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  • 1. Asensio Ramos, A.
    et al.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sparse inversion of Stokes profiles I. Two-dimensional Milne-Eddington inversions2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 577, article id A140Article in journal (Refereed)
    Abstract [en]

    Context. Inversion codes are numerical tools used to infer physical properties from observations. Despite their success, the quality of current spectropolarimetric observations and those expected in the near future presents a challenge to current inversion codes. Aims. The pixel-by-pixel strategy of inverting spectropolarimetric data that we currently use needs to be surpassed and improved. The inverted physical parameters have to take into account the spatial correlation that is present in the data and that contains valuable physical information. Methods. We used the concept of sparsity or compressibility to develop a new generation of inversion codes for the Stokes parameters. The inversion code uses numerical optimization techniques based on the idea of proximal algorithms to impose sparsity. In so doing, we allow for the first time exploiting the spatial correlation on the maps of physical parameters. Sparsity also regularizes the solution by reducing the number of unknowns. Results. We compare the results of the new inversion code with pixel-by-pixel inversions to demonstrate the increased robustness of the solution. We also show how the method can easily compensate for the effect of the telescope point spread function, producing solutions with an enhanced contrast.

  • 2. Asensio Ramos, A.
    et al.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Martinez Gonzalez, M. J.
    Pastor Yabar, A.
    Inversion of Stokes profiles with systematic effects2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 590, article id A87Article in journal (Refereed)
    Abstract [en]

    Quantitative thermodynamical, dynamical and magnetic properties of the solar and stellar plasmas are obtained by interpreting their emergent non-polarized and polarized spectrum. This inference requires the selection of a set of spectral lines that are particularly sensitive to the physical conditions in the plasma and a suitable parametric model of the solar/stellar atmosphere. Nonlinear inversion codes are then used to fit the model to the observations. However, the presence of systematic effects, like nearby or blended spectral lines, telluric absorption, or incorrect correction of the continuum, among others, can strongly affect the results. We present an extension to current inversion codes that can deal with these effects in a transparent way. The resulting algorithm is very simple and can be applied to any existing inversion code with the addition of a few lines of code as an extra step in each iteration.

  • 3. Asensio Ramos, A.
    et al.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Martínez González, M. J.
    Socas-Navarro, H.
    Inference of the chromospheric magnetic field orientation in the Ca II 8542 angstrom line fibrils2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, article id A133Article in journal (Refereed)
    Abstract [en]

    Context. Solar chromospheric fibrils, as observed in the core of strong chromospheric spectral lines, extend from photospheric field concentrations suggesting that they trace magnetic field lines. These images have been historically used as proxies of magnetic fields for many purposes.

    Aims. Use statistical analysis to test whether the association between fibrils and magnetic field lines is justified.

    Methods. We use a Bayesian hierarchical model to analyze several tens of thousands of pixels in spectro- polarimetric chromospheric images of penumbrae and chromospheric fibrils. We compare the alignment between the field azimuth inferred from the linear polarization signals through the transverse Zeeman effect and the direction of the fibrils in the image.

    Results. We conclude that, in the analyzed fields of view, fibrils are often well aligned with the magnetic field azimuth. Despite this alignment, the analysis also shows that there is a non-negligible dispersion. In penumbral filaments, we find a dispersion with a standard deviation of similar to 16 degrees, while this dispersion goes up to similar to 34 degrees in less magnetized regions.

  • 4.
    Bjørgen, Johan P.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sukhorukov, Andrii V.
    Stockholm University, Faculty of Science, Department of Astronomy. National Academy of Sciences of Ukraine, Ukraine.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, Mats
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scharmer, Göran B.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hansteen, Viggo H.
    Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 611, article id A62Article in journal (Refereed)
    Abstract [en]

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

  • 5. Cristaldi, A.
    et al.
    Guglielmino, S. L.
    Zuccarello, F.
    Romano, P.
    Falco, M.
    van der Voort, L. Rouppe
    Rodriguez, Jaime de la Cruz
    Stockholm University, Faculty of Science, Department of Astronomy.
    Ermolli, I.
    Criscuoli, S.
    DYNAMIC PROPERTIES ALONG THE NEUTRAL LINE OF A DELTA SPOT INFERRED FROM HIGH-RESOLUTION OBSERVATIONS2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 789, no 2, p. 162-Article in journal (Refereed)
    Abstract [en]

