Author name code: sainz-dalda ADS astronomy entries on 2022-09-14 author:"Sainz Dalda, Alberto" ------------------------------------------------------------------------ Title: Solar surges related to UV bursts: Characterization through k-means, inversions, and density diagnostics Authors: Nóbrega Siverio, Daniel; Guglielmino, Salvatore Luigi; Sainz Dalda, A. Bibcode: 2022cosp...44.2530N Altcode: Surges are dynamic, cool and dense ejections typically observed in chromospheric lines and closely related to other solar phenomena like UV bursts or coronal jets. Even though surges have been observed for decades now, fundamental questions regarding the temperature and density distribution, as well as their connection and impact on upper layers of the solar atmosphere remain open. Our aim is to characterize the chromospheric and transition region properties of these phenomena taking advantage of high-resolution observations combined with advanced techniques. We have analyzed four surges that appear related to UV bursts observed with the Interface Region Imaging Spectrograph (IRIS) on 2016 April. We have studied the mid- and low-chromosphere of the surges by getting their representative Mg II h$&$k line profiles through the k-means algorithm and performing inversions on them using the STIC code. We have also studied the far-UV spectra, focusing on the O IV 1399.8 and 1401.2 Å lines, and carrying out density diagnostics. We obtain that the mid- and low-chromosphere of the surges are characterized by temperatures between 5.5 and 6.9 kK, electronic number densities from ∼1.5$\times$10$^{11}$ to 2.5$\times$10$ ^{12}$ cm$^{‑3}$, and line-of-sight velocities of a few km/s at optical depths ranging from log$ _{10}$($\tau$)=‑6.0 to ‑3.2. We find, for the first time, observational evidence of O IV emission within the surges, indicating that these phenomena have a transition region counterpart even in the weakest lines. The O IV emitting layers of the surges have an electron number density between 2.5$\times$10$ ^{10}$ and 10$ ^{12}$ cm$ ^{‑3}$. Title: Emergence of internetwork magnetic fields into the chromosphere and transition region Authors: Gosic, Milan; De Pontieu, Bart; Bellot Rubio, L. R.; Sainz Dalda, A.; Esteban Pozuelo, Sara Bibcode: 2022cosp...44.2511G Altcode: Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic structures that populate the interior of supergranular cells. Since they are spread all over the Sun, they may hold a significant fraction of the total magnetic energy stored in the photosphere. Therefore, it is crucial to understand their role in the quiet Sun magnetism and impact on the energetics and dynamics of the solar atmosphere. To provide new insights into this topic, we studied three flux emergence events and followed them as they emerge into the photosphere and reach the chromosphere and transition region. We used coordinated, high-resolution, multiwavelength observations obtained with the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). SST Fe I 6173 and Mg I b$_2$ 5173 magnetograms show the footpoints of the IN bipoles emerging at the solar surface and rising toward the upper solar atmosphere. For the first time, our spectropolarimetric measurements in the Ca II 8542 \AA\/ line provide direct observational evidence that IN fields are capable of reaching the chromosphere. IRIS observations reveal another important piece of information - small-scale IN loops can even reach transition region heights, and locally heat the upper solar atmosphere. Title: Unsupervised Machine Learning for the Identification of Preflare Spectroscopic Signatures Authors: Woods, Magnus M.; Sainz Dalda, Alberto; De Pontieu, Bart Bibcode: 2021ApJ...922..137W Altcode: The study of the preflare environment is of great importance to understanding what drives solar flares. k-means clustering, an unsupervised machine-learning technique, has the ability to cluster large data set in a way that would be impractical or impossible for a human to do. In this paper we present a study using k-means clustering to identify possible preflare signatures in spectroscopic observations of the Mg II h and k spectral lines made by NASA's Interface Region Imaging Spectrometer. Our analysis finds that spectral profiles showing single-peak Mg II h and k and single-peaked emission in the Mg II UV triplet lines are associated with preflare activity up to 40 minutes prior to flaring. Subsequent inversions of these spectral profiles reveal increased temperature and electron density in the chromosphere, which suggest that significant heating events in the chromosphere may be associated with precursor signals to flares. Title: Solar surges related to UV bursts. Characterization through k-means, inversions, and density diagnostics Authors: Nóbrega-Siverio, D.; Guglielmino, S. L.; Sainz Dalda, A. Bibcode: 2021A&A...655A..28N Altcode: 2021arXiv210813960N Context. Surges are cool and dense ejections typically observed in chromospheric lines and closely related to other solar phenomena such as UV bursts or coronal jets. Even though surges have been observed for decades now, questions regarding their fundamental physical properties such as temperature and density, as well as their impact on upper layers of the solar atmosphere remain open.
Aims: Our aim is to address the current lack of inverted models and diagnostics of surges, as well as to characterize the chromospheric and transition region plasma of these phenomena.
Methods: We have analyzed an episode of recurrent surges related to UV bursts observed with the Interface Region Imaging Spectrograph (IRIS) in April 2016. The mid- and low-chromosphere of the surges were unprecedentedly examined by getting their representative Mg IIh&k line profiles through the k-means algorithm and performing inversions on them using the state-of-the-art STiC code. We have studied the far-UV spectra focusing on the O IV 1399.8 Å and 1401.2 Å lines, which were previously unexplored for surges, carrying out density diagnostics to determine the transition region properties of these ejections. We have also used numerical experiments performed with the Bifrost code for comparisons.
Results: Thanks to the k-means clustering, we reduced the number of Mg IIh&k profiles to invert by a factor 43.2. The inversions of the representative profiles show that the mid- and low-chromosphere of the surges are characterized, with a high degree of reliability, by temperatures mainly around T = 6 kK at −6.0 ≤ log10(τ)≤ − 3.2. For the electronic number density, ne, and line-of-sight velocity, VLOS, the most reliable results from the inversions are within −6.0 ≤ log10(τ)≤ − 4.8, with ne ranging from ∼1.6 × 1011 cm−3 up to 1012 cm−3, and VLOS of a few km s−1. We find, for the first time, observational evidence of enhanced O IV emission within the surges, indicating that these phenomena have a considerable impact on the transition region even in the weakest far-UV lines. The O IV emitting layers of the surges have an electron number density ranging from 2.5 × 1010 cm−3 to 1012 cm−3. The numerical simulations provide theoretical support in terms of the topology and location of the O IV emission within the surges.

