arXiv subjects

Alberto Sainz Dalda

Publications and source records attributed to Alberto Sainz Dalda.

At least 19 recordsLinked to original sources

arXiv2026

The connection between solar coronal abundances and the underlying lower atmospheric properties

Elemental abundances in the solar corona and solar wind are often observed to differ from those in the solar photosphere, most commonly showing an enhancement of low first ionization-potential (FIP) elements (the FIP effect). The observational evidence of the connection between the chemical fractionation in the solar atmosphere with FIP suggests that the mechanisms responsible for this effect take place in the chromosphere, where low-FIP elements are mostly ionized, while high-FIP elements remain mostly neutral. We discuss the findings of recent observational studies that have investigated the possible footprint of coronal abundance anomalies in the lower atmosphere. We also discuss the limitations of current observations, and future perspectives on addressing this important open issue in solar physics.

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arXiv2026

Predicting the thermodynamics in the chromosphere from the translation of SDO data into the IRIS$^{2}$ inversion results using a visual transformer model

We present SDO2IRIS$^2$: a visual transformer model that translates a combination of images of the chromosphere and transition region (TR), observed by AIA, and a line-of-sight magnetogram, provided by HMI, into temperature, line-of-sight velocity (v$_{los}$), velocity of the turbulent motions (v$_{turb}$), and electron density (n$_{e}$) in the chromosphere. Using the thermodynamic variables obtained from the inversion of the chromospheric lines Mg II h&k, observed by IRIS, as the target of the model, and the intensity images in the chromosphere and TR, and the photospheric magnetogram as the input, the predicted T and n$_{e}$ show a strong correlation ($\approx 0.80$) for $\approx$80% of the test inverted data, a moderate-to-strong correlation ($\approx0.63$) for 70% of the v$_{turb}$ of the target test inverted data, while for the $v_{los}$, the correlation is weak. Therefore, the predicted values by SDO2IRIS$^2$ may be used as an estimation of the thermodynamics in the chromosphere, either as a stand-alone result or as complementary information to other chromospheric data observed simultaneously. The execution time employed by SDO2IRIS$^2$ to obtain the thermodynamic values in the chromosphere is of the order of a few minutes, being $\le10$ minutes when using a CPU, and $\le5$ minutes when using a GPU. SDO2IRIS$^2$ opens a new avenue for the use of SDO data thanks to the inversions provided by IRIS observables.

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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.

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arXiv2025

Bifrost Models of the Quiet Sun. I. Comparison with Solar Observations

Studying the emergence of magnetic fields is essential for understanding the physical mechanisms behind various phenomena in the solar atmosphere. Most importantly, the emerging fields offer valuable insights into how energy and mass are transferred to the upper solar atmosphere. As a result, they have garnered significant attention from both observational and theoretical perspectives. In this article, we present two models of quiet-Sun-like magnetic fields generated by the Bifrost code. We compare these models with observations from the Swedish 1-meter Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). By tracking the magnetic features in both the SST and Bifrost data, we determine the similarities and differences between the fields identified in the models and those observed. We conduct a quantitative comparison of various properties, such as flux content, flux densities, horizontal and line-of-sight velocities, lifetimes, sizes, and surface interactions. Additionally, we identify and analyze the properties of the largest emerging bipoles in the SST and Bifrost data. Our findings indicate that the magnetic bipoles in the Bifrost simulations are generally stronger than those observed with the SST. However, a qualitative comparison of the chromospheric and transition region responses to the emerging fields in the Bifrost models, SST, and IRIS observations shows similar heating processes occurring above and around the emerging fields. Finally, we outline our plans for future work aimed at studying the emergence of magnetic fields in the quiet Sun, with a particular focus on the chromosphere and upper atmospheric layers.

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arXiv2025

Global impact of emerging internetwork fields on the low solar atmosphere

Small-scale, newly emerging internetwork (IN) magnetic fields are considered a viable source of energy and mass for the solar chromosphere and possibly the corona. Multiple studies show that single events of flux emergence can indeed locally heat the low solar atmosphere through interactions of the upward propagating magnetic loops and the preexisting ambient field lines. However, the global impact of the newly emerging IN fields on the solar atmosphere is still unknown. In this paper, we study the spatio-temporal evolution of IN bipolar flux features and analyze their impact on the energetics and dynamics of the quiet Sun atmosphere. We use high resolution, multi-wavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph (IRIS), Hinode, and the Solar Dynamics Observatory (SDO) to identify emerging IN magnetic fields and follow their evolution. Our observational results suggest that only the largest IN bipoles are capable of heating locally the low solar atmosphere, while the global contribution of these bipoles appears to be marginal. However, the total number of bipoles detected and their impact estimated in this work is limited by the sensitivity level, spatial resolution, and duration of our observations. To detect smaller and weaker IN fields that would maintain the basal flux, and examine their contribution to the chromospheric heating, we will need higher resolution, higher sensitivity and longer time series obtained with current and next-generation ground- and space-based telescopes.