    Delta (delta) spots are complex magnetic configurations of sunspots characterized by umbrae of opposite polarity sharing a common penumbra. In order to investigate the fine structure of the region separating the two magnetic polarities of a delta spot, we studied the morphology, the magnetic configuration, and the velocity field in such a region using observations of active region (AR) NOAA 11267 obtained with the CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish Solar Telescope on 2011 August 6. The analysis of CRISP data shows upflows and downflows of similar to +/- 3 km s(-1) in proximity of the delta spot polarity inversion line (PIL), and horizontal motions along the PIL of the order of similar to 1 km s(-1). The results obtained from the SIR inversion of CRISP data also indicate that the transverse magnetic field in the brighter region separating the two opposite magnetic polarities of the delta spot is tilted about similar to 45 degrees with respect to the PIL. Solar Dynamics Observatory/Helioseismic and Magnetic Imager observations confirm the presence of motions of similar to +/- 3 km s(-1) in proximity of the PIL, which were observed to last 15 hr. From the data analyzed, we conclude that the steady, persistent, and subsonic motions observed along the delta spot PIL can be interpreted as being due to Evershed flows occurring in the penumbral filaments that show a curved, wrapped configuration. The fluting of the penumbral filaments and their bending, continuously increased by the approaching motion of the negative umbra toward the positive one, give rise to the complex line-of-sight velocity maps that we observed.

  • 6.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hansteen, Viggo
    Bellot-Rubio, Luis
    Ortiz, Ada
    EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. II. NON-LTE CHROMOSPHERIC DIAGNOSTICS AND INVERSIONS2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 810, no 2, article id 145Article in journal (Refereed)
    Abstract [en]

    Magnetic flux emergence into the outer layers of the Sun is a fundamental mechanism for releasing energy into the chromosphere and the corona. In this paper, we study the emergence of granular-sized flux concentrations and the structuring of the corresponding physical parameters and atmospheric diagnostics in the upper photosphere and in the chromosphere. We make use of a realistic 3D MHD simulation of the outer layers of the Sun to study the formation of the Ca II 8542 line. We also derive semi-empirical 3D models from non-LTE inversions of our observations. These models contain information on the line-of-sight stratifications of temperature, velocity, and the magnetic field. Our analysis explains the peculiar Ca II 8542 angstrom profiles observed in the flux emerging region. Additionally, we derive detailed temperature and velocity maps describing the ascent of a magnetic bubble from the photosphere to the chromosphere. The inversions suggest that, in active regions, granular-sized bubbles emerge up to the lower chromosphere where the existing large-scale field hinders their ascent. We report hints of heating when the field reaches the chromosphere.

  • 7.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Asensio Ramos, Andrés
    NON-LTE INVERSIONS OF THE Mg II h & k AND UV TRIPLET LINES2016In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 830, no 2, article id L30Article in journal (Refereed)
    Abstract [en]

    The Mg II h & k lines are powerful diagnostics for studying the solar chromosphere. They have become particularly popular with the launch of the Interface Region Imaging Spectrograph (IRIS) satellite, and a number of studies that include these lines have lead to great progress in understanding chromospheric heating, in many cases thanks to the support from 3D MHD simulations. In this study, we utilize another approach to analyze observations: non-LTE inversions of the Mg II h & k and UV triplet lines including the effects of partial redistribution. Our inversion code attempts to construct a model atmosphere that is compatible with the observed spectra. We have assessed the capabilities and limitations of the inversions using the FALC atmosphere and a snapshot from a 3D radiation-MHD simulation. We find that Mg II h & k allow reconstructing a model atmosphere from the middle photosphere to the transition region. We have also explored the capabilities of a multi-line/multi-atom setup, including the Mg IIh & k, the Ca II 854.2. nm, and the Fe I. 630.25 lines to recover the full stratification of physical parameters, including the magnetic field vector, from the photosphere to the chromosphere. Finally, we present the first inversions of observed IRIS spectra from quiet-Sun, plage, and sunspot, with very promising results.

