Context. The driving and excitation mechanisms of decay-less kink oscillations in coronal loops remain under active debate. The photospheric dynamics may provide the continuous energy supply required to sustain these oscillations. Aims. We aim to quantify and provide simple observational constraints on the photospheric driving of coronal loops in a few typical active region configurations: sunspot, plage, pores, and enhanced-network regions. We then aim to investigate the possible interplay between the photospheric driving and the properties of kink oscillations in the connected coronal loops. Methods. We analysed two unique datasets of the corona and photosphere taken at a high spatial and temporal resolution during the first coordinated observation campaign between Solar Orbiter and the Swedish 1-m Solar Telescope (SST). We applied a local correlation tracking method on the SST/CRISP data to quantify the photospheric motions at the base of coronal loops. The same loops were then analysed in the corona by exploiting data from the Extreme Ultraviolet Imager (EUI) on Solar Orbiter and by using a wavelet analysis to characterise the detected kink oscillations. Results. Each type of photospheric region shows varying dynamics but with an overall increase in strength going from pore to plage to enhanced-network to sunspot regions. Differences can also be seen in the amplitudes of the fundamental kink mode measured in the corresponding coronal loops. This suggests the photosphere is involved in the driving of coronal kink oscillations. However, the few samples available do not allow the excitation mechanism to be further established yet. Conclusions. Despite oscillating coronal loops being anchored in seemingly a statica strong magnetic field regions, as seen from coronal EUV observations, photospheric observations provide evidence for a continuous and significant driving at their base. The precise connection between photospheric driving and coronal kink oscillations remains to be further investigated. Upcoming coordinated observations between Solar Orbiter and ground-based telescopes will provide crucial additional observational constraints, with this pilot study serving as a baseline for future works. Finally, this study provides critical constraints on both the quasi-steady and broadband photospheric driving that can be tested in existing numerical models of coronal loops.

Context. Flux emergence in the solar atmosphere is a complex process that causes a release of magnetic energy as heat and acceleration of solar plasma on a variety of spatial scales.

Aims. We aim to investigate temperatures and velocities in small-scale reconnection episodes during flux emergence.

Methods. We analyzed imaging spectropolarimetric data taken in the He I 1083 nm line with a spatial resolution of 0.26″, a time cadence of 2.8 s, and a spectral range corresponding to ±220 km s⁻¹ around the line. This line is sensitive to temperatures higher than 15 kK, unlike diagnostics such as Mg II h&k, Ca II H&K, and Hα, which lose sensitivity already at 15 kK. The He I data is complemented by imaging spectropolarimetry in the Fe I 617.3 nm and Ca II 854.2 nm lines and imaging spectroscopy in Ca II K and Hα at a cadence between 12 s and 36 s. We employed inversions to determine the magnetic field and vertical velocity in the solar atmosphere. We computed He I 1083 nm profiles from a radiation-magneto-hydrodynamics simulation of the solar atmosphere to help in the interpretation of the observations.

Results. We find fast-evolving blob-like emission features in the He I 1083 nm triplet at locations where the magnetic field is rapidly changing direction, and these are likely sites of magnetic reconnection. We fit the line with a model consisting of an emitting layer located below a cold layer representing the fibril canopy. The modeling provides evidence that this model, while simple, catches the essential characteristics of the line formation. The morphology of the emission in the He I 1083 nm is localized and blob-like, unlike the emission in the Ca II K line, which is more filamentary.

Conclusions. The modeling shows that the He I 1083 nm emission features and their Doppler shifts can be caused by opposite-polarity reconnection and/or horizontal current sheets below the canopy layer in the chromosphere. Based on the high observed Doppler width and the blob-like appearance of the emission features, we conjecture that at least a fraction of them are produced by plasmoids. We conclude that transition-region-like temperatures in the deeper layers of the active region chromosphere are more common than previously thought.

Magnetic reconnection at small spatial scales is a fundamental driver of energy release and plasma dynamics in the lower solar atmosphere. We present observations of a brightening in an active region, captured in high-resolution data from the Daniel K. Inouye Solar Telescope using the Visible Broadband Imager and the Visible Spectro-Polarimeter (ViSP). The event exhibits Ellerman bomb−like morphology in the Hβ filter, associated with flux cancellation between a small negative-polarity patch and an opposite polarity plage. Additionally, it displays enhanced emissions in Ca ii K, hot elongated features containing Alfvénic plasma flows, and cooler blueshifted structures. We employ multiline, nonlocal thermodynamic equilibrium inversions of the spectropolarimetric data to infer the stratification of the physical parameters of the atmosphere. Furthermore, we use the photospheric vector magnetogram inferred from the ViSP spectra as a boundary condition for nonlinear force-free field extrapolations, revealing the three-dimensional distribution of squashing factors. We find significant enhancements in temperature, velocity, and microturbulence confined to the upper photosphere and low chromosphere. Our findings provide observational evidence of low-altitude magnetic reconnection along quasi-separatrix layers in a compact fan-spine-type configuration, highlighting the complex interplay between magnetic topology, energy release, and plasma flows.

