Context. Synthetic spectra from 3D models of the solar atmosphere have become increasingly successful at reproducing observations, but there are still some outstanding discrepancies for chromospheric spectral lines, such as Ca II and Mg II, particularly regarding the width of the line cores. It has been demonstrated that using sufficiently high spatial resolution in the simulations significantly diminishes the differences in width between the mean spectra in observations and simulations, but a detailed investigation into how this impacts subgroups of individual profiles is currently lacking.

Aims. We compare and contrast the typical shapes of synthetic Ca II 854.2 nm spectra found in Bifrost simulations having different magnetic activity with the spectral shapes found in a quiet-Sun observation from the Swedish 1-m Solar Telescope (SST).

Methods. We used clustering techniques to extract the typical Ca II 854.2 nm profile shapes synthesized from Bifrost simulations with varying amounts of magnetic activity. We degraded the synthetic profiles to observational conditions and repeated the clustering, and we compared our synthetic results with actual observations. Subsequently, we examined the atmospheric structures in our models for some select sets of clusters, with the intention of uncovering why they do or do not resemble actual observations.

Results. While the mean spectra for our high resolution simulations compare reasonably well with the observations, we find that there are considerable differences between the clusters of observed and synthetic intensity profiles, even after the synthetic profiles have been degraded to match observational conditions. The typical absorption profiles from the simulations are both narrower and display a steeper transition from the inner wings to the line core. Furthermore, even in our most quiescent simulation, we find a far larger fraction of profiles with local emission around the core, or other exotic profile shapes, than in the quiet-Sun observations. Looking into the atmospheric structure for a selected set of synthetic clusters, we find distinct differences in the temperature stratification for the clusters most and least similar to the observations. The narrow and steep profiles are associated with either weak gradients in temperature or temperatures rising to a local maximum in the line wing forming region before sinking to a minimum in the line core forming region. The profiles that display less steep transitions show extended temperature gradients that are steeper in the range−3 ≲ log τ₅₀₀₀ ≲ −1.

Context. Observations of center-to-limb variations (CLVs) of spectral lines and continua provide a good test for the accuracy of models with a solar and stellar atmospheric structure and spectral line formation. They are also widely used to constrain elemental abundances, and are becoming increasingly more important in atmospheric studies of exoplanets. However, only a few such data sets exist for chromospheric lines.

Aims. We aim to create a set of standard profiles by means of mosaics made with the CRISP and CHROMIS instruments of the Swedish 1-m Solar Telescope (SST), as well as to explore the robustness of said profiles obtained using this method.

Methods. For each spectral line, we used a mosaic that ranges from the center to the limb. Each of these mosaics were averaged down to 50 individual spectral profiles and spaced by 0.02 in the μ scale. These profiles were corrected for p-mode oscillations, and their line parameters (equivalent width, line shift, full-width at half-maximum, and line depth) were then compared against literature values whenever possible.

Results. We present a set of 50 average profiles that are spaced equidistantly along the cosine of the heliocentric angle (μ) by steps of 0.02 for five continuum points between 4001 and 7772 Å, as well as ten of the most commonly observed spectral lines at the SST (Ca II H & K, Ηβ, Mg I 5173 Å, C I 5380 Å, Fe I 6173 Å, Fe I 6301 Å, Ha, O I 7772 Å, and Ca II 8542 Å).

Conclusions. The CLV of line profiles and continua are shared in the CDS as machine readable tables, providing a quantitative constraint on theoretical models that aim to model stellar atmospheres.

Context. The shapes of Stokes profiles contain a great deal of information about the atmospheric conditions that produced them. However, a variety of different atmospheric structures can produce very similar profiles. Thus, it is important for a proper interpretation of the observations to have a good understanding of how the shapes of Stokes profiles depend on the underlying atmosphere. An excellent tool in this regard is forward modeling, namely, computing and studying synthetic spectra from realistic simulations of the solar atmosphere. Modern simulations routinely produce several hundred thousand spectral profiles per snapshot. With such numbers, it becomes necessary to use automated procedures in order to organize the profiles according to their shape. Here, we illustrate the use of two complementary methods, k-means and k-Shape, to cluster similarly shaped profiles and demonstrate how the resulting clusters can be combined with knowledge of the simulation's atmosphere to interpret spectral shapes.

Aims. We aim to showcase the use of clustering analysis for forward modeling. In particular, we wish to introduce the k-Shape clustering method to the solar physics community as a complement to the well-known k-means method.

