arXiv subjects

Jaime de la Cruz Rodríguez

Publications and source records attributed to Jaime de la Cruz Rodríguez.

At least 19 recordsLinked to original sources

arXiv2026

Investigating the inferred spatiotemporal evolution of magnetic fields during an X1-class flare

Several observational studies have reported sudden temporal changes in chromospheric magnetic fields inferred during solar flares using the weak-field approximation (WFA) and nonlocal thermodynamic equilibrium (NLTE) inversions. We investigate whether these variations reflect the rapid evolution of the solar atmosphere or can be influenced by opacity changes caused by flare heating. We perform spatially coupled and regularized NLTE inversions of high-resolution observations and use the inferred atmospheric models to study the formation of the Ca II 854.2 nm line. We also analyze snapshots from a 3D radiative magnetohydrodynamic flare simulation to investigate the evolution of the mapping between column mass and geometrical height. We find that opacity effects alone cannot, in many cases, explain the temporal variations in the reconstructed magnetic fields, suggesting that they reflect real solar evolution. However, in approximately 36% of the regions exhibiting rapid magnetic-field changes, opacity effects are sufficiently strong to bias their detection. Ejected or condensed cold material can also shift the line formation to a different depth, producing an apparent sudden change in the inferred magnetic field. The simulation further shows that, once the flare ribbon forms, a given column mass maps to deeper geometrical layers. These results highlight the need for caution when interpreting the temporal evolution of chromospheric magnetic fields during flares, as neither the WFA nor NLTE inversions based on 1D hydrostatic equilibrium can fully account for these effects.

astro-ph.SR↗

arXiv2026

The IRIS$^{2+}$ inversion tool: recovering the radiative losses and the thermodynamics in the lower solar atmosphere

We introduce an improved and fast inversion tool that is able to provide the thermodynamics of the solar atmosphere from the photosphere to the top of the chromosphere, as well as the integrated radiative losses in the chromosphere for data observed by the Interface Region Imaging Spectrograph (IRIS). This NASA mission has been observing the Sun and providing, among other kinds of data, multi-line spectral observations sensitive to changes in the lower solar atmosphere since 2013. In this paper, we explain the new inversion tool IRIS$^{2+}$ based on the IRIS$^{2+}$ database, which is based on 135,472 synthetic representative profiles (RP), each of them consisting of 6 chromospheric lines and 6 photospheric lines, their corresponding representative model atmospheres (RMA), and the integrated radiative losses (IRL) associated with these atmospheres. A nearest neighbor (k-nn) model algorithm is trained with the synthetic representative profiles to predict the closest RP in the database to the one observed, at which point IRIS$^{2+}$ assigns the RMA and the IRL to the location of that observed profile. We have compared the results obtained by IRIS$^{2+}$ with results obtained from the state-of-the-art inversion code STiC, which is also used to build the IRIS$^{2+}$ database. We find that the thermodynamics and the IRL obtained with both methods are comparable in most cases. Therefore, IRIS$^{2+}$ is a fast and reliable inversion tool that provides approximate values of the thermodynamic state and the radiative losses in the lower solar atmosphere for a large variety of solar scenes observed with IRIS.

astro-ph.SR↗

arXiv2024

Transverse oscillations in 3D along Ca II K bright fibrils in the solar chromosphere

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 $\mathrm{kW~m}^{-2}$, which is a significant fraction of the average chromospheric radiative energy losses. We aim to determine oscillation properties of fibrils not only in the plane-of-the-sky (horizontal) direction, but also along the line-of-sight (vertical) direction. We obtained imaging-spectroscopy data in $\mathrm{Fe~I}$, $\mathrm{Ca~II~IR}$, and $\mathrm{Ca~II~K}$ with the Swedish 1-m Solar Telescope. We created a sample of 605 bright $\mathrm{Ca~II~K}$ fibrils and measured their horizontal motions. Their vertical motion was determined through 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. The bright $\mathrm{Ca~II~K}$ fibrils carry transverse waves with a mean period of $2.1\times10^2~$s, and a horizontal velocity amplitude of 1$~\mathrm{km~s}^{-1}$, consistent with earlier results. The mean vertical velocity amplitude is 1.1$~\mathrm{km~s}^{-1}$. We find that 77% of the fibrils carry waves in both the vertical and horizontal directions, and 80% of this subsample exhibits 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 polarized waves (phase difference of $0$ or $π$). 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.

