM. In this framework, the nonequilibrium rate equations, non-LTE radiative transfer, and hydrodynamic equations are solved self-consistently and iteratively. In the equation of state, the kinetic temperature is determined by treating molecular hydrogen formation and dissociation in nonequilibrium, while the remaining elements contributing to the gas are treated using collisional rates only.  The module is implemented for one-, two-, and three- dimensional geometries, and is currently tested in one dimension. In this talk, I will present the first results of the dynamic evolution of the solar atmosphere obtained from these radiative (M)HD simulations.

Statistical relationship between subsurface convective flows and the magnetic energy build-up in sunspots

Strong solar flares occur in delta spots characterized by the opposite-polarity magnetic fluxes in a single penumbra. Sunspot formation via flux emergence from the convection zone to the photosphere can be strongly affected by convective turbulent flows. It has not yet been shown how crucial convective flows are for the formation of delta spots. To reveal the impact of convective flows in the convection zone on the formation and evolution of sunspot magnetic fields, we simulated the emergence and transport of magnetic flux tubes in the convection zone using radiative magnetohydrodynamics code R2D2. 93 simulations were carried out by allocating the twisted flux tubes to different positions in the convection zone. As a result, both delta-type and beta-type magnetic distributions were reproduced only by the differences in the convective flows surrounding the flux tubes. The delta spots were formed by the collision of positive and negative magnetic fluxes on the photosphere. The unipolar and bipolar rotations of the delta spots were driven by magnetic twist and writhe, transporting magnetic helicity from the convection zone to the corona. We detected a strong correlation between the distribution of the non-potential magnetic field in the photosphere and the position of the downflow plume in the convection zone. 

Stability of large sunspots

The stability of sunspots with acoustic scatterers refers to how sunspots interact with solar acoustic waves (p-modes) in the solar interior. Sunspots act as acoustic scatterers, meaning they absorb, deflect, and modify the propagation of these waves. Studying this interaction helps scientists understand the subsurface magnetic structure and stability of sunspots, suggesting that sunspots may consist of bundles of magnetic flux elements rather than a single solid structure. This is important for helioseismology and understanding the dynamics of solar magnetic fields and solar activity. 

Understanding the Solar Chromosphere’s Fine Structure: Bridging MURaM Simulations and Observations 

Sanghita Chandra, Robert H. Cameron, Damien Przybylski, Sami K. Solanki

Spicules have long been suggested to contribute to coronal heating, with reports of an association with transition region (e.g. Si IV emission) and coronal emission. However, interpreting spicule observations remains challenging due to line-of-sight superposition, Doppler-dependent visibility, and the difficulty of inferring three-dimensional structure from two-dimensional diagnostics. Realistic 3D simulations now help overcome these limits. We present the first statistical analysis of synthetic spicules from MURaM-ChE simulations, which reproduce key features of chromospheric dynamics. The non-equilibrium hydrogen treatment enables examination of 3D spicule geometry. Using a new Hα proxy, we identify numerous off-limb spicules and their on-disk counterparts in an enhanced-network simulation. 

We statistically analyze 58 spicules (types I and II) and compare them with earlier high-resolution studies using Ca II H and Hα observations. The synthetic spicules show morphological properties and lifetimes broadly consistent with observations. To probe the underlying structure, we examine selected spicules using the Hα-proxy opacity. One illustrative spicule, with an apparent 190 km/s velocity, shows a sheet-like morphology. We demonstrate that such extreme apparent speeds arise not from true mass motion but from a rippling plasma sheet seen along the line of sight. The sheet lies at a quasi-separatrix layer (QSL). Whether a spicule appears sheet- or thread-like depends strongly on the Doppler shift selected for observation, and their on-disk counterparts often exhibit structures distinct from the spicules themselves. We further investigate the transition-region response of these structures using synthetic Si IV emission.

By combining physically motivated synthetic diagnostics with MURaM-ChE simulations, this work provides a physically consistent framework for comparing observed and simulated spicule morphology and apparent dynamics, thereby substantially narrowing the gap between observations and models of the solar chromosphere.

Intermittent turbulent fluctuations in solar coronal mass ejections

Localized regions of high intensity fluctuations are known to be signatures of intermittency in fluid and plasma turbulence. We investigate such turbulent spots using in-situ spacecraft observations of a sample of 125 Earth-directed solar coronal mass ejections (CMEs). We present statistical results which suggest that the intensity of the strongest turbulent spot and the turbulent spot occurrence rate are reliable indicators of the onset of the leading part of the CME event. The findings of this study can enhance our understanding of intermittence in collisionless plasma turbulence and can improve CME/sheath-driven space weather impact prediction models.

Solar flares as electric circuits detected in multi-wavelength  observations and sunquakes: Seismic responses in the solar interior 

We survey the sunquakes observed so far in cycles 23-25 including the first sunquake with a double bounce or doble ridge in time-distance diagram. We derive the key properties of sunquakes required to explain from the point of view of physical processes in flaring atmospheres and their effects on the  generation of acoustic waves in the solar interior. We explore the outcomes of the hydrodynamic responses of flaring atmospheres to injection of energetic particle, generation of high density shocks traveling towards the solar interior, and the conditions (speed, depth, timing, frequencies) of the generation by them of acoustic waves during propagation in the interior beneath flaring atmospheres. We present first a theory of acoustic waves generated by a point source in stratified plasma and evaluate analytical parametric solutions for a monochromatic source derived for plane-parallel polytrope model of the solar interior. The solution would be used to gain insights into the properties of the generated wavefront as a function of the excitation frequency of acoustic waves and depth of their formation. Then we also consider a varying pressure perturbation, or dense  shock, moving into the solar interior with a supersonic speed. The moving supersonic sources will be shown to excite acoustic waves with a geometry of the generated wavefront constrained by the source depth and its Mach number. The results are discussed in relation to flare simulated for the semiempirical models of the solar interior (Christiansen-Dalsgaard et al, 2003, 2016) and examples of generated acoustic waves in the interior will be presented. The directivity of generated acoustic waves and observational conditions will be also discussed and linked to physical processes in the flaring atmospheres. The simulations results will be compared with some observation of sunquakes.