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arXiv2022

$IRIS^{2+}$: A comprehensive database of stratified thermodynamic models in the low solar atmosphere

Our knowledge of the low solar atmosphere, i.e., the photosphere and chromosphere, is based on the knowledge gained from the observations and the theoretical and numerical modeling of these layers. In this sense, the thermodynamical and magnetic semi-empirical models of the solar atmosphere have significantly contributed to the advance in the understanding of the physics of the Sun. In the past, many of these models have been used as a reference that helps us to, e.g., constrain the theoretical and numerical modeling, or to verify the goodness of physical parameters obtained from the inversion of the spectral lines. Nevertheless, semi-empirical models are quite limited by the assumptions that are inherent to the approach and do not necessarily provide an accurate view of the instantaneous and local thermodynamic conditions in the solar atmosphere. In this work, we provide an extensive collection of thermodynamic model atmospheres for active regions (ARs) obtained from the simultaneous inversion of 6 lines sensitive to changes in the thermodynamical conditions in the chromosphere, and another 6 lines sensitive to changes in the thermodynamical conditions in the photospere. These inversions were made using 320 representative profiles (RP) obtained by clustering the profiles in the umbra, penumbra, pore-like, plage, and surrounding quiet-sun in 126 active regions. Due to the simultaneous inversion of the selected lines, the resulting representative model atmosphere (RMA) samples the thermodynamics from the bottom of the photosphere to the top of the chromosphere. As a result, this database, named $IRIS^{2+}$ and formed by $40,320$ RP-RMA pairs, represents the most comprehensive collection of stratified-in-optical-depth thermodynamic models of the low solar atmosphere.

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arXiv2022

Small-scale chromospheric spectropolarimetric observables in the internetwork from a 3D radiative MHD model

The presence of the magnetic field is critical to transport energy through the solar atmosphere. The new generation of telescopes will provide new insight into how the magnetic field arrives into the chromosphere and its role in the energy balance of the solar atmosphere. We have used a 3D radiative MHD numerical model of the solar atmosphere with high spatial resolution (4~km) calculated with the Bifrost code. This code solves the full MHD equations with non-grey and non-LTE radiative transfer and thermal conduction along magnetic field lines. The model shows how the lower chromosphere in the internetwork, a region dominated by magneto-acoustic shocks and where plasma beta is greater than 1, is able to generate magnetic field in-situ Martinez-Sykora et al 2019. We have synthesized full-polarimetric Stokes profiles from this model for several spectral lines formed in the photosphere and the chromosphere. These synthetic profiles illustrate the types of observables expected from DKIST and IRIS. Our work provides insight into how to interpret observations from these observatories. We find that in order to discern the chromospheric magnetic observables it is crucial to compensate for Doppler shift rather than use fix wavelength range.

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arXiv2022

Chromospheric and Coronal heating in active region plage by dissipation of currents from braiding

It remains unclear which physical processes are responsible for the dramatic increase with height of the temperature in stellar atmospheres, known as the chromospheric ($\sim$10,000 K) and coronal (several million K) heating problems. Statistical studies of sun-like stars reveal that chromospheric and coronal emissions are correlated on a global scale, constraining, in principle, theoretical models of potential heating mechanisms. However, so far, spatially resolved observations of the Sun have surprisingly failed to show a similar correlation on small spatial scales, leaving models poorly constrained. Here we use unique coordinated high-resolution observations of the chromosphere (from the Interface Region Imaging Spectrograph or IRIS satellite) and low corona (from the Hi-C 2.1 sounding rocket), and machine-learning based inversion techniques to show a strong correlation on spatial scales of a few hundred km between heating in the chromosphere and low corona for regions with strong magnetic field ("plage"). These results are compatible with recent advanced 3D radiative magnetohydrodynamic simulations in which dissipation of current sheets formed due to the braiding of the magnetic field lines deep in the atmosphere is responsible for heating the plasma simultaneously to chromospheric and coronal temperatures. Our results provide deep insight into the nature of the heating mechanism in solar active regions.

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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.