astro-ph.SR↗

arXiv2024

Improved reconstruction of solar magnetic fields from imaging spectropolarimetry through spatio-temporal regularisation

Determination of solar magnetic fields with a spatial resolution set by the diffraction limit of a telescope is difficult because the time required to measure the Stokes vector with sufficient signal-to-noise is long compared to the solar evolution timescale. This difficulty gets worse with increasing telescope size as the photon flux per diffraction-limited resolution element remains constant but the evolution timescale decreases linearly with the diffraction-limited resolution. We aim to improve magnetic field reconstruction at the diffraction limit without averaging the observations in time or space, and without applying noise filtering. The magnetic field vector tends to evolve slower than the temperature, velocity and microturbulence. We exploit this by adding temporal regularisation terms for the magnetic field to the linear least-squares fitting used in the weak-field approximation, as well as to the Levenberg-Marquardt algorithm used in inversions. The other model parameters are allowed to change in time without constraints. We infer the chromospheric magnetic field from Ca II 854.2 nm observations using the weak field approximation and the photospheric magnetic field from Fe I 617.3 nm observations using Milne-Eddington inversions, both with and without temporal regularisation. Temporal regularisation reduce the noise in the reconstructed maps of the magnetic field and provides a better coherency in time in both the weak-field approximation and Milne-Eddington inversions. Temporal regularisation markedly improves magnetic field determination from spatially and temporally resolved observations.

astro-ph.SR↗

arXiv2023

Solar Atmospheric Heating Due to Small-scale Events in an Emerging Flux Region

We investigate the thermal, kinematic and magnetic structure of small-scale heating events in an emerging flux region (EFR). We use high-resolution multi-line observations (including Ca II 8542~Å, Ca II K, and Fe I 6301~Åline pair) of an EFR located close to the disk center from the CRISP and CHROMIS instruments at the Swedish 1-m Solar Telescope. We perform non-LTE inversions of multiple spectral lines to infer the temperature, velocity, and magnetic field structure of the heating events. Additionally, we use the data-driven Coronal Global Evolutionary Model to simulate the evolution of the 3D magnetic field configuration above the events and understand their dynamics. Furthermore, we analyze the differential emission measure to gain insights into the heating of the coronal plasma in the EFR. Our analysis reveals the presence of numerous small-scale heating events in the EFR, primarily located at polarity inversion lines of bipolar structures. These events not only heat the lower atmosphere but also significantly heat the corona. The data-driven simulations, along with the observed enhancement of currents and Poynting flux, suggest that magnetic reconnection in the lower atmosphere is likely responsible for the observed heating at these sites.

astro-ph.SR↗

arXiv2023

Decay of a photospheric transient filament at the boundary of a pore and the chromospheric response

Intermediate stages between pores and sunspots are a rare phenomenon and can manifest with the formation of transient photospheric penumbral-like filaments. Although the magnetic field changes rapidly during the evolution of such filaments, they have not been shown to be connected to magnetic reconnection events yet. We analyzed observations of a pore in NOAA AR 12739 from the Swedish Solar Telescope including spectropolarimetric data of the Fe I 6173 Å and the Ca II 8542 Å line and spectroscopic data of the Ca II K 3934 Å line. The VFISV Milne-Eddington inversion code and the multi-line Non-LTE inversion code STiC were utilized to obtain atmospheric parameters in the photosphere and the chromosphere. Multiple filamentary structures of inclined magnetic fields are found in photospheric inclination maps at the boundary of the pore, although the pore never developed a penumbra. One of the filaments shows a clear counterpart in continuum intensity maps in addition to photospheric blueshifts. During its decay, a brightening in the blue wing of the Ca II 8542 Å line is observed. The Ca II K 3934 Å and the Ca II 8542 Å lines show complex spectral profiles in this region. Depth-dependent STiC inversion results using data from all available lines yield a temperature increase (roughly 1000 Kelvin) and bidirectional flows (magnitudes up to 8 km/s) at log tau=-3.5. The temporal and spatial correlation of the decaying filament (observed in the photosphere) to the temperature increase and the bidirectional flows in the high photosphere/low chromosphere suggests that they are connected. We propose scenarios in which magnetic reconnection happens at the edge of a rising magnetic flux tube in the photosphere. This leads to both the decay of the filament in the photosphere and the observed temperature increase and the bidirectional flows in the high photosphere/low chromosphere.