  • 8.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Uppsala University, Sweden.
    Löfdahl, Mats G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sütterlin, Peter
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hillberg, Tomas
    Stockholm University, Faculty of Science, Department of Astronomy.
    van der Voort, L. Rouppe
    CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 573, article id A40Article in journal (Refereed)
    Abstract [en]

    The production of science-ready data from major solar telescopes requires expertise beyond that of the typical observer. This is a consequence of the increasing complexity of instruments and observing sequences, which require calibrations and corrections for instrumental and seeing effects that are not only difficult to measure, but are also coupled in ways that require careful analysis in the design of the correction procedures. Modern space-based telescopes have data-processing pipelines capable of routinely producing well-characterized data products. High resolution imaging spectropolarimeters at ground-based telescopes need similar data pipelines. We present new methods for flat-fielding spectropolarimetric data acquired with telecentric Fabry-Perot instruments and a new approach for accurate camera co-alignment for image restoration. We document a procedure that forms the basis of current state-of- the-art processing of data from the CRISP imaging spectropolarimeter at the Swedish 1 m Solar Telescope (SST). By collecting, implementing, and testing a suite of computer programs, we have defined a data reduction pipeline for this instrument. This pipeline, CRISPRED, streamlines the process of making science-ready data. It is implemented and operated in IDL, with time-consuming steps delegated to C. CRISPRED will also be the basis for the data pipeline of the forthcoming CHROMIS instrument.

  • 9.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Socas-Navarro, Hector
    Instituto de Astrofisica de Canarias.
    Are solar chromospheric fibrils tracing the magnetic field?Article in journal (Refereed)
  • 10.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Socas-Navarro, Hector
    Instituto de Astrofisica de Canarias.
    Carlsson, Mats
    University of Oslo.
    Leenaarts, Jorrit
    Utrecht University.
    Non-LTE inversions from a 3D MHD chromospheric modelManuscript (preprint) (Other academic)
  • 11.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    SST/CRISP observations of Ca II 854.2 nm2008In: 12th European Solar Physics Meeting, 2008Conference paper (Other academic)
  • 12.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Measuring the solar atmosphere2010Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The new CRISP filter at the Swedish 1-m Solar Telescope provides opportunities for observing the solar atmosphere with unprecedented spatial resolution and cadence. In order to benefit from the high quality of observational data from this instrument, we have developed methods for calibrating and restoring polarized Stokes images, obtained at optical and near infrared wavelengths, taking into account field-of-view variations of the filter properties.

    In order to facilitate velocity measurements, a time series from a 3D hydrodynamical granulation simulation is used to compute quiet Sun spectral line profiles at different heliocentric angles. The synthetic line profiles, with their convective blueshifts, can be used as absolute references for line-of-sight velocities.

    Observations of the Ca II 8542 Å line are used to study magnetic fields in chromospheric fibrils. The line wings show the granulation pattern at mid-photospheric heights whereas the overlying chromosphere is seen in the core of the line. Using full Stokes data, we have attempted to observationally verify the alignment of chromospheric fibrils with the magnetic field. Our results suggest that in most cases fibrils are aligned along the magnetic field direction, but we also find examples where this is not the case.

    Detailed interpretation of Stokes data from spectral lines formed in the chromospheric data can be made using non-LTE inversion codes. For the first time, we use a realistic 3D MHD chromospheric simulation of the quiet Sun to assess how well NLTE inversions recover physical quantities from spectropolarimetric observations of Ca II 8542 Å. We demonstrate that inversions provide realistic estimates of depth-averaged quantities in the chromosphere, although high spectral resolution and high sensitivity are needed to measure quiet Sun chromospheric magnetic fields.