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arXiv2022

Chromospheric thermodynamic conditions from inversions of complex Mg II h&k profiles observed in flares

The flare activity of the Sun has been studied for decades, using both space- and ground-based telescopes. In particular, the Interface Region Imaging Spectrograph (IRIS) provides unique diagnostics to investigate the thermodynamics of flares in the solar atmosphere. The Mg II h&k and Mg II UV triple lines provide key information about the thermodynamics of low to upper chromosphere, while the C II 1334 & 1335 AA lines cover the upper-chromosphere and low transition region. The Mg II h&k and Mg II UV triplet lines show a peculiar, pointy shape before and during the flare activity. The physical interpretation that can explain these profiles has remained elusive. In this paper, we show the results of a non-LTE inversion of such peculiar profiles. To better constrain the atmospheric conditions, the Mg II h&k and Mg II UV triple lines are simultaneously inverted with the C II 1334 & 1335 AA lines. This combined inversion leads to more accurate derived thermodynamic parameters, especially the temperature and the turbulent motions (micro-turbulence velocity). We use the inversion code STiC to look for the best fit between the observed profile and a synthetic profile obtained by solving the radiative transfer problem considering non-local thermodynamic equilibrium and partial frequency redistribution of the radiation due to scattered photons. We are able to conclude that these unique, pointy profiles are associated with a simultaneous increase of the temperature and the electron density in the chromosphere, while the micro-turbulence velocity has values between 5-15 km/s, which seem to be more realistic values than the ones suggested in previous work. More importantly, the line-of-sight velocity shows a large gradient along the optical depth in the high chromosphere. This seems to be the parameter that gives the pointy aspect to these profiles.

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arXiv2022

Recovering thermodynamics from spectral profiles observed by IRIS (II): improved calculation of the uncertainties based on Monte Carlo experiments

Observations by the Interface Region Imaging Spectrograph (IRIS) in the Mg II h & k spectral lines have provided a new diagnostic window towards the knowledge of the complex physical conditions in the solar chromosphere. Theoretical efforts focused on understanding the behavior of these lines have allowed us to obtain a better and more accurate vision of the chromosphere. These efforts include forward modeling, numerical simulations, and inversions. In this paper, we focus our attention on the uncertainties associated with the thermodynamic model atmosphere obtained after the inversion of the Mg II h & k lines. We have used ~ 50;000 synthetic representative profiles of the IRIS2 database to characterize the most important source of uncertainties in the inversion process, viz.: the inherent noise of the observations, the random initialization of process, and the selection criteria in a high-dimensional space. We have applied a Monte Carlo approach to this problem. Thus, for a given synthetic representative profile, we have created five randomized noise realizations (representative of the most popular exposure times in the IRIS observations), and inverted these profiles five times with different inversion initializations. The resulting 25 inverted profiles, fits to noisy data, and model atmospheres are then used to determine the uncertainty in the model atmosphere, based on the standard deviation and empirical selection criteria for the goodness of fit. With this approach, the new uncertainties of the models available in the IRIS2 database are more reliable at the optical depths where the Mg II h & k lines are sensitive to changes in the thermodynamics.

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arXiv2021

Compression of Solar Spectroscopic Observations: a Case Study of Mg II k Spectral Line Profiles Observed by NASA's IRIS Satellite

In this study we extract the deep features and investigate the compression of the Mg II k spectral line profiles observed in quiet Sun regions by NASA's IRIS satellite. The data set of line profiles used for the analysis was obtained on April 20th, 2020, at the center of the solar disc, and contains almost 300,000 individual Mg II k line profiles after data cleaning. The data are separated into train and test subsets. The train subset was used to train the autoencoder of the varying embedding layer size. The early stopping criterion was implemented on the test subset to prevent the model from overfitting. Our results indicate that it is possible to compress the spectral line profiles more than 27 times (which corresponds to the reduction of the data dimensionality from 110 to 4) while having a 4 DN average reconstruction error, which is comparable to the variations in the line continuum. The mean squared error and the reconstruction error of even statistical moments sharply decrease when the dimensionality of the embedding layer increases from 1 to 4 and almost stop decreasing for higher numbers. The observed occasional improvements in training for values higher than 4 indicate that a better compact embedding may potentially be obtained if other training strategies and longer training times are used. The features learned for the critical four-dimensional case can be interpreted. In particular, three of these four features mainly control the line width, line asymmetry, and line dip formation respectively. The presented results are the first attempt to obtain a compact embedding for spectroscopic line profiles and confirm the value of this approach, in particular for feature extraction, data compression, and denoising.