astro-ph.SR↗

arXiv2023

Does H$α$ Stokes~$V$ profiles probe the chromospheric magnetic field? An observational perspective

We investigated the diagnostic potential of the Stokes $V$ profile of the H$α$ line to probe the chromospheric line-of-sight (LOS) magnetic field ($B_{\mathrm{LOS}}$) by comparing the $B_{\mathrm{LOS}}$ inferred from the weak field approximation (WFA) with that of inferred from the multi-line inversions of the Ca II 8542 Å, Si I 8536 Å and Fe I 8538 Å lines using the STiC inversion code. Simultaneous spectropolarimetric observations of a pore in the Ca II 8542 Å and H$α$ spectral lines obtained from the SPINOR at the Dunn Solar Telescope on the 4th of December, 2008 are used in this study. The WFA was applied on the Stokes $I$ and $V$ profiles of H$α$ line over three wavelength ranges viz.: around line core ($Δλ=\pm0.35$ Å), line wings ($Δλ=[-1.5, -0.6]$ and $[+0.6, +1.5]$ Å) and full spectral range of the line ($Δλ=\pm1.5$ Å) to derive the $B_{\mathrm{LOS}}$. We found the maximum $B_{\mathrm{LOS}}$ strengths of $\sim+800$ and $\sim+600$ G at $\logτ_{\mathrm{500}}$ = $-$1 and $-$4.5, respectively in the pore. The morphological map of the $B_{\mathrm{LOS}}$ inferred from the H$α$ line core is similar to the $B_{\mathrm{LOS}}$ map at $\logτ_{\mathrm{500}}$ = $-$4.5 inferred from multi-line inversions. The $B_{\mathrm{LOS}}$ map inferred from the H$α$ line wings and full spectral range have a similar morphological structure to the $B_{\mathrm{LOS}}$ map inferred at $\logτ_{\mathrm{500}}$ = $-$1. The $B_{\mathrm{LOS}}$ estimated from H$α$ using WFA is weaker by a factor of $\approx 0.53$ than that of inferred from the multi-line inversions.

astro-ph.SR↗

arXiv2022

Chromospheric Heating from Local Magnetic Growth and Ambipolar Diffusion Under Non-Equilibrium Conditions

The heating of the chromosphere in internetwork regions remains one of the foremost open questions in solar physics. In the present study we tackle this old problem by using a very high spatial-resolution simulation of quiet-Sun conditions performed with radiative MHD numerical models and IRIS observations. We have expanded a previously existing 3D radiative MHD numerical model of the solar atmosphere, which included self-consistently locally driven magnetic amplification in the chromosphere, by adding ambipolar diffusion and time-dependent non-equilibrium hydrogen ionization to the model. The energy of the magnetic field is dissipated in the upper chromosphere, providing a large temperature increase due to ambipolar diffusion and the non-equilibrium ionization (NEQI). At the same time, we find that adding the ambipolar diffusion and NEQI in the simulation has a minor impact on the local growth of the magnetic field in the lower chromosphere and its dynamics. Our comparison between synthesized Mg II profiles from these high spatial resolution models, with and without ambipolar diffusion and NEQI, and quiet Sun and coronal hole observations from IRIS now reveal a better correspondence. The intensity of profiles is increased and the line cores are slightly broader when ambipolar diffusion and NEQI effects are included. Therefore, the Mg II profiles are closer to those observed than in previous models, but some differences still remain.

astro-ph.SR↗

arXiv2020

ALMA and IRIS Observations of the Solar Chromosphere II: Structure and Dynamics of Chromospheric Plage