Context. Hα 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 Hα 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 Hα 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 Hα proxy by comparing it against features detected in the wings of Hα synthesised using MULTI3D. We detect numerous small-scale structures rooted at the network patches, similar to observations in Hα. 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 Alfvén 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 Hα spectral line. We demonstrate its applicability by studying the formation, dynamics and properties of an RBE.

Context. The heating and structure of the solar chromosphere depends on the underlying magnetic field, among other parameters. The lowest magnetic flux of the solar atmosphere is found in the quiet Sun internetwork and is thought to be provided by the small-scale dynamo (SSD) process.

Aims. Our aim is to understand the chromospheric structure and dynamics in a simulation with purely SSD generated magnetic fields.

Methods. We performed a 3D radiation-magnetohydrodynamic (rMHD) simulation of the solar atmosphere, including the necessary physics to simulate the solar chromosphere. No magnetic field was imposed beyond that generated by an SSD process. We analysed the magnetic field in the chromosphere, and the resulting energy balance.

Results. Plasma at chromospheric temperatures reaches high into the atmosphere, with small, transient regions reaching coronal temperatures. An average Poynting flux of 5 × 10⁶ erg cm⁻³ s⁻¹ is found at the base of the chromosphere. The magnetic field in the chromosphere falls off more slowly with height than predicted by a potential field extrapolation from the radial component of the photospheric field. Starting in the middle chromosphere, the magnetic energy density is an order of magnitude higher than the kinetic energy density and, in the upper chromosphere, is also higher than the thermal energy density. Nonetheless, even in the high chromosphere, the plasma-β in shock fronts and low-field regions can locally reach values above unity.

Conclusions. The interactions between shocks and the magnetic field are essential to understanding the dynamics of the internetwork chromosphere. The SSD generated magnetic fields are strong enough to dominate the energy balance in the mid to upper chromosphere. The energy flux into the chromosphere is 8.16 × 10⁶ erg cm⁻² s⁻¹, higher than the canonical values required to heat the quiet Sun chromosphere and corona. Possibly due to the limited box size, the simulation is unable to maintain a million-degree corona.

Context. Fibrils in the solar chromosphere carry transverse oscillations as determined from non-spectroscopic imaging data. They are estimated to carry an energy flux of several kW m⁻², which is a significant fraction of the average chromospheric radiative energy losses.

Aims. We aim to determine the oscillation properties of fibrils not only in the plane-of-the-sky (horizontal) direction, but also along the line-of-sight (vertical) direction.

Methods. We obtained imaging-spectroscopy data in Fe I 6173 Å, Ca II 8542 Å, and Ca II K with the Swedish 1-m Solar Telescope. We created a sample of 605 bright Ca II K fibrils and measured their horizontal motions. Their vertical motion was determined through non-local thermodynamic equilibrium (non-LTE) inversion of the observed spectra. We determined the periods and velocity amplitudes of the fibril oscillations, as well as phase differences between vertical and horizontal oscillations in the fibrils.

Results. The bright Ca II K fibrils carry transverse waves with a mean period of 2.1 × 10² s, and a horizontal velocity amplitude of 1 km s⁻¹, consistent with earlier results. The mean vertical velocity amplitude is 1.1 km s⁻¹. We find that 77% of the fibrils carry waves in both the vertical and horizontal directions, and 80% of this subsample exhibit oscillations with similar periods in both horizontal and vertical directions. For the latter, we find that all phase differences between 0 and 2π occur with a mild but significant preference for linearly polarised waves (a phase difference of 0 or π).

Conclusions. The results are consistent with the scenario where transverse waves are excited by granular buffeting at the photospheric footpoints of the fibrils. Estimates of transverse wave flux based only on imaging data are too low because they ignore the contribution of the vertical velocity.

The chromosphere is a dynamic and complex layer where all the relevant physical processes happen on very small spatio-temporal scales. A few spectral lines that can be used as chromospheric diagnostics give us convoluted information that is hard to interpret without realistic theoretical models. What are the key ingredients that these models need to contain? The magnetic field has a paramount effect on chromospheric structuring. This is obvious from the ubiquitous presence of chromospheric dynamic fibrilar structures visible on the solar disk and at the limb. The numerical experiments presented in this manuscript illustrate the present state of modeling. They showcase to what extent our models reproduce various chromospheric features and their dynamics. The publication describes the effect different ingredients have on chromospheric models and provides a recipe for building one-to-one models. Combining these models with observations will provide insight into the physical processes that take place in the solar atmosphere. 

Context. Rapid blue- and redshifted excursions (RBEs and RREs) may play an important role in mass-loading and heating the solar corona, but their nature and origin are still debatable.