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arXiv2021

Emergence of internetwork magnetic fields through the solar atmosphere

Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they emerge all over the Sun, these small-scale fields bring a substantial amount of flux, and therefore energy, to the solar surface. Because of this, IN fields are crucial for understanding the quiet Sun (QS) magnetism. However, they are weak and produce very small polarization signals, which is the reason why their properties and impact on the energetics and dynamics of the solar atmosphere are poorly known. Here we use coordinated, high-resolution, multiwavelength observations obtained with the Swedish 1-m Solar Telescope (SST) and the \textit{Interface Region Imaging Spectrograph} (IRIS) to follow the evolution of IN magnetic loops as they emerge into the photosphere and reach the chromosphere and transition region. We studied in this paper three flux emergence events having total unsigned magnetic fluxes of $1.9\times10^{18}$, $2.5\times10^{18}$, and $5.3\times10^{18}$~Mx. The footpoints of the emerging IN bipoles are clearly seen to appear in the photosphere and to rise up through the solar atmosphere, as observed in \ion{Fe}{1} 6173 Å\/ and \ion{Mg}{1} b$_2$ 5173 Å\/ magnetograms, respectively. For the first time, our polarimetric measurements taken in the chromospheric \ion{Ca}{2} 8542 Å\/ line provide direct observational evidence that IN fields are capable of reaching the chromosphere. Moreover, using IRIS data, we study the effects of these weak fields on the heating of the chromosphere and transition region.

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arXiv2021

Mapping Solar Magnetic Fields from the Photosphere to the Base of the Corona

Routine ultraviolet imaging of the Sun's upper atmosphere shows the spectacular manifestation of solar activity; yet we remain blind to its main driver, the magnetic field. Here we report unprecedented spectropolarimetric observations of an active region plage and its surrounding enhanced network, showing circular polarization in ultraviolet (Mg II $h$ & $k$ and Mn I) and visible (Fe I) lines. We infer the longitudinal magnetic field from the photosphere to the very upper chromosphere. At the top of the plage chromosphere the field strengths reach more than 300 gauss, strongly correlated with the Mg II $k$ line core intensity and the electron pressure. This unique mapping shows how the magnetic field couples the different atmospheric layers and reveals the magnetic origin of the heating in the plage chromosphere.

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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.

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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.

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arXiv2018

A Statistical Comparison between Photospheric Vector Magnetograms Obtained by SDO/HMI and Hinode/SP

Since May 1, 2010, we have been able to study (almost) continuously the vector magnetic field in the Sun, thanks to two space-based observatories: the Solar Dynamics Observatory (SDO) and Hinode. Both are equipped with instruments able to measure the Stokes parameters of Zeeman-induced polarization of photospheric line radiation. But the observation modes, the spectral lines, the spatial, spectral and temporal sampling, and even the inversion codes used to recover magnetic and thermodynamic information from the Stokes profiles are different. We compare the vector magnetic fields derived from observations with the HMI instrument on board SDO, with those observed by the SP instrument on Hinode. We have obtained relationships between components of magnetic vectors in the umbra, penumbra and plage observed in 14 maps of NOAA AR 11084. Importantly, we have transformed SP data into observables comparable to those of HMI, to explore possible influences of the different modes of operation of the two instruments, and the inversion schemes used to infer the magnetic fields. The assumed filling factor (fraction of each pixel containing a Zeeman signature) produces the most significant differences in derived magnetic properties, especially in the plage. The spectral and angular samplings have the next largest effects. We suggest to treat the disambiguation in the same way in the data provided by HMI and SP. That would make the relationship between the vector magnetic field recovered from these data stronger, what would favor the simultaneous or complementary use of both instruments.

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arXiv2015

The Coronal Global Evolutionary Model: Using HMI Vector Magnetogram and Doppler Data to Model the Buildup of Free Magnetic Energy in the Solar Corona

The most violent space weather events (eruptive solar flares and coronal mass ejections) are driven by the release of free magnetic energy stored in the solar corona. Energy can build up on timescales of hours to days, and then may be suddenly released in the form of a magnetic eruption, which then propagates through interplanetary space, possibly impacting the Earth's space environment. Can we use the observed evolution of the magnetic and velocity fields in the solar photosphere to model the evolution of the overlying solar coronal field, including the storage and release of magnetic energy in such eruptions? The objective of CGEM, the Coronal Global Evolutionary Model, funded by the NASA/NSF Space Weather Modeling program, is to develop and evaluate such a model for the evolution of the coronal magnetic field. The evolving coronal magnetic field can then be used as a starting point for magnetohydrodynamic (MHD) models of the corona, which can then be used to drive models of heliospheric evolution and predictions of magnetic field and plasma density conditions at 1AU.

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