We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate plage from its surrounding regions compared to other methods commonly used. We caution that isolating plage from its immediate surroundings must be done with care in order to successfully mitigate statistical biases that, for instance, can impact quantitative comparisons between different chromospheric observables. Using this methodology, our analysis suggests that 1.25 mm wavelength free-free emission in plage regions observed with ALMA/Band6 may not form in the low chromosphere as previously thought, but rather in the upper chromospheric parts of dynamic plage features (such as spicules and other bright structures), i.e., near geometric heights of transition region temperatures. We investigate the high degree of similarity between chromospheric plage features observed in ALMA/Band6 (at 1.25 mm wavelength) and IRIS/Si IV at 1393Å. We also show that IRIS/Mg II h and k is not as well correlated with ALMA/Band6 as was previously thought, and we discuss the discrepancies with previous works. Lastly, we report indications for chromospheric heating due to propagating shocks supported by the ALMA/Band6 observations.

astro-ph.SR↗

arXiv2020

ALMA and IRIS Observations of the Solar Chromosphere I: an On-Disk Type II Spicule

We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph (IRIS). The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that considers non-equilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with ALMA and confirm its multithermal nature.

astro-ph.SR↗

arXiv2020

Signatures of ubiquitous magnetic reconnection in the lower solar atmosphere

Ellerman Bomb-like brightenings of the hydrogen Balmer line wings in the quiet Sun (QSEBs) are a signature of the fundamental process of magnetic reconnection at the smallest observable scale in the solar lower atmosphere. We analyze high spatial resolution observations (0.1 arcsec) obtained with the Swedish 1-m Solar Telescope to explore signatures of QSEBs in the H$β$ line. We find that QSEBs are ubiquitous and uniformly distributed throughout the quiet Sun, predominantly occurring in intergranular lanes. We find up to 120 QSEBs in the FOV for a single moment in time; this is more than an order of magnitude higher than the number of QSEBs found in earlier H$α$ observations. This suggests that about half a million QSEBs could be present in the lower solar atmosphere at any given time. The QSEB brightening found in the H$β$ line wings also persist in the line core with a temporal delay and spatial offset towards the nearest solar limb. Our results suggest that QSEBs emanate through magnetic reconnection along vertically extended current sheets in the solar lower atmosphere. The apparent omnipresence of small-scale magnetic reconnection may play an important role in the energy balance of the solar chromosphere.

astro-ph.SR↗

arXiv2020

Physical properties of bright Ca II K fibrils in the solar chromosphere

Broad-band images of the solar chromosphere in the Ca II H&K line cores around active regions are covered with fine bright elongated structures called bright fibrils. The mechanisms that form these structures and cause them to appear bright are still unknown. We aim to investigate the physical properties, such as temperature, line-of-sight velocity, and microturbulence, in the atmosphere that produces bright fibrils and to compare those to the properties of their surrounding atmosphere. We used simultaneous observations of a plage region in Fe I 6301-2 Å, Ca II 8542 Å, Ca II K, and H$α$ acquired by the CRISP and CHROMIS instruments on the Swedish 1-m Solar Telescope. We manually selected a sample of 282 Ca II K bright fibrils. We compared the appearance of the fibrils in our sample to the Ca II 8542 Å and H$α$ data. We performed non-local thermodynamic equilibrium (non-LTE) inversions using the inversion code STiC on the Fe I 6301-2 Å, Ca II 8542 Å, Ca II K lines to infer the physical properties of the atmosphere. The line profiles in bright fibrils have a higher intensity in their K$_2$ peaks compared to profiles formed in the surrounding atmosphere. The inversion results show that the atmosphere in fibrils is on average $100-200$~K hotter at an optical depth log$(τ) = -4.3$ compared to their surroundings. The line-of-sight velocity at chromospheric heights in the fibrils does not show any preference towards upflows or downflows. The microturbulence in the fibrils is on average 0.5 km s$^{-1}$ higher compared to their surroundings. Our results suggest that the fibrils have a limited extent in height, and they should be viewed as hot threads pervading the chromosphere.

astro-ph.SR↗

arXiv2019

Three-dimensional modeling of chromospheric spectral lines in a simulated active region