Methods. We generated synthetic Stokes profiles for the Ca II 854.2 nm line using the Multi3D code from a Bifrost simulation snapshot. We then applied the k-means and k-Shape clustering techniques to group the profiles together according to their shape and investigated the within-group correlations of temperature, line-of-sight velocity, and line-of-sight magnetic field strengths.

Results. We show and compare the classes of profile shapes we retrieved from applying both k-means and k-Shape to our synthetic intensity spectra. We then show the structure of the underlying atmosphere for two particular classes of profile shapes retrieved by the clustering and demonstrate how this leads to an interpretation for the formation of those profile shapes. We applied both methods to the subset of our profiles containing the strongest Stokes V signals and we demonstrate how k-Shape can be qualitatively better than k-means at retrieving complex profile shapes when using a small number of clusters.

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.

Solar oxygen abundance measurements based on the O I near-infrared triplet have been a much debated subject for several decades, since non-local thermodynamic equilibrium (NLTE) calculations with 3D radiation-hydrodynamics model atmospheres introduced a large change to the 1D LTE modeling. In this work, we aim to test solar line formation across the solar disk using new observations obtained with the SST/CRISP instrument. The observed data set is based on a spectroscopic mosaic that stretches from disk center to the solar limb. By comparing the state-of-the-art 3D NLTE models with the data, we find that the 3D NLTE models provide an excellent description of the line formation across the disk. We obtain an abundance value of A(O)=(8.73 +/- 0.03) dex, with a very small angular dispersion across the disk. We conclude that spectroscopic mosaics are excellent probes for geometric and physical properties of hydrodynamics models and NLTE line formation.

Context. Fan-shaped jets sometimes form above light bridges and are believed to be driven by the reconnection of the vertical umbral field with the more horizontal field above the light bridges. Because these jets are not fully opaque in the wings of most chromospheric lines, it is not possible to study their spectra without highly complex considerations of radiative transfer in spectral lines from the atmosphere behind the fan.

Aims. We take advantage of a unique set of observations of the Hα line along with the Ca II 8542 Å and Ca II K lines obtained with the CRISP and CHROMIS instrument of the Swedish 1-m Solar Telescope to study the physical properties of a fan-shaped jet that was backlit by an X9.3 flare. For what we believe to be the first time, we report an observationally derived estimate of the mass and density of material in a fan-shaped jet.

Methods. The Hα flare ribbon emission profiles from behind the fan are highly broadened and flattened, allowing us to investigate the fan with a single slab via Beckers’ cloud model, as if it were backlit by a flat spectral profile of continuum emission. Using this model we derived the opacity and velocity of the material in the jet. Using inversions of Ca II 8542 Å emission via the STockholm inversion Code, we were also able to estimate the temperature and to cross-check the velocity of the material in the jet. Finally, we used the masses and the plane-of-sky and line-of-sight velocities as functions of time to investigate the downward supply of energy and momentum to the photosphere in the collapse of this jet, and evaluated it as a potential driver for a sunquake beneath.

Results. We find that the physical properties of the fan material are reasonably chromospheric in nature, with a temperature of 7050 ± 250 K and a mean density of 2 ± 0.3 × 10⁻¹¹ g cm⁻³.

Conclusions. The total mass observed in Hα was found to be 3.9 ± 0.7 × 10₁₃ g and the kinetic energy delivered to the base of the fan in its collapse was nearly two orders of magnitude below typical sunquake energies. We therefore rule out this jet as the sunquake driver, but cannot completely rule out larger fan jets as potential drivers.