Movie associated with Fig. 2 is available at https://www.aanda.org Title: Emergence of Internetwork Magnetic Fields through the Solar Atmosphere Authors: Gošić, M.; De Pontieu, B.; Bellot Rubio, L. R.; Sainz Dalda, A.; Pozuelo, S. Esteban Bibcode: 2021ApJ...911...41G Altcode: 2021arXiv210302213G 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 and the 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 × 1018, 2.5 × 1018, and 5.3 × 1018 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 Fe I 6173 Å and Mg I b2 5173 Å magnetograms, respectively. For the first time, our polarimetric measurements taken in the chromospheric Ca II 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. Title: Compression of Solar Spectroscopic Observations: a Case Study of Mg II k Spectral Line Profiles Observed by NASA's IRIS Satellite Authors: Sadykov, Viacheslav M; Kitiashvili, Irina N; Sainz Dalda, Alberto; Oria, Vincent; Kosovichev, Alexander G; Illarionov, Egor Bibcode: 2021arXiv210307373S Altcode: 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. Title: ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora, Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.; Sainz Dalda, Alberto Bibcode: 2021ApJ...906...83C Altcode: 2020arXiv201205970C We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate a plage from its surrounding regions than other methods commonly used. We caution that isolating a 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 λ = 1.25 mm free-free emission in plage regions observed with the Atacama Large Millimeter/submillimeter Array (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 wavelengths) and the Interface Region Imaging Spectrograph (IRIS)/Si IV at 1393 Å. We also show that IRIS/Mg II h and k are not as well correlated with ALMA/Band6 as was previously thought, and we discuss discrepancies with previous works. Lastly, we report indications of chromospheric heating due to propagating shocks supported by the ALMA/Band6 observations. Title: ALMA and IRIS Observations of the Solar Chromosphere. I. An On-disk Type II Spicule Authors: Chintzoglou, Georgios; De Pontieu, Bart; Martínez-Sykora, Juan; Hansteen, Viggo; de la Cruz Rodríguez, Jaime; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Wedemeyer, Sven; Bastian, Timothy S.; Sainz Dalda, Alberto Bibcode: 2021ApJ...906...82C Altcode: 2020arXiv200512717C We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph. 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 nonequilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with ALMA and confirm its multithermal nature. Title: Expected spectropolarimetric observables in the lower solar atmosphere from 3D radiative MHD models Authors: Sainz Dalda, A.; Gosic, M.; Martinez-Sykora, J. Bibcode: 2020AGUFMSH0010019S Altcode: No abstract at ADS Title: ALMA and IRIS Observations Highlighting the Dynamics and Structure of Chromospheric Plage Authors: Chintzoglou, G.; De Pontieu, B.; Martinez-Sykora, J.; Hansteen, V. H.; de la Cruz Rodriguez, J.; Szydlarski, M.; Jafarzadeh, S.; Wedemeyer, S.; Bastian, T.; Sainz Dalda, A. Bibcode: 2020AGUFMSH0010009C Altcode: 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. In addition, we discuss the strikingly high degree of similarity between chromospheric plage features observed in ALMA/Band6 and IRIS/\ion{Si}{4} (also reproduced in our model) suggesting that ALMA/Band6 does not observe in the low chromosphere as previously thought but rather observes the upper chromospheric parts of structures such as spicules and other bright structures above plage at geometric heights near transition region temperatures. We also show that IRIS/\ion{Mg}{2} is not as well correlated with ALMA/Band6 as was previously thought. For these comparisons, we propose and employ a novel empirical method for the determination of plage regions, which seems to better isolate plage from its surrounding regions as compared to other methods commonly used. We caution that isolating plage from its immediate surroundings must be done with care to mitigate statistical bias in quantitative comparisons between different chromospheric observables. Lastly, we report indications for chromospheric heating due to traveling shocks supported by the ALMA/Band6 observations. Title: Chromospheric response to emergence of internetwork magnetic fields Authors: Gosic, M.; De Pontieu, B.; Bellot Rubio, L.; Sainz Dalda, A. Bibcode: 2020AGUFMSH0010006G Altcode: Internetwork (IN) magnetic fields are weak, short-lived, but highly dynamic magnetic structures that emerge all over the Sun. They bring an enormous amount of magnetic flux and energy to the solar surface. Therefore, IN fields are of paramount importance for maintenance of the QS magnetism. Since these fields are ubiquitous, they may have a substantial impact on the energetics and dynamics of the solar atmosphere. In this work, we use coordinated, high-resolution, multiwavelength observations obtained with the Interface Region Imaging Spectrograph (IRIS) and the Swedish 1 m Solar Telescope (SST) to follow the evolution of IN magnetic loops as they emerge into the photosphere. The footpoints of the emerging IN bipoles are clearly visible as they appear in the photosphere and rise up through the solar atmosphere, as seen in SST magnetograms taken in the Fe I 6173 Å and Mg I b2 5173 Å lines, respectively. Our polarimetric measurements, taken in the Ca II 8542 Å line, provide the first direct observational evidence that IN fields are capable of reaching the chromosphere. Moreover, using IRIS data, we describe in detail how individual IN bipoles affect the dynamics and energetics of the chromosphere and transition region. Title: High-resolution observations of the solar photosphere, chromosphere, and transition region. A database of coordinated IRIS and SST observations Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Carlsson, M.; de la Cruz Rodríguez, J.; Bose, S.; Chintzoglou, G.; Drews, A.; Froment, C.; Gošić, M.; Graham, D. R.; Hansteen, V. H.; Henriques, V. M. J.; Jafarzadeh, S.; Joshi, J.; Kleint, L.; Kohutova, P.; Leifsen, T.; Martínez-Sykora, J.; Nóbrega-Siverio, D.; Ortiz, A.; Pereira, T. M. D.; Popovas, A.; Quintero Noda, C.; Sainz Dalda, A.; Scharmer, G. B.; Schmit, D.; Scullion, E.; Skogsrud, H.; Szydlarski, M.; Timmons, R.; Vissers, G. J. M.; Woods, M. M.; Zacharias, P. Bibcode: 2020A&A...641A.146R Altcode: 2020arXiv200514175R NASA's Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe I lines and spectrally resolved imaging in the chromospheric Ca II 8542 Å, Hα, and Ca II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes. Title: Investigating Pre-flare Signatures with K-means Clustering Authors: Woods, M.; De Pontieu, B.; Sainz Dalda, A. Bibcode: 2019AGUFMSH31E3348W Altcode: We present the results of a large statistical study of pre-flare activity using spectroscopic data from the IRIS spacecraft, analysed using K-means clustering. Solar flares are large energy releases whose effects are observed throughout the solar atmosphere. They are also heavily correlated with eruptions and CMEs, which extend their influence into the greater heliosphere. Many models exist that attempt to explain the triggering of flares and eruptions, but not all have clear observational signatures related to them. The identification of reliable pre-flare signatures is therefore highly important to not only furthering our understanding the processes that lead to flaring and eruptions, but also to efforts to predict the occurrence of such events.

We compiled a data set of over 100 flare events with pre-flare coverage, comprising a range of GOES classes and eruptivities. The individual spectra from these data were then standardised for direct comparison, and the unsupervised machine learning technique K-means clustering was run upon them. This allowed us to investigate the types of spectra that were observed prior to flaring, and how these spectra are distributed both spatially and temporally. Additionally we investigated how these spectral clusters are related to the eruptivity of the flares that they precede. Title: A comprehensive three-dimensional radiative magnetohydrodynamic simulation of a solar flare Authors: Cheung, M. C. M.; Rempel, M.; Chintzoglou, G.; Chen, F.; Testa, P.; Martínez-Sykora, J.; Sainz Dalda, A.; DeRosa, M. L.; Malanushenko, A.; Hansteen, V.; De Pontieu, B.; Carlsson, M.; Gudiksen, B.; McIntosh, S. W. Bibcode: 2019NatAs...3..160C Altcode: 2018NatAs...3..160C Solar and stellar flares are the most intense emitters of X-rays and extreme ultraviolet radiation in planetary systems1,2. On the Sun, strong flares are usually found in newly emerging sunspot regions3. The emergence of these magnetic sunspot groups leads to the accumulation of magnetic energy in the corona. When the magnetic field undergoes abrupt relaxation, the energy released powers coronal mass ejections as well as heating plasma to temperatures beyond tens of millions of kelvins. While recent work has shed light on how magnetic energy and twist accumulate in the corona4 and on how three-dimensional magnetic reconnection allows for rapid energy release5,6, a self-consistent model capturing how such magnetic changes translate into observable diagnostics has remained elusive. Here, we present a comprehensive radiative magnetohydrodynamics simulation of a solar flare capturing the process from emergence to eruption. The simulation has sufficient realism for the synthesis of remote sensing measurements to compare with observations at visible, ultraviolet and X-ray wavelengths. This unifying model allows us to