Because of the complex physics that governs the formation of chromospheric lines, interpretation of solar chromospheric observations is difficult. The origin and characteristics of many chromospheric features are, because of this, unresolved. We focus here on studying two prominent features: long fibrils and flare ribbons. To model them, we use a 3D MHD simulation of an active region which self-consistently reproduces both of them. We model the H$α$, Mg II k, Ca II K, and Ca II 8542 Å lines using the 3D non-LTE radiative transfer code Multi3D. This simulation reproduces long fibrils that span between the opposite-polarity sunspots and go up to 4 Mm in height. They can be traced in all lines due to density corrugation. Opposite to previous studies, H$α$, Mg II h&k, and Ca II H&K, are formed at similar height in this model. Magnetic field lines are aligned with the H$α$ fibrils, but the latter holds to a lesser extent for the Ca II 8542 Å line. The simulation shows structures in the H$α$ line core that look like flare ribbons. The emission in the ribbons is caused by a dense chromosphere and a transition region at high column mass. The ribbons are visible in all chromospheric lines, but least prominent in Ca II 8542 Å line. In some pixels, broad asymmetric profiles with a single emission peak are produced, similar to the profiles observed in flare ribbons. They are caused by a deep onset of the chromospheric temperature rise and large velocity gradients. The simulation produces long fibrils similar to what is seen in observations. It also produces structures similar to flare ribbons despite the lack of non-thermal electrons in the simulation. The latter suggests that thermal conduction might be a significant agent in transporting flare energy to the chromosphere in addition to non-thermal electrons.

astro-ph.SR↗

arXiv2019

Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach

Inversion codes allow reconstructing a model atmosphere from observations. With the inclusion of optically thick lines that form in the solar chromosphere, such modelling is computationally very expensive because a non-LTE evaluation of the radiation field is required. In this study, we combine the results provided by these traditional methods with machine and deep learning techniques to obtain similar-quality results in an easy-touse, much faster way. We have applied these new methods to Mg II h&k lines observed by IRIS. As a result, we are able to reconstruct the thermodynamic state (temperature, line-of-sight velocity, non-thermal velocities, electron density, etc.) in the chromosphere and upper photosphere of an area equivalent to an active region in a few CPU minutes, speeding up the process by a factor of $10^5$-$10^6$. The open-source code accompanying this paper will allow the community to use IRIS observations to open a new window to a host of solar phenomena.

astro-ph.SR↗

arXiv2018

Magnetic field variations associated with umbral flashes and penumbral waves

Umbral flashes (UF) and running penumbral waves (RPWs) in sunspot chromospheres leave a dramatic imprint in the intensity profile of the Ca II 854.2 nm line. Recent studies have focussed on also explaining the observed polarization profiles, that show even more dramatic variations during the passage of these shock fronts. While most of these variations can be explained with an almost constant magnetic field as a function of time, several studies have reported changes in the inferred magnetic field strength during UF phases. In this study we investigate the origin of these periodic variations of the magnetic field strength by analyzing a time-series of high temporal cadence observations acquired in the Ca II line with the CRISP instrument at the Swedish 1-m Solar Telescope. In particular, we analyze how the inferred geometrical height scale changes between quiescent and UF phases, and whether those changes are enough to explain the observed changes in $B$. We have performed non-LTE data inversions with the NICOLE code of a time-series of very high spatio-temporal resolution observations in the Ca II and Fe I 630.15\630.25 nm lines. Our results indicate that the Ca II line in sunspots is greatly sensitive to magnetic fields at $\logτ_{500}=-5$ during UFs and quiescence. However, this optical depth value does not correspond to the same geometrical height during the two phases. Our results indicate that during UFs and RPWs the $\logτ=-5$ is located at a higher geometrical height than during quiescence. Additionally, the inferred magnetic field values are higher in UFs (~270 G) and in RPWs (~100 G). Our results suggest that opacity changes caused by UFs and RPWs cannot explain the observed temporal variations in the magnetic field, as the line seems to form at higher geometrical heights where the field is expected to be lower.