We present high-resolution and multiline observations of a C2-class solar flare (SOL2019-05-06T08:47), which occurred in NOAA AR 12740 on May 6, 2019. The rise, peak, and decay phases of the flare were recorded continuously and quasi-simultaneously in the Ca II K line with the CHROMIS instrument and in the Ca II 8542 Å and Fe I 6173 Å lines with the CRISP instrument at the Swedish 1 m Solar Telescope. The observations in the chromospheric Ca II lines exhibit intense brightening near the flare footpoints. At these locations, a nonlocal thermodynamic equilibrium inversion code was employed to infer the temperature, magnetic field, line-of-sight (LOS) velocity, and microturbulent velocity stratification in the flaring atmosphere. The temporal analysis of the inferred temperature at the flare footpoints shows that the flaring atmosphere from log τ₅₀₀ ∼ −2.5 to −3.5 is heated up to 7 kK, whereas from log τ₅₀₀ ∼ −3.5 to −5 the inferred temperature ranges between ∼7.5 kK and ∼11 kK. During the flare peak time, the LOS velocity shows both upflows and downflows around the flare footpoints in the upper chromosphere and lower chromosphere, respectively. Moreover, the temporal analysis of the LOS magnetic field at the flare points exhibits a maximum change of ∼600 G. After the flare, the LOS magnetic field decreases to the non-flaring value, exhibiting no permanent or step-wise change. The analysis of response functions to the temperature, LOS magnetic field, and velocity shows that the Ca II lines exhibit enhanced sensitivity to the deeper layers (i.e., log τ₅₀₀ ∼ −3) of the flaring atmosphere, whereas for the non-flaring atmosphere they are mainly sensitive around log τ₅₀₀ ∼ −4. We suggest that a fraction of the apparent increase in the LOS magnetic field at the flare footpoints may be due to the increase in the sensitivity of the Ca II 8542 Å line in the deeper layers, where the field strength is relatively strong. The rest may be due to magnetic field reconfiguration during the flare. In the photosphere, we do not notice significant changes in the physical parameters during the flare or non-flare times. Our observations illustrate that even a less intense C-class flare can heat the deeper layers of the solar chromosphere, mainly at the flare footpoints, without affecting the photosphere.

Context. Observations show that small-scale vortical plasma motions are ubiquitous in the quiet solar atmosphere. They have received increasing attention in recent years because they are a viable candidate mechanism for the heating of the outer solar atmospheric layers. However, the true nature and the origin of these swirls, and their effective role in the energy transport, are still unclear.

Aims. We investigate the evolution and origin of chromospheric swirls by analyzing numerical simulations of the quiet solar atmosphere. In particular, we are interested in finding their relation with magnetic field perturbations and in the processes driving their evolution.

Methods. The radiative magnetohydrodynamic code CO5BOLD is used to perform realistic numerical simulations of a small portion of the solar atmosphere, ranging from the top layers of the convection zone to the middle chromosphere. For the analysis, the swirling strength criterion and its evolution equation are applied in order to identify vortical motions and to study their dynamics. As a new criterion, we introduce the magnetic swirling strength, which allows us to recognize torsional perturbations in the magnetic field.

Results. We find a strong correlation between swirling strength and magnetic swirling strength, in particular in intense magnetic flux concentrations, which suggests a tight relation between vortical motions and torsional magnetic field perturbations. Furthermore, we find that swirls propagate upward with the local Alfven speed as unidirectional swirls driven by magnetic tension forces alone. In the photosphere and low chromosphere, the rotation of the plasma co-occurs with a twist in the upwardly directed magnetic field that is in the opposite direction of the plasma flow. All together, these are clear characteristics of torsional Alfven waves. Yet, the Alfven wave is not oscillatory but takes the form of a unidirectional pulse. The novelty of the present work is that these Alfven pulses naturally emerge from realistic numerical simulations of the solar atmosphere. We also find indications of an imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects being at the origin of the swirls. At the base of the chromosphere, we find a mean net upwardly directed Poynting flux of 12.86.5 kW m(-2), which is mainly due to swirling motions. This energy flux is mostly associated with large and complex swirling structures, which we interpret as the superposition of various small-scale vortices.

Conclusions. We conclude that the ubiquitous swirling events observed in numerical simulations are tightly correlated with perturbations of the magnetic field. At photospheric and chromospheric levels, they form Alfven pulses that propagate upward and may contribute to chromospheric heating.

Using high-resolution spectropolarimetric data recorded with the Swedish 1 m Solar Telescope, we have identified several instances of granular lanes traveling into granules. These are believed to be the observational signature of underlying tubes of vortical flow with their axis oriented parallel to the solar surface. Associated with these horizontal vortex tubes, we detect in some cases a significant signal in linear polarization, located at the trailing dark edge of the granular lane. The linear polarization appears at a later stage of the granular lane development, and is flanked by patches of circular polarization. Stokes inversions show that the elongated patch of linear polarization signal arises from the horizontal magnetic field aligned with the granular lane. We analyze snapshots of a magnetohydrodynamic numerical simulation and find cases in which the horizontal vortex tube of the granular lane redistributes and transports the magnetic field to the solar surface causing a polarimetric signature similar to what is observed. We thus witness a mechanism capable of transporting magnetic flux to the solar surface within granules. This mechanism is probably an important component of the small-scale dynamo supposedly acting at the solar surface and generating the quiet-Sun magnetic field.