explain a number of well-known features of solar flares7, including the time profile of the X-ray flux during flares, origin and temporal evolution of chromospheric evaporation and condensation, and sweeping of flare ribbons in the lower atmosphere. Furthermore, the model reproduces the apparent non-thermal shape of coronal X-ray spectra, which is the result of the superposition of multi-component super-hot plasmas8 up to and beyond 100 million K. Title: Radiative MHD Simulation of a Solar Flare Authors: Cheung, Mark; Rempel, Matthias D.; Chintzoglou, Georgios; Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto; DeRosa, Marc L.; Malanushenko, Anna; Hansteen, Viggo; Carlsson, Mats; De Pontieu, Bart; Gudiksen, Boris; McIntosh, Scott W. Bibcode: 2019AAS...23431005C Altcode: We present a radiative MHD simulation of a solar flare. The computational domain captures the near-surface layers of the convection zone and overlying atmosphere. Inspired by the observed evolution of NOAA Active Region (AR) 12017, a parasitic bipolar region is imposed to emerge in the vicinity of a pre-existing sunspot. The emergence of twisted magnetic flux generates shear flows that create a pre-existing flux rope underneath the canopy field of the sunspot. Following erosion of the overlying bootstrapping field, the flux rope erupts. Rapid release of magnetic energy results in multi-wavelength synthetic observables (including X-ray spectra, narrowband EUV images, Doppler shifts of EUV lines) that are consistent with flare observations. This works suggests the super-position of multi-thermal, superhot (up to 100 MK) plasma may be partially responsible for the apparent non-thermal shape of coronal X-ray sources in flares. Implications for remote sensing observations of other astrophysical objects is also discussed. This work is an important stepping stone toward high-fidelity data-driven MHD models. Title: Recovering Thermodynamics from Spectral Profiles observed by IRIS: A Machine and Deep Learning Approach Authors: Sainz Dalda, Alberto; de la Cruz Rodríguez, Jaime; De Pontieu, Bart; Gošić, Milan Bibcode: 2019ApJ...875L..18S Altcode: 2019arXiv190408390S Inversion codes allow the reconstruction of a model atmosphere from observations. With the inclusion of optically thick lines that form in the solar chromosphere, such modeling 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-to-use, much faster way. We have applied these new methods to Mg II h and k lines observed by the Interface Region Imaging Spectrograph (IRIS). As a result, we are able to reconstruct the thermodynamic state (temperature, line-of-sight velocity, nonthermal 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 105 - 106. The open-source code accompanying this Letter will allow the community to use IRIS observations to open a new window to a host of solar phenomena. Title: A Statistical Comparison between Photospheric Vector Magnetograms Obtained by SDO/HMI and Hinode/SP Authors: Sainz Dalda, Alberto Bibcode: 2017ApJ...851..111S Altcode: 2018arXiv180107374S Since 2010 May 1, 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 Active Region 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, which would favor the simultaneous or complementary use of both instruments. Title: Realistic radiative MHD simulation of a solar flare Authors: Rempel, Matthias D.; Cheung, Mark; Chintzoglou, Georgios; Chen, Feng; Testa, Paola; Martinez-Sykora, Juan; Sainz Dalda, Alberto; DeRosa, Marc L.; Viktorovna Malanushenko, Anna; Hansteen, Viggo H.; De Pontieu, Bart; Carlsson, Mats; Gudiksen, Boris; McIntosh, Scott W. Bibcode: 2017SPD....4840001R Altcode: We present a recently developed version of the MURaM radiative MHD code that includes coronal physics in terms of optically thin radiative loss and field aligned heat conduction. The code employs the "Boris correction" (semi-relativistic MHD with a reduced speed of light) and a hyperbolic treatment of heat conduction, which allow for efficient simulations of the photosphere/corona system by avoiding the severe time-step constraints arising from Alfven wave propagation and heat conduction. We demonstrate that this approach can be used even in dynamic phases such as a flare. We consider a setup in which a flare is triggered by flux emergence into a pre-existing bipolar active region. After the coronal energy release, efficient transport of energy along field lines leads to the formation of flare ribbons within seconds. In the flare ribbons we find downflows for temperatures lower than ~5 MK and upflows at higher temperatures. The resulting soft X-ray emission shows a fast rise and slow decay, reaching a peak corresponding to a mid C-class flare. The post reconnection energy release in the corona leads to average particle energies reaching 50 keV (500 MK under the assumption of a thermal plasma). We show that hard X-ray emission from the corona computed under the assumption of thermal bremsstrahlung can produce a power-law spectrum due to the multi-thermal nature of the plasma. The electron energy flux into the flare ribbons (classic heat conduction with free streaming limit) is highly inhomogeneous and reaches peak values of about 3x1011 erg/cm2/s in a small fraction of the ribbons, indicating regions that could potentially produce hard X-ray footpoint sources. We demonstrate that these findings are robust by comparing simulations computed with different values of the saturation heat flux as well as the "reduced speed of light". Title: Physics & Diagnostics of the Drivers of Solar Eruptions Authors: Cheung, Mark; Rempel, Matthias D.; Martinez-Sykora, Juan; Testa, Paola; Hansteen, Viggo H.; Viktorovna Malanushenko, Anna; Sainz Dalda, Alberto; DeRosa, Marc L.; De Pontieu, Bart; Carlsson, Mats; Chen, Feng; McIntosh, Scott W.; Gudiksen, Boris Bibcode: 2016SPD....47.0607C Altcode: We provide an update on our NASA Heliophysics Grand Challenges Research (HGCR) project on the ‘Physics & Diagnostics of the Drivers of Solar Eruptions’. This presentation will focus on results from a data-inspired, 3D radiative MHD model of a solar flare. The model flare results from the interaction of newly emerging flux with a pre-existing active region. Synthetic observables from the model reproduce observational features compatible with actual flares. These include signatures of coronal magnetic reconnection, chromospheric evaporation, EUV flare arcades, sweeping motion of flare ribbons and sunquakes. Title: SDO/HMI Vector Magnetic Field Observations of the Solar Polar Region Authors: Sun, X.; Hoeksema, J. T.; Liu, Y.; Norton, A. A.; Sainz Dalda, A.; Hayashi, K. Bibcode: 2015AGUFMSH23A2429S Altcode: SDO/HMI is now providing full-disk vector magnetograms of the Sun. Although the instrument is optimized for strong field in active regions, data from the quieter regions can still provide valuable diagnostics if treated carefully. Here we present our first attempt at inferring the vector field in the polar regions. Through deep averaging (96 min) of the Stokes profiles, we find that many unipolar patches reach 5-sigma signal-to-noise ratio, so magnetic field can be inferred with confidence. The inclination of the field in these patches appears to deviate from the radial direction. We discuss the implications for global coronal field topology and our next steps of work. Title: A self-consistent combined radiative transfer hydrodynamic and particle acceleration model for the X1.0 class flare on March 29, 2014 Authors: Rubio da Costa, F.; Kleint, L.; Sainz Dalda, A.; Petrosian, V.; Liu, W. Bibcode: 2015AGUFMSH31B2419R Altcode: The X1.0 flare on March 29, 2014 was well observed, covering its emission at several wavelengths from the photosphere to the corona. The RHESSI spectra images allow us to estimate the temporal variation of the electron spectra using regularized inversion techniques. Using this as input for a combined particle acceleration and transport (Stanford-Flare) and radiative transfer hydrodynamic (Radyn) code, we calculate the response of the atmosphere to the electron heating. We will present the evolution of the thermal continuum and several line emissions. Comparing them with GOES soft X-ray and high resolution observations from IRIS, SDO and DST/IBIS allows us to test the basic mechanism(s) of acceleration and to constrain its characteristics. We will also present perspectives on how to apply this methodology and related diagnostics to other flares. Title: On Helium 1083 nm Line Polarization during the Impulsive Phase of an X1 Flare Authors: Judge, Philip G.; Kleint, Lucia; Sainz Dalda, Alberto Bibcode: 2015ApJ...814..100J Altcode: 2015arXiv151009218J We analyze spectropolarimetric data of the He i 1083 nm multiplet (1s2s{}3{S}1-1s2p{}3{P}2,1,0o) during the X1 flare SOL2014-03-29T17:48, obtained with the Facility Infrared Spectrometer (FIRS) at the Dunn Solar Telescope. While scanning active region NOAA 12017, the FIRS slit crossed a flare ribbon during the impulsive phase, when the helium line intensities turned into emission at ≲twice the continuum intensity. Their linear polarization profiles are of the same sign across the multiplet including 1082.9 nm, intensity-like, at ≲5% of the continuum intensity. Weaker Zeeman-induced linear polarization is also observed. Only the strongest linear polarization coincides with hard X-ray (HXR) emission at 30-70 keV observed by RHESSI. The polarization is generally more extended and lasts longer than the HXR emission. The upper J = 0 level of the 1082.9 nm component is unpolarizable thus, lower-level polarization is the culprit. We make non-LTE radiative