2026 PDG seminars

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.