astro-ph.SR↗

arXiv2018

Temperature constraints from inversions of synthetic solar optical, UV and radio spectra

High-resolution observations of the solar chromosphere at millimeter wavelengths are now possible with the Atacama Large Millimeter Array (ALMA), promising to tackle many open problems in solar physics. Observations from other ground and space-based telescopes will greatly benefit from coordinated endeavors with ALMA, yet the diagnostic potential of combined optical, ultraviolet and mm observations has remained mostly unassessed. In this paper we investigate whether mm-wavelengths could aid current inversion schemes to retrieve a more accurate representation of the temperature structure of the solar atmosphere. We performed several non-LTE inversion experiments of the emergent spectra from a snapshot of 3D radiation-MHD simulation. We included common line diagnostics such as CaII H, K, 8542 Å$\,$ and MgII h and k, taking into account partial frequency redistribution effects, along with the continuum around 1.2 mm and 3 mm. We found that including the mm-continuum in inversions allows a more accurate inference of temperature as function of optical depth. The addition of ALMA bands to other diagnostics should improve the accuracy of the inferred chromospheric temperatures between $\log τ\sim[-6,-4.5]$ where the CaII and MgII lines are weakly coupled to the local conditions. However, we found that simultaneous multi-atom, non-LTE inversions of optical and UV lines present equally strong constraints in the lower chromosphere and thus are not greatly improved by the 1.2 mm band. Nonetheless, the 3 mm band is still needed to better constrain the mid-upper chromosphere.

astro-ph.SR↗

arXiv2018

Chromospheric heating due to cancellation of quiet Sun internetwork fields

The heating of the solar chromosphere remains one of the most important questions in solar physics. Our current understanding is that small-scale internetwork (IN) magnetic fields play an important role as a heating agent. Indeed, cancellations of IN magnetic elements in the photosphere can produce transient brightenings in the chromosphere and transition region. These bright structures might be the signature of energy release and plasma heating, probably driven by magnetic reconnection of IN field lines. Although single events are not expected to release large amounts of energy, their global contribution to the chromosphere may be significant due to their ubiquitous presence in quiet Sun regions. In this paper we study cancellations of IN elements and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1-m Solar Telescope (SST) to identify cancellations of IN magnetic flux patches and follow their evolution. We find that, on average, these events live for ~3 minutes in the photosphere and ~12 minutes in the chromosphere and/or transition region. Employing multi-line inversions of the Mg II h & k lines we show that cancellations produce clear signatures of heating in the upper atmospheric layers. However, at the resolution and sensitivity accessible to the SST, their number density still seems to be one order of magnitude too low to explain the global chromospheric heating.

astro-ph.SR↗

arXiv2017

Three-dimensional modeling of the Ca II H&K lines in the solar atmosphere

CHROMIS, a new imaging spectrometer at the Swedish 1-m Solar Telescope (SST), can observe the chromosphere in the H and K lines of Ca II at high spatial and spectral resolution. Accurate modeling as well as an understanding of the formation of these lines are needed to interpret the SST/CHROMIS observations. Such modeling is computationally challenging because these lines are influenced by strong departures from local thermodynamic equilibrium, three-dimensional radiative transfer, and partially coherent resonance scattering of photons. We aim to model the CaII H&K lines in 3D model atmospheres to understand their formation and to investigate their diagnostic potential for probing the chromosphere. We model the synthetic spectrum of Ca II using the radiative transfer code Multi3D in three different radiation-magnetohydrodynamic model atmospheres computed with the Bifrost code. We classify synthetic intensity profiles according to their shapes and study how their features are related to the physical properties in the model atmospheres. We investigate whether the synthetic data reproduce the observed spatially-averaged line shapes, center-to-limb variation and compare with SST/CHROMIS images. The spatially-averaged synthetic line profiles show too low central emission peaks, and too small separation between the peaks. The trends of the observed center-to-limb variation of the profiles properties are reproduced by the models. The Ca II H&K line profiles provide a temperature diagnostic of the temperature minimum and the temperature at the formation height of the emission peaks. The Doppler shift of the central depression is an excellent probe of the velocity in the upper chromosphere.

astro-ph.SR↗