transfer calculations in thermal slabs optimized to fit only intensities. The linear polarizations are naturally reproduced, through a systematic change of sign with wavelength of the radiation anisotropy when slab optical depths of the 1082.9 component are ≲1. Neither are collisions with beams of particles needed, nor can they produce the same sign of polarization of the 1082.9 and 1083.0 nm components. The He i line polarization merely requires heating sufficient to produce slabs of the required thickness. Widely different polarizations of Hα, reported previously, are explained by different radiative anisotropies arising from slabs of different optical depths. Title: The Fast Filament Eruption Leading to the X-flare on 2014 March 29 Authors: Kleint, Lucia; Battaglia, Marina; Reardon, Kevin; Sainz Dalda, Alberto; Young, Peter R.; Krucker, Säm Bibcode: 2015ApJ...806....9K Altcode: 2015arXiv150400515K We investigate the sequence of events leading to the solar X1 flare SOL2014-03-29T17:48. Because of the unprecedented joint observations of an X-flare with the ground-based Dunn Solar Telescope and the spacecraft IRIS, Hinode, RHESSI, STEREO, and the Solar Dynamics Observatory, we can sample many solar layers from the photosphere to the corona. A filament eruption was observed above a region of previous flux emergence, which possibly led to a change in magnetic field configuration, causing the X-flare. This was concluded from the timing and location of the hard X-ray emission, which started to increase slightly less than a minute after the filament accelerated. The filament showed Doppler velocities of ∼2-5 km s-1 at chromospheric temperatures for at least one hour before the flare occurred, mostly blueshifts, but also redshifts near its footpoints. Fifteen minutes before the flare, its chromospheric Doppler shifts increased to ∼6-10 km s-1 and plasma heating could be observed before it lifted off with at least 600 km s-1 as seen in IRIS data. Compared to previous studies, this acceleration (∼3-5 km s-2) is very fast, while the velocities are in the common range for coronal mass ejections. An interesting feature was a low-lying twisted second filament near the erupting filament, which did not seem to participate in the eruption. After the flare ribbons started on each of the second filament’s sides, it seems to have untangled and vanished during the flare. These observations are some of the highest resolution data of an X-class flare to date and reveal some small-scale features yet to be explained. Title: Solar Flare Chromospheric Line Emission: Comparison Between IBIS High-resolution Observations and Radiative Hydrodynamic Simulations Authors: Rubio da Costa, Fatima; Kleint, Lucia; Petrosian, Vahé; Sainz Dalda, Alberto; Liu, Wei Bibcode: 2015ApJ...804...56R Altcode: 2015ApJ...804...56D; 2014arXiv1412.1815R Solar flares involve impulsive energy release, which results in enhanced radiation over a broad spectral range and a wide range of heights. In particular, line emission from the chromosphere can provide critical diagnostics of plasma heating processes. Thus, a direct comparison between high-resolution spectroscopic observations and advanced numerical modeling results could be extremely valuable, but has not yet been attempted. In this paper, we present such a self-consistent investigation of an M3.0 flare observed by the Dunn Solar Telescope’s Interferometric Bi-dimensional Spectrometer (IBIS) on 2011 September 24 which we have modeled using the radiative hydrodynamic code RADYN. We obtained images and spectra of the flaring region with IBIS in Hα 6563 Å and Ca ii 8542 Å, and with RHESSI in X-rays. The latter observations were used to infer the non-thermal electron population, which was passed to RADYN to simulate the atmospheric response to electron collisional heating. We then synthesized spectral lines and compared their shapes and intensities to those observed by IBIS and found a general agreement. In particular, the synthetic Ca ii 8542 Å profile fits well to the observed profile, while the synthetic Hα profile is fainter in the core than for the observation. This indicates that Hα emission is more responsive to the non-thermal electron flux than the Ca ii 8542 Å emission. We suggest that it is necessary to refine the energy input and other processes to resolve this discrepancy. Title: Study of a sunspot umbra using spatially deconvolved Hinode spectropolarometric data Authors: Sainz Dalda, A.; Cheung, C. M. M. Bibcode: 2014AGUFMSH41C4160S Altcode: We have studied the properties of the umbra of NOAA AR 10933 using a 2-step deconvolution method to improve the quality of the data. We have implemented a simple, fast deconvolution technique for observations from the Solar Optical Telescope (SOT, Tsuneta et al 2008) on-board Hinode (Kosugi et al 2007). By performing Richardson-Lucy iterative deconvolution (Richardson 1972, Lucy 1974) on the polarization images one wavelength at a time, we remove the diffraction pattern of SOT from SP data before feeding them into a conventional inversion scheme (SIR, Ruiz Cobo & del Toro Iniesta 1992), which performs the inversion independently pixel-by-pixel. As result, we obtain well-contrasted, well-detailed physical maps of the vector magnetic field and other thermodynamic parameters that may shed light on the nature of the umbra. Title: Comparison between IBIS Observations and Radiative Transfer Hydrodynamic Simulations of a Solar Flare Authors: Rubio da Costa, F.; Kleint, L.; Liu, W.; Sainz Dalda, A.; Petrosian, V. Bibcode: 2014AGUFMSH13B4104R Altcode: High-resolution spectroscopic observations of solar flares are rare but can provide valuable diagnostics. On September 24, 2011 an M3.0 class flare was observed by the Interferometric BIdimensional Spectropolarimeter (IBIS) in chromospheric Hα and CaII 8542 Å lines and by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in X-rays. We fitted the RHESSI spectra at different times with a power-law plus isothermal component. We then used the fitted real-time spectral parameters of nonthermal electrons as the input to the RADYN radiative hydrodynamic code (Carlsson et al, 1992, 1996; Allred et al, 2005) to simulate the low-chromospheric response to collisional heating by energetic electrons. We synthesized both the Hα and CaII 8542 Å lines from the simulation results and compare them with the IBIS observations. We discuss the constraints from this comparison on particle acceleration mechanisms in solar flares. Title: High-resolution Observations of the X-flare on 2014-03-29 Authors: Kleint, L.; Battaglia, M.; Krucker, S.; Reardon, K.; Sainz Dalda, A. Bibcode: 2014AGUFMSH31C..06K Altcode: We investigate the sequence of events leading to the X1 flare SOL2014-03-29T17:48. Because of the unprecedented joint observations of an X-flare with the ground-based Dunn Solar Telescope and the spacecraft IRIS, Hinode, RHESSI, STEREO, and SDO, we can sample many solar layers from the photosphere to the corona. We find that a filament eruption, which was possibly caused by a thermal instability, was the cause of this X-flare. The filament was rising in the chromosphere for at least one hour before the flare occurred with a velocity of ∼2--5 km/s. 15 minutes before the flare, its chromospheric rise velocity increased to ∼6--10 km/s, before it lifted off with at least 600 km/s, as seen by IRIS in the transition region. Doppler velocities from H-alpha images reveal intriguing small scale flows along the filament and enable us to derive its probable shape. An unusual feature was a low-lying twisted flux rope near the filament, which did not participate in the filament eruption. After the flare ribbons started on each of its sides, the flux rope seems to have untangled and vanished during the flare. We present a comprehensive overview of the flare, including polarimetric and spectroscopic data at subarcsecond resolution. Title: On the Origin of a Sunquake during the 2014 March 29 X1 Flare Authors: Judge, Philip G.; Kleint, Lucia; Donea, Alina; Sainz Dalda, Alberto; Fletcher, Lyndsay Bibcode: 2014ApJ...796...85J Altcode: 2014arXiv1409.6268J Helioseismic data from the Helioseismic Magnetic Imager instrument have revealed a sunquake associated with the X1 flare SOL2014-03-29T17:48 in active region NOAA 12017. We try to discover if acoustic-like impulses or actions of the Lorentz force caused the sunquake. We analyze spectropolarimetric data obtained with the Facility Infrared Spectrometer (FIRS) at the Dunn Solar Telescope (DST). Fortunately, the FIRS slit crossed the flare kernel close to the acoustic source during the impulsive phase. The infrared FIRS data remain unsaturated throughout the flare. Stokes profiles of lines of Si I 1082.7 nm and He I 1083.0 nm are analyzed. At the flare footpoint, the Si I 1082.7 nm core intensity increases by a factor of several, and the IR continuum increases by 4% ± 1%. Remarkably, the Si I core resembles the classical Ca II K line's self-reversed profile. With nLTE radiative models of H, C, Si, and Fe, these properties set the penetration depth of flare heating to 100 ± 100 km (i.e., photospheric layers). Estimates of the non-magnetic energy flux are at least a factor of two less than the sunquake energy flux. Milne-Eddington inversions of the Si I line show that the local magnetic energy changes are also too small to drive the acoustic pulse. Our work raises several questions. Have we missed the signature of downward energy propagation? Is it intermittent in time and/or non-local? Does the 1-2 s photospheric radiative damping time discount compressive modes?

