Alfvén waves are widely regarded as important carriers of energy through the partially ionised layers of the solar atmosphere. In these plasmas, ion-neutral collisions introduce dissipation and make the wave frequency complex, so that the associated group speed also becomes complex. In this paper, we investigate the complex group speed of Alfvén waves in a partially ionised plasma with steady, field-aligned background flow. We aim to determine how collisional coupling, ionisation degree, and background flow modify the transport of wave energy in the lower solar atmosphere. We adopt a two-fluid model for ions and neutrals and consider transverse Alfvén perturbations propagating along a uniform magnetic field. Using the linearised governing equations, we derive the dispersion relation and obtain the corresponding complex group speed from its kinematic definition. We then complement this description with an energetic interpretation based on the wave energy balance of the system. We show that the real and imaginary parts of the group speed have distinct physical meanings. The real part governs the propagation of the wave packet, while the imaginary part measures how the damping rate varies with wavenumber. In weakly ionised plasmas, the flow mode displays a much stronger sensitivity to neutral drift than the forward and backward Alfvén branches, and may develop a positive imaginary part of the group speed. In strongly ionised plasmas, the overall behaviour is simpler, but the flow mode still retains a distinct transport signature. In the mode-conversion regime, the flow and backward Alfvén branches exchange not only their phase speed properties, but also their transport and dissipative characteristics. Our results show that ion-neutral collisions and steady flows modify not only the damping of Alfvén waves, but also the way their energy is transported. The complex group speed therefore provides a useful diagnostic of wave propagation and energy transport in partially ionised regions of the solar atmosphere.
Galileo Galilei’s 1613 description of small bright patches “piazzette più chiare” accompanying sunspots is among the earliest telescopic records of what are now known as solar faculae. We show that it was Giovanni Battista Baliani, in a letter to Galileo dated 31 January 1614, who first proposed that these dark and bright solar features might cause genuine variations in the Sun’s radiative output, arguably the first scientific hypothesis of solar variability on record. We resume the subsequent three-century history of the facula concept, from its informal use as an observational label by Scheiner and Hevelius, through the eighteenth-century debate between Lalande and Cassini over its physical nature, to Herschel’s empirical test against historical wheatprice data and his facule/lucule distinction, and to Humboldt’s and Meyer’s early proposals of a facular compensation mechanism offsetting sunspot darkening. This historical reconstruction places Baliani’s largely overlooked letter as the conceptual origin of the opposing contribution of sunspots and faculae that underlies reconstructions of Total Solar Irradiance.
Coronal holes (CHs) are low-density regions in the solar atmosphere, characterised by open magnetic field lines driving the high-speed solar wind. While appearing dark in EUV/X-rays, their radio signatures are complex and frequency-dependent, showing either brightness depressions or enhancements relative to the quiet-Sun (QS). This variability reflects the complex inner temperature, density and magnetic structures of the solar atmosphere.Within the SunDish project, we investigate the CH phenomenology using the National Institute for Astrophysics (INAF) large single-dish radio telescopes (Medicina 'Gavril Grueff' 32-m and Sardinia Radio Telescope 'SRT' 64-m), aiming to bridge the observational gap between the chromosphere and corona by exploring the under-investigated K-band (18 - 26 GHz ). This study analyses three representative scenarios: expected brightness temperature ( Tb ) depressions, radio-enhanced CHs, and radio-dark regions (RDRs) unrelated to standard CHs.Our analysis reveals a non-uniform phenomenology where radio-enhanced CHs and RDRs identify deep-penetrating structures affecting the upper chromosphere, including the first radio counterpart to a dark halo. Spectral indices and weak circular polarisation confirm thermal free-free emission as the primary mechanism, excluding significant gyro-magnetic contributions. However, magnetic field estimates suggest that a purely thermal model in a simple stratified atmosphere cannot fully describe these regions, where radio enhancements likely arise from unresolved structures like coronal bright points and magnetic flux tubes. These results highlight multi-frequency diagnostics as an essential tool for probing the coupling between different layers of the solar atmosphere, demonstrating that CHs are dynamic environments where complex magnetic activity drives their vertical structure and evolution.
Solar flares (SF) are energetic solar events that can influence space weather and disrupt space-based technologies. Many data-driven forecasting models rely on parameterized, hand-engineered features derived from active regions (ARs). However, such parameterization compresses the spatial structure of ARs and may discard information relevant to flare initiation. In this work, we investigate the spatial distributions of several AR properties for flare forecasting using deep convolutional neural networks. To standardize input dimensions across ARs, we propose a resizing technique called Pad-crop. Our results show that models trained on different physical properties achieve comparable forecasting performance, indicating that the morphology of ARs in-its-own may encode meaningful flare signatures. Gradient-weighted Class Activation Mapping (Grad-CAM) analysis further indicates that the models focus on physically meaningful zones of the ARs, suggesting that relevant features are being captured. In addition, an analysis of false-positive predictions suggests that intermediate C-class flares during the prediction window may contribute to model confusion. These findings highlight the importance of the spatial structure in AR properties and motivate further exploration of morphology-based approaches for SF forecasting. The source code associated with this work will be made publicly available.
The early evolutionary stages of some active regions (ARs) are analysed with the aim of resolving one of the paramount-importance problems of heliophysics: what plays a primary, active role in the formation of active regions and sunspots – magnetic fields or motion of solar plasma? In particular, the first view underlies the popular model of rising Ω -shaped, twisted flux-tube loop of strong magnetic field. In this study, optical images obtained by the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory are used along with full-vector maps of magnetic field based on HMI data. Maps of fluid-particle tracks are constructed using local correlation tracking of photospheric brightness elements. Comparisons of optical and magnetic maps for three developing active regions confirm the conclusion made by Bumba and Howard in the mid-1960s that the supergranulation pattern is not destroyed by the emerging magnetic field of the developing AR, which would be inevitable if a loop of the strong-magnetic-field flux tube rose through the subphotospheric layers. There are no signs of spreading flows on scales comparable to the size of the whole active region. Various forms of dissymmetry between the leading and trailing parts of ARs are revealed. Next, an example of a well-developed sunspot group is considered, which demonstrates that the twist of the magnetic configuration may develop much later than the group starts growing. It is concluded that plasma flows appear to be determinant with respect to magnetic fields. The considered scenarios of the origin of ARs are in sharp contradiction with the rising-tube model.