Aims. We aim to model these features to learn more about their properties, formation, and origin.

Methods. We created a realistic three-dimensional (3D) magnetohydrodynamic model of a solar plage region. Synthetic Hα spectra were generated and the spectral signatures of these features identified. The magnetic field lines associated with these events were traced, and the underlying dynamic was studied.

Results. The model reproduces many properties of RBEs and RREs well, such as spatial distribution, lateral movement, length, and lifetimes. Synthetic Hα line profiles, similarly to observed ones, show a strong blue- or redshift as well as asymmetries. These line profiles are caused by the vertical component of velocities higher than 30 − 40 km s⁻¹, which mostly appear in the height range 2 − 4 Mm. By tracing magnetic field lines, we show that the vertical velocity that causes the appearance of RBEs or RREs is always associated with the component of velocity perpendicular to the magnetic field lines.

Conclusions. The study confirms the hypothesis that RBEs and RREs are signs of Alfvénic waves with, in some cases, a significant contribution from slow magneto-acoustic modes.

Context. Recent observations have revealed loop-like structures at very small scales visible in observables that sample the transition region (TR) and even coronal temperatures. These structures are referred to as either ‘unresolved fine structures’, ‘dynamic cool loops’, ‘miniature hot loops’ or ‘campfires’ depending on the observables in which they are detected. Their formation remains unclear.

Aims. Realistic magnetohydrodynamic simulations and forward synthesis of spectral lines are used to investigate how these features occur.

Methods. Computations were carried out using the MURaM code to generate model atmospheres. The synthetic Hα and Si IV spectra are calculated at two angles (μ = 1, μ = 0.66) using the Multi3D code. We traced magnetic field lines in the model and examined the evolution of the underlying field topology.

Results. The synthetic Hα Dopplergrams reveal loops that evolve dramatically within a few minutes. The synthetic Hα line profiles show observed asymmetries and Doppler shifts in the line core. However, they also show strong emission peaks in the line wings, even at the slanted view. The synthetic Si IV emission features partly coincide with structures visible in Hα Dopplergrams and partly follow separate magnetic field threads. Some are even visible in the emission measure maps for the lg(T/K) = [5.8, 6.2] temperature interval. The emission areas trace out the magnetic field lines rooted in opposite polarities in a bipolar region.

Conclusions. The model shows that a loop-like structure in a bipolar system with footpoints undergoing rapid movement and shuffling can produce many small-scale recurrent events heated to high temperatures. It demonstrates that heating to different temperatures occurs and can be confined to a small part of the loop, at the location where resistive and viscous heating increases. The model largely reproduces the observed features in terms of size, lifetime and morphology in chromospheric, TR and coronal observables. The morphology and evolution of the resulting observable features can vary depending on the viewing angle.

Context. A proper estimate of the chromospheric magnetic fields is thought to improve modelling of both active region and coronal mass ejection evolution. However, because the chromospheric field is not regularly obtained for sufficiently large fields of view, estimates thereof are commonly obtained through data-driven models or field extrapolations, based on photospheric boundary conditions alone and involving pre-processing that may reduce details and dynamic range in the magnetograms.

Aims. We investigate the similarity between the chromospheric magnetic field that is directly inferred from observations and the field obtained from a magnetohydrostatic (MHS) extrapolation based on a high-resolution photospheric magnetogram.

Methods. Based on Swedish 1-m Solar Telescope Fe I 6173 Å and Ca II 8542 Å observations of NOAA active region 12723, we employed the spatially regularised weak-field approximation (WFA) to derive the vector magnetic field in the chromosphere from Ca II, as well as non-local thermodynamic equilibrium (non-LTE) inversions of Fe I and Ca II to infer a model atmosphere for selected regions. Milne-Eddington inversions of Fe I serve as photospheric boundary conditions for the MHS model that delivers the three-dimensional field, gas pressure, and density self-consistently.

Results. For the line-of-sight component, the MHS chromospheric field generally agrees with the non-LTE inversions and WFA, but tends to be weaker by 16% on average than these when larger in magnitude than 300 G. The observationally inferred transverse component is systematically stronger, up to an order of magnitude in magnetically weaker regions, but the qualitative distribution with height is similar to the MHS results. For either field component, the MHS chromospheric field lacks the fine structure derived from the inversions. Furthermore, the MHS model does not recover the magnetic imprint from a set of high fibrils connecting the main polarities.

Conclusions. The MHS extrapolation and WFA provide a qualitatively similar chromospheric field, where the azimuth of the former is better aligned with Ca II 8542 Å fibrils than that of the WFA, especially outside strong-field concentrations. The amount of structure as well as the transverse field strengths are, however, underestimated by the MHS extrapolation. This underscores the importance of considering a chromospheric magnetic field constraint in data-driven modelling of active regions, particularly in the context of space weather predictions.