The National Center for Atmospheric Research is sponsored by the National Science Foundation. Title: A particular seismic event generated during the solar flare 2014 March 29 Authors: Donea, Alina C.; Judge, P.; Kleint, L.; Sainz-Dalda, Alberto Bibcode: 2014shin.confE..49D Altcode: The X1.3 solar flare of 2014 March 29 from AR 2017 was extremely well observed, from both space and the ground. Helioseismic observations from the Helioseismic Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO) indicate that this flare generated a weak seismic transient. All previous strong seismic transients to date have emanated from sunspot penumbrae, but the source of this transient lay outside the active-region penumbra close to a magnetic pore.

Uniquely, Kleint and Sainz Dalda captured ground based imaging and slit spectropolarimetry of this flare using the IBIS and FIRS instruments respectively, at the Dunn Solar Telescope in Sunspot, New Mexico. Here we report only on FIRS data along with space-based data, IBIS data will be reported elsewhere. The FIRS infrared data are not saturated even during the flare, which was observed through the rise and decay phases. We are still investigating the origins of peculiar, Zeeman-induced polarization in the He I 1083 nm multiplet.

Using spectropolarimetric data of Si I and He I lines from FIRS, we investigate the evolution of both photosphere and chromosphere above and around the seismic source. Together with data from AIA and from RHESSI, these data offer unique new insight into how the flare energy is channeled into and through the photosphere into the Sun's interior as a seismic transient. We present acoustic properties of the seismic event and their relationship to photospheric and chromospheric plasma and magnetic fields from FIRS, and to the evolving plasmas seen from space from UV to X-ray wavelengths. Title: Relationship between unusual features in umbrae and flares Authors: Sainz Dalda, Alberto; Kleint, Lucia Bibcode: 2014AAS...22412314S Altcode: The influence of photospheric and chromospheric dynamics and morphologies on flare activity are still unclear. We present a study of two flaring active regions (ARs) with complementary instruments (DST/IBIS, Hinode/SOT-SP, SDO/HMI and SDO/AIA) to investigate the temporal evolution of the sunspots and their magnetic and thermodynamic properties. In spite of vast differences in flare occurrence and flare magnitudes, both ARs show similar features in the lower solar atmosphere during flares. We investigate common magnetic topologies and dynamics, which may favor flare activity. Title: Unusual Filaments inside the Umbra Authors: Kleint, L.; Sainz Dalda, A. Bibcode: 2013ApJ...770...74K Altcode: 2013arXiv1305.7263K We analyze several unusual filamentary structures which appeared in the umbra of one of the sunspots in AR 11302. They do not resemble typical light bridges in morphology or in evolution. We analyze data from SDO/HMI to investigate their temporal evolution, Hinode/SP for photospheric inversions, IBIS for chromospheric imaging, and SDO/AIA for the overlying corona. Photospheric inversions reveal a horizontal, inverse Evershed flow along these structures, which we call umbral filaments. Chromospheric images show brightenings and energy dissipation, while coronal images indicate that bright coronal loops seem to end in these umbral filaments. These rapidly evolving features do not seem to be common, and are possibly related to the high flare-productivity of the active region. Their analysis could help to understand the complex evolution of active regions. Title: Differences between the vector magnetic field provided by the SDO/HMI and Hinode-SOT/SP databases. Authors: Sainz Dalda, A. Bibcode: 2013enss.confE.115S Altcode: Since April 7, 2010, SDO/HMI has been observing the photosphere of the full solar disk with almost continuous coverage. Because of its high temporal cadence, SDO/HMI data are especially useful to study those dynamic phenomena that involve the vector magnetic field and its derived quantities, e.g.: electric current density or helicity. Hinode-SOT/SP has a better spectral and spatial resolution, but a worse temporal sampling than SDO/HMI. Hinode SOT/SP takes about 0.5-1h to scan a typical AR. We can overcome both instruments' disadvantages by combining their data. We compare the vector magnetic field of NOAA AR 11410, obtained from inversions of data from both instruments. We found that the magnetic field observed by both instruments is mainly the same for the umbra and penumbra, but it is slightly different for the plage. The most important difference is due to the spectral sampling. However, the spatial sampling also plays an important role in the derived quantities, i.e. those that require calculating derivatives on the spatial coordinates. While we cannot validate which inversion code or instrument gives 'better' estimates of the vector magnetic field, we can provide a conversion between SDO/HMI and Hinode-SOT/SP data. Title: Towards Measuring the Magnetic Energy Spectrum at Sub-Resolution Scales Authors: López Ariste, A.; Sainz Dalda, A. Bibcode: 2012ASPC..463..243L Altcode: Area asymmetries in the Stokes V profile of the Zeeman-sensitive Fe I line at 630.25 nm shed light on gradients of velocity and magnetic field along the photon path. We use that information on observations of Hinode-SOT/SP of the quiet sun at different heliocentric angles to further investigate turbulent field models in those regions. The relationship between the asymmetry value and a correlation length scale for the field strength in the framework of stochastic radiative transfer for polarized light allows us to identify in the data subresolution scales of change of the magnetic field. From these we make a crude first attempt to determine the energy spectrum of the turbulent magnetic field down to the km scale as a proof-of-concept of the potential of this technique. Title: The Dynamic Polar Magnetic Field Before Its Polarity Reversal Authors: Sun, X.; Hoeksema, J. T.; Liu, Y.; Sainz Dalda, A.; Norton, A.; Hayashi, K. Bibcode: 2012AGUFMSH41D2130S Altcode: We characterize the magnetic field in the Sun's polar region using the spectropolarimetric measurement from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). With HMI's high cadence, continuous time coverage, and moderate spectral and spatial resolution, we are able to estimate the polar magnetic flux, its latitudinal distribution, and its temporal variation over three years (2010-2012) during Cycle 24's rising phase. A comparison with higher spectral resolution observations from Hinode SOT/SP provides constraints on the flux estimates. The tracking of individual magnetic element movements yields new insight on the polar field's dynamic behavior leading up to the polarity reversal. We compare the result with that from the HMI line-of-sight data, as well as MDI data for Cycle 23. All observations indicate an earlier reversal of the northern hemisphere owing to more solar activity in the rising phase, which resulted in a significant hemispheric asymmetry. Title: Bipolar Magnetic Structures in Sunspot Penumbrae Authors: Sainz Dalda, A.; Bellot Rubio, L. R. Bibcode: 2012ASPC..454..221S Altcode: We present a study of bipolar, sea-serpent-like structures in the penumbra of sunspots. Our analysis is based on longitudinal magnetograms and full Stokes spectra of AR 10923 taken with the NFI and SP instruments of Hinode. The circular polarization maps reveal the presence of many elongated structures in the mid penumbra and beyond. They consist of two opposite-polarity patches that move together as a single entity toward the outer penumbral boundary, where they become moving magnetic features. Their Stokes profiles suggest a complex magnetic topology. The existence of opposite polarities in the penumbra is well known from previous analyses, but this is the first time that their bipolar nature is unveiled. Our observations also demonstrate that they are intimately connected with the Evershed flow. These structures provide new constraints to theoretical and numerical models of sunspots. Title: Scales of the magnetic fields in the quiet Sun Authors: López Ariste, A.; Sainz Dalda, A. Bibcode: 2012A&A...540A..66L Altcode: 2012arXiv1202.5436L Context. The presence of a turbulent magnetic field in the quiet Sun has been unveiled observationally using different techniques. The magnetic field is quasi-isotropic and has field strengths weaker than 100 G. It is pervasive and may host a local dynamo.
Aims: We aim to determine the length scale of the turbulent magnetic field in the quiet Sun.
Methods: The Stokes V area asymmetry is sensitive to minute variations in the magnetic topology along the line of sight. Using data provided by Hinode-SOT/SP instrument, we performed a statistical study of this quantity. We classified the different magnetic regimes and infer properties of the turbulent magnetic regime. In particular we measured the correlation length associated to these fields for the first time.
Results: The histograms of Stokes V area asymmetries reveal three different regimes: one organized, quasi-vertical and strong field (flux tubes or other structures of the like); a strongly asymmetric group of profiles found around field concentrations; and a turbulent isotropic field. For the last, we confirm its isotropy and measure correlation lengths from hundreds of kilometers down to 10 km, at which point we lost sensitivity. A crude attempt to measure the power spectra of these turbulent fields is made.
Conclusions: In addition to confirming the existence of a turbulent field in the quiet Sun, we give further prove of its isotropy. We also measure correlation lengths down to 10 km. The combined results show magnetic fields with a large span of length scales, as expected from a turbulent cascade. Title: Spectropolarimetry of the photosphere and the chromosphere with IBIS Authors: Kleint, L.; Sainz Dalda, A. Bibcode: 2012decs.confE...4K Altcode: We have obtained quasi-simultaneous spectropolarimetric imaging observations of various chromospheric and photospheric features in the lines Fe I 6302 A, Ca II 8542 A, H-alpha 6563 A and Na I 5896 A with the IBIS instrument at Sac Peak. Our targets include the quiet Sun, pores, sunspots, and flaring regions and our goal is to analyze the 3D magnetic field structure of the solar atmosphere. We carry out NTLE inversions with the NICOLE code to investigate interpretation techniques for chromospheric spectropolarimetric observations. The very faint polarization signatures make chromospheric inversions of the quiet Sun challenging. On the other hand, they are quite pronounced during flares and show us that the chromospheric magnetic structure is seemingly unrelated to the photosphere during these events. Title: Observation, inversion and numerical simulation of single-lobed Stokes V profiles in the quiet sun. Authors: Sainz Dalda, A.; Martínez-Sykora, J.; Bellot Rubio, L.; Title, A. Bibcode: 2012decs.confE..89S Altcode: We have studied characteristics and statistics of strong asymmetric profiles in Stokes V, i.e., single-lobed profiles, in quiet sun using Hinode/SOT. These profiles require the existence of a velocity gradient along the line-of-sight, possibly associated with gradients of magnetic field strength, inclination and/or azimuth. For a better understanding, observations, inversions and numerical simulations are compared. We focus our analysis of the observations on the statistical properties of the single-lobed Stokes V profiles and the results provided by the inversions using SIRJUMP, which is an LTE inversion code that can reproduce sharp discontinuities or jump in the magnetic field and line-of-sight velocity of the atmosphere model. In the quiet sun, magnetic field is continuously appearing and disappearing at small scales due to the convective motions and the input of new flux from deeper layers. From radiative MHD 3D simulations, using Bifrost code, we note that most of these small scale processes have stratifications with gradients of magnetic field strength, inclination and velocities. As result, those stratifications showing jumps in the magnetic field configuration are associated with the existence of single-lobe Stokes V profiles in the solar photosphere, as we previously assumed for the inversions. We show that most of these profiles come from emerging and disappearance magnetic flux in small scales in the simulations. Finally, we emphasize importance of the comparison between the synthetic profiles from the simulations with the observed ones and the atmospheres that produce them. This comparison will ultimately improve the realism of the simulations and quantify the emerging and disappearance flux in the quiet sun. Title: Study of Single-lobed Circular Polarization Profiles in the Quiet Sun Authors: Sainz Dalda, A.; Martínez-Sykora, J.; Bellot Rubio, L.; Title, A. Bibcode: 2012ApJ...748...38S Altcode: 2012arXiv1202.0593S The existence of asymmetries in the circular polarization (Stokes V) profiles emerging from the solar photosphere has been known since the 1970s. These profiles require the presence of a velocity gradient along the line of sight (LOS), possibly associated with gradients of magnetic field strength, inclination, and/or azimuth. We have focused our study on the Stokes V profiles showing extreme asymmetry in the form of only one lobe. Using Hinode spectropolarimetric measurements, we have performed a statistical study of the properties of these profiles in the quiet Sun. We show their spatial distribution, their main physical properties, how they are related with several physical observables, and their behavior with respect to their position on the solar disk. The single-lobed Stokes V profiles occupy roughly 2% of the solar surface. For the first time, we have observed their temporal evolution and have retrieved the physical conditions of the atmospheres from which they emerged using an inversion code implementing discontinuities of the atmospheric parameters along the LOS. In addition, we use synthetic Stokes profiles from three-dimensional magnetoconvection simulations to complement the results of the inversion. The main features of the synthetic single-lobed profiles are in general agreement with the observed ones, lending support to the magnetic and dynamic topologies inferred from the inversion. The combination of all these different analyses suggests that most of the single-lobed Stokes V profiles are signals coming from the magnetic flux emergence and/or submergence processes taking place in small patches in the photosphere of the quiet Sun. Title: Magnetic Topology of a Naked Sunspot: Is It Really Naked? Authors: Sainz Dalda, A.; Vargas Domínguez, S.; Tarbell, T. D. Bibcode: 2012ApJ...746L..13S Altcode: 2012arXiv1202.0591S The high spatial, temporal, and spectral resolution achieved by Hinode instruments gives much better understanding of the behavior of some elusive solar features, such as pores and naked sunspots. Their fast evolution and, in some cases, their small sizes have made their study difficult. The moving magnetic features (MMFs) have been studied during the last 40 years. They have been always associated with sunspots, especially with the penumbra. However, a recent observation of a naked sunspot (one with no penumbra) has shown MMF activity. The authors of this reported observation expressed their reservations about the explanation given to the bipolar MMF activity as an extension of the penumbral filaments into the moat. How can this type of MMF exist when a penumbra does not? In this Letter, we study the full magnetic and (horizontal) velocity topology of the same naked sunspot, showing how the existence of a magnetic field topology similar to that observed in sunspots can explain these MMFs, even when the intensity map of the naked sunspot does not show a penumbra. Title: Spectropolarimetric Comparison Between SDO/HMI and Hinode-SOT/SP Through THEMIS/MTR Authors: Sainz Dalda, A.; Lopez Ariste, A.; Gelly, B.; Tarbell, T. D.; Centeno, R.; DeRosa, M. L.; Hoeksema, J. T. Bibcode: 2011AGUFMSH31A1986S Altcode: In the golden age of solar spacecraft observatories, the use of similar instruments observing same targets offers us the possibility to get more accurate information of the physical processes taking place on them. We present a comparison between the vector magnetic field and thermodynamic quantities obtained by three different spectropolarimetric instruments. We have used the simultaneous multi-wavelength capabilities of THEMIS/MTR as bridge between the observations at Fe I 6173 Å provided by SDO/HMI and at Fe I 6301 & 6302 Å by Hinode-SOT/SP observations. The official inversion codes for these instruments (PCA based-on, VFISV and MERLIN respectively) have been used with the data properly arranged for them. Therefore, we compare the final products usually offered to the community, i.e. after the inversion, using different codes and these different wavelengths. The cross-calibration of these products shall allow us to go forward from one instrument result to other one in an easy, convenient way. Title: Spectropolarimetric Study of Sea-serpent Penumbral Filaments and a Naked Sunspot Authors: Sainz Dalda, Alberto; Tarbell, T.; Title, A.; Vargas Dominguez, S.; Bellot Rubio, L. R. Bibcode: 2011SPD....42.0303S Altcode: 2011BAAS..43S.0303S We present a spectropolarimetric study of the sea-serpent penumbral filaments in AR NOAA 10944 and of a naked sunspot (i.e. a sunspot-like feature without penumbra) in AR NOAA 10977. Both active regions were observed by Hinode-SOT/SP in the photospheric lines Fe I 6301 & 6302 [[Unable to Display Character: &#506]]. The high spatial and temporal resolution combined with the high polarimetric sensitivity of these observations enables us to get a better understanding of the dynamics of the penumbra and the moving magnetic feature (herafter MMF) activity in and around both traditional and naked sunspots. Our results show how the temporal evolution of the sea-serpent filaments fits very well with the thin-tube flux model for the penumbra presented by Schlichenmaier (2003). In addition, the spectropolarmetric analysis of the naked sunspot addresses the issue posed by Zuccarello et al. (2009) about the existence of bipolar MMFs around naked sunspots even when they cannot be explained as an extension of the penumbral filaments. Title: Temporal Evolution of the Sea-Serpent Penumbral Filaments Authors: Sainz Dalda, A.; Bellost Rubio, L. Bibcode: 2010AGUFMSH11B1644S Altcode: The high spectral and spatial resolution provided by Hinode allows us to investigate the temporal evolution of magnetic structures in and around of the sunspot penumbra. Such structures show behavior similar to the penumbral bright points, i.e. they migrate inward from 2/3 of the penumbra and outward from the outer part of the penumbra. This dynamical process can be explained by the so-called thin-flux tube penumbral model that takes into account the magneto-convective overshoot. Title: Sunspot Umbra Atmosphere from Full Stokes Inversion Authors: Wenzel, R.; Berdyugina, S. V.; Fluri, D. M.; Arnaud, J.; Sainz-Dalda, A. Bibcode: 2010ASPC..428..117W Altcode: 2010arXiv1003.5114W Sunspots are prominent manifestations of the solar cycle and provide key constraints for understanding its operation. Also, knowing the internal structure of sunspots allows us to gain insights on the energy transport in strong magnetic fields and, thus, on the processes inside the convection zone, where solar magnetic fields are generated and amplified before emerging at the surface on various scales, even during solar minima. In this paper, we present results of a spectropolarimetric analysis of a sunspot observed during the declining phase of solar cycle 23. By inversion of the full Stokes spectra, observed in several spectral regions in the optical at the THEMIS facility, we infer the height dependence of physical quantities such as the temperature and the magnetic field strength for different sunspot regions. The simultaneous use of atomic (Fe I 5250.2 and 5250.6 Å) and highly temperature-sensitive molecular (TiO 7055 Å and MgH 5200 Å) lines allows us to improve a model of the sunspot umbra. Title: Explanation of the Sea-serpent Magnetic Structure of Sunspot Penumbrae Authors: Kitiashvili, I. N.; Bellot Rubio, L. R.; Kosovichev, A. G.; Mansour, N. N.; Sainz Dalda, A.; Wray, A. A. Bibcode: 2010ApJ...716L.181K Altcode: 2010arXiv1003.0049K Recent spectro-polarimetric observations of a sunspot showed the formation of bipolar magnetic patches in the mid-penumbra and their propagation toward the outer penumbral boundary. The observations were interpreted as being caused by sea-serpent magnetic fields near the solar surface. In this Letter, we develop a three-dimensional radiative MHD numerical model to explain the sea-serpent structure and the wave-like behavior of the penumbral magnetic field lines. The simulations reproduce the observed behavior, suggesting that the sea-serpent phenomenon is a consequence of magnetoconvection in a strongly inclined magnetic field. It involves several physical processes: filamentary structurization, high-speed overturning convective motions in strong, almost horizontal magnetic fields with partially frozen field lines, and traveling convective waves. The results demonstrate a correlation of the bipolar magnetic patches with high-speed Evershed downflows in the penumbra. This is the first time that a three-dimensional numerical model of the penumbra results in downward-directed magnetic fields, an essential ingredient of sunspot penumbrae that has eluded explanation until now. Title: Sea-Serpent Magnetic Structure of Sunspot Penumbrae: Observations and MHD Simulations Authors: Kitiashvili, Irina; Bellot Rubio, L. R.; Kosovichev, A. G.; Mansour, N. N.; Sainz Dalda, A.; Wray, A. A. Bibcode: 2010AAS...21631706K Altcode: 2010BAAS...41..899K Recent high-resolution spectro-polarimetric observations of a sunspot detected formation of bipolar magnetic patches in the mid penumbra and propagation of these patches toward the outer penumbral boundary. The observations have been interpreted as an evidence of sea-serpent field lines near the solar surface. Using a radiative 3D MHD code, we model the behavior of solar magnetoconvection in strongly inclined magnetic field of penumbra. The numerical simulation results reproduce the moving bipolar magnetic elements observed in high-resolution SOHO/MDI and Hinode/SOT data and also their physical properties, supporting the sea-serpent model. The simulations explain the sea-serpent structure and dynamics of the penumbral field as a consequence of turbulent magnetoconvection in a highly inclined, strong magnetic field, which forms filamentary structures and has properties of traveling convective wave. The model also shows that the appearance of the sea-serpent magnetic field lines is closely related to high-speed patches ("Evershed clouds") of the penumbra radial outflow. Title: Decaimiento de flujo magnético en el Sol: Manchas, MMFs y ERs Title: Decaimiento de flujo magnético en el Sol: Manchas, MMFs y ERs Title: Magnetic Flux Decay in the Sun: Spots, MMFs [moving magnetic features] and ERs; Authors: Sainz Dalda, Alberto Bibcode: 2009PhDT.......573S Altcode: No abstract at ADS Title: Sunspot Model Atmosphere from Inversion of Stokes Profiles Authors: Wenzel, R.; Berdyugina, V. S.; Fluri, D. M.; Arnaud, J.; Sainz Dalda, A. Bibcode: 2008ESPM...12.2.24W Altcode: We present results of a spectropolarimetric analysis of sunspots. By inversion of full Stokes spectra observed in serveral spectral regions in the optical at the THEMIS facility we infer the height dependence of physical quantities such as the temperature, LOS velocity and magnetic field for different sunspot regions. The wide spectral range and the use of TiO and MgH transitions, which are extremely temperature sensitive and can be treated in LTE even in higher layers, allow us to extend and improve a sunspot model atmosphere. Title: Comet McNaught C/2006 P1: observation of the sodium emission by the solar telescope THEMIS Authors: Leblanc, F.; Fulle, M.; López Ariste, A.; Cremonese, G.; Doressoundiram, A.; Sainz Dalda, A.; Gelly, B. Bibcode: 2008A&A...482..293L Altcode: Comet McNaught C/2006 P1 was the brightest comet of the last forty years when reaching its perihelion at an heliocentric distance of 0.17 AU. Two days before this perihelion, at an heliocentric distance of 0.2 AU, Themis, a French-Italian solar telescope in the Canary Islands, Spain, observed the Comet sodium emission of McNaught. The measured maximum sodium brightness of the D2 emission line peaked at 900 Mega-Rayleigh. The spatial distribution of the sodium emission with respect to the nucleus of the comet is in agreement with previous observations. It displays a clear sunward-tailward asymmetry that suggests a dichotomy of the sodium sources between a source close to the nucleus and an extended source most probably corresponding to the dust tail. The spatial distribution along the slit of the width and speed of the Doppler Na distribution also suggests such a dichotomy. The sodium ejection rate inferred from this observation agrees with the value of the ejection rate extrapolated from comet Hale-Bopp, taking into account the heliocentric distance of comet McNaught and its significantly larger dust release. If we suppose a similar concentration of sodium atoms in both comets, this observation suggests that the sodium ejection rate from comets McNaught and Hale-Bopp is proportional to the solar flux. Therefore the most probable ejection mechanisms are photo-sputtering, solar wind sputtering, or cometary ion sputtering, and not thermal desorption. Title: Detection of sea-serpent field lines in sunspot penumbrae Authors: Sainz Dalda, A.; Bellot Rubio, L. R. Bibcode: 2008A&A...481L..21S Altcode: 2007arXiv0712.2983S Aims:We investigate the spatial distribution of magnetic polarities in the penumbra of a spot observed very close to disk center.
Methods: High angular and temporal resolution magnetograms taken with the Narrowband Filter Imager aboard Hinode are used in this study. They provide continuous and stable measurements in the photospheric Fe I 630.25 line for long periods of time.
Results: Our observations show small-scale, elongated, bipolar magnetic structures that appear in the mid penumbra and move radially outward. They occur in between the more vertical fields of the penumbra, and can be associated with the horizontal fields that harbor the Evershed flow. Many of them cross the outer penumbral boundary, becoming moving magnetic features in the sunspot moat. We determine the properties of these structures, including their sizes, proper motions, footpoint separation, and lifetimes.
Conclusions: The bipolar patches can be interpreted as being produced by sea-serpent field lines that originate in the mid penumbra and eventually leave the spot in the form moving magnetic features. The existence of such field lines has been inferred from Stokes inversions of spectropolarimetric measurements at lower angular resolution, but this is the first time they are imaged directly. Our observations add another piece of evidence in favor of the uncombed structure of penumbral magnetic fields.

A movie is only available in electronic form at http://www.aanda.org Title: Chromospheric reversals in the emergence of an ephemeral region Authors: Sainz Dalda, A.; López Ariste, A. Bibcode: 2007A&A...469..721S Altcode: Context: The behaviour of both ephemeral regions and moving magnetic features has been often described for the photospheric layer, but not for the chromosphere. Both magnetic structures are related to the decay of active regions, but their actual role is not yet clear.
Aims: Our aim is to observe and understand the behaviour of these structures in the chromosphere.
Methods: We performed simultaneous photospheric and chromospheric spectropolarimetric observations of an ephemeral region and a moving magnetic feature. A new code developed for the reduction of spectropolarimetric data of several wavelengths observed simultaneously was used here for the first time and will be described here for future reference. The resulting Stokes profiles are analysed in detail.
Results: The Stokes V profiles of Fe i 6301 and 6302 Å are reversed in polarity with respect to the chromospheric Ca ii 8498 and 8542 Å in the cases shown. They reveal a reversed magnetic field topology between these layers for both structures. The time evolution of the ephemeral region results in a cancellation of the chromospheric signal, while the ratio of Stokes V amplitudes of Fe i 6301 and 6302 Å simultaneously informs us of a strengthening of the photospheric field. A scenario of the evolution of the ephemeral region is consequently suggested.
Conclusions: The moving magnetic feature reveals itself as an exclusively photospheric feature, similar to the ephemeral region in the initial stages of its emergence. As the loop emerges into the chromosphere it is slowed down and does not reach the corona. Most probably, the opposite polarity background in the chromosphere prevents ascent into corona. Title: Using reduction and inversion tools for THEMIS-MTR data: chromospheric reversals of a moving magnetic feature and an ephemeral region . Authors: Sainz Dalda, A.; López Ariste, A. Bibcode: 2007MmSAI..78..154S Altcode: New tools have been developed for THEMIS spectropolarimetric data. In this paper we present how these tools work and can be used in order to understand two interesting observed phenomena: a moving magnetic feature and an ephemeral region. Title: First observation of bald patches in a filament channel and at a barb endpoint Authors: López Ariste, A.; Aulanier, G.; Schmieder, B.; Sainz Dalda, A. Bibcode: 2006A&A...456..725L Altcode: The 3D magnetic field topology of solar filaments/prominences is strongly debated, because it is not directly measureable in the corona. Among various prominence models, several are consistent with many observations, but their related topologies are very different. We conduct observations to address this paradigm. We measure the photospheric vector magnetic field in several small flux concentrations surrounding a filament observed far from disc center. Our objective is to test for the presence/absence of magnetic dips around/below the filament body/barb, which is a strong constraint on prominence models, and that is still untested by observations. Our observations are performed with the THEMIS/MTR instrument. The four Stokes parameters are extracted, from which the vector magnetic fields are calculated using a PCA inversion. The resulting vector fields are then deprojected onto the photospheric plane. The 180° ambiguity is then solved by selecting the only solution that matches filament chirality rules. Considering the weakness of the resulting magnetic fields, a careful analysis of the inversion procedure and its error bars was performed, to avoid over-interpretation of noisy or ambiguous Stokes profiles. Thanks to the simultaneous multi-wavelength THEMIS observations, the vector field maps are coaligned with the Hα image of the filament. By definition, photospheric dips are identifiable where the horizontal component of the magnetic field points from a negative toward a positive polarity. Among six bipolar regions analyzed in the filament channel, four at least display photospheric magnetic dips, i.e. bald patches. For barbs, the topology of the endpoint is that of a bald patch located next to a parasitic polarity, not of an arcade pointing within the polarity. The observed magnetic field topology in the photosphere tends to support models of prominence based on magnetic dips located within weakly twisted flux tubes. Their underlying and lateral extensions form photospheric dips both within the channel and below barbs. Title: Moving Magnetic Features as Prolongation of Penumbral Filaments Authors: Sainz Dalda, A.; Martínez Pillet, V. Bibcode: 2005ApJ...632.1176S Altcode: A sequence of 633 high spatial resolution magnetograms and continuum images from SOHO MDI of NOAA AR 0330 is used to study moving magnetic feature (MMF) activity in the moat surrounding a mature leader sunspot. The time-averaged frame shows that the moat region is covered by a magnetic field that exhibits the same polarity distribution as that observed in the penumbra. The moat field displays the true polarity of the spot in the sector where the penumbra displays it. Similarly, on the side where the penumbra shows a polarity opposite the true one (due to projection effects after the so-called apparent neutral line), the moat field also displays a polarity opposite the true one. This is only compatible with a moat field that is horizontal almost everywhere, as in the outer penumbra. Indeed, this horizontal moat field is seen to be physically connected with the penumbra. This connection is made evident when analyzing the individual structures detected in the averaged images, which we call moat filaments. The filaments stretch out for 12" in the moat and can be traced back into the penumbra. The observed polarity distribution along them is only compatible with mean inclinations in the range of 80°-90°. Inside the spot, these filaments are linked to the more horizontal magnetic field component that is thought to carry a large part of the Evershed flow. Several bipolar MMFs are seen to originate inside the penumbra and cross the sunspot outer boundary to enter the moat region, following the paths outlined by the moat filaments. These results are discussed in the frame of our current theoretical understanding of the Evershed flow and MMF activity. Title: Flux Cancellation in a Decaying Active Region Authors: Martinez Pillet, V.; Sainz Dalda, A.; van Driel-Gesztelyi, L. Bibcode: 2004cosp...35.1133M Altcode: 2004cosp.meet.1133M Flux Cancellation in a Decaying Active Region Flux cancellation is observed in many regions on the Sun as internetwork, network and active regions fields. It clearly plays a crucial role in the constant flux processing observed in the solar surface. During the decay of an active region, we have observed the in-situ dissapearance of 70 % of its flux (from SOHO/MDI). Active region flux decay is a global, large-scale, process crucial to the solar cycle. But the flux cancellations, where the flux actually disappears, do take place in very small scale regions. There opposite polarities meet and vanish. The process needs of observations with sufficient sensitivity and angular resolution. In the example presented here, we show how up to 4 of these cancellations are associated with outward moving material in the Corona (as observed by TRACE), including a major active region filament eruption. Solar Orbiter, profiting from the advantage observing position and near-corotation can follow these subtle, but crucial, processes with the necessary set of instruments: Magnetographs, Coronal imagers and spectrographs. For those events occurring in the spacecraft solar vertical, one should not exclude the detection of the phenomena in the in-situ instruments.