  • 13.
    de la Cruz Rodríguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, Mats
    University of Oslo.
    Solar velocity references from 3D HD photospheric modelsArticle in journal (Refereed)
  • 14.
    de la Cruz Rodríguez, Jaime
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, M.
    Radiative Diagnostics in the Solar Photosphere and Chromosphere2017In: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 210, no 1-4, p. 109-143Article, review/survey (Refereed)
    Abstract [en]

    Magnetic fields on the surface of the Sun and stars in general imprint or modify the polarization state of the electromagnetic radiation that is leaving from the star. The inference of solar/ stellar magnetic fields is performed by detecting, studying and modeling polarized light from the target star. In this review we present an overview of techniques that are used to study the atmosphere of the Sun, and particularly those that allow to infer magnetic fields. We have combined a small selection of theory on polarized radiative transfer, inversion techniques and we discuss a number of results from chromospheric inversions.

  • 15. Esteban Pozuelo, S.
    et al.
    Bellot Rubio, L. R.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    LATERAL DOWNFLOWS IN SUNSPOT PENUMBRAL FILAMENTS AND THEIR TEMPORAL EVOLUTION2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 803, no 2, article id 93Article in journal (Refereed)
    Abstract [en]

    We study the temporal evolution of downflows observed at the lateral edges of penumbral filaments in a sunspot located very close to the disk center. Our analysis is based on a sequence of nearly diffraction-limited scans of the Fe I 617.3 nm line taken with the CRisp Imaging Spectro-Polarimeter instrument at the Swedish 1 m Solar Telescope. We compute Dopplergrams from the observed intensity profiles using line bisectors and filter the resulting velocity maps for subsonic oscillations. Lateral downflows appear everywhere in the center-side penumbra as small, weak patches of redshifts next to or along the edges of blueshifted flow channels. These patches have an intermittent life and undergo mergings and fragmentations quite frequently. The lateral downflows move together with the hosting filaments and react to their shape variations, very much resembling the evolution of granular convection in the quiet Sun. There is a good relation between brightness and velocity in the center-side penumbra, with downflows being darker than upflows on average, which is again reminiscent of quiet Sun convection. These results point to the existence of overturning convection in sunspot penumbrae, with elongated cells forming filaments where the flow is upward but very inclined, and weak lateral downward flows. In general, the circular polarization profiles emerging from the lateral downflows do not show sign reversals, although sometimes we detect three-lobed profiles that are suggestive of opposite magnetic polarities in the pixel.

  • 16.
    Esteban Pozuelo, Sara
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. CSIC, Spain.
    Bellot Rubio, L. R.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    PROPERTIES OF SUPERSONIC EVERSHED DOWNFLOWS2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 832, no 2, article id 170Article in journal (Refereed)
    Abstract [en]

    We study supersonic Evershed downflows in a sunspot penumbra by means of high spatial resolution spectropolarimetric data acquired in the Fe I. 617.3 nm line with the CRISP instrument at the Swedish 1 m Solar Telescope. Physical observables, such as Dopplergrams calculated from line bisectors and Stokes. V zero-crossing wavelengths, and Stokes. V maps in the far red-wing, are used to find regions where supersonic Evershed downflows may exist. We retrieve the line-of-sight velocity and the magnetic field vector in these regions using two-component inversions of the observed Stokes profiles with the help of the SIR code. We follow these regions during their lifetime to study their temporal behavior. Finally, we carry out a statistical analysis of the detected supersonic downflows to characterize their physical properties. Supersonic downflows are contained in compact patches moving outward, which are located in the mid-and outer penumbra. They are observed as bright, roundish structures at the outer end of penumbral filaments that resemble penumbral grains. The patches may undergo fragmentations and mergings during their lifetime; some of them are recurrent. Supersonic downflows are associated with strong and rather vertical magnetic fields with a reversed polarity compared to that of the sunspot. Our results suggest that downflows returning back to the solar surface with supersonic velocities are abruptly stopped in dense deep layers and produce a shock. Consequently, this shock enhances the temperature and is detected as a bright grain in the continuum filtergrams, which could explain the existence of outward-moving grains in the mid-and outer penumbra.

  • 17. Gošić, M.
    et al.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    De Pontieu, B.
    Bellot Rubio, L. R.
    Carlsson, M.
    Esteban Pozuelo, Sara
    Stockholm University, Faculty of Science, Department of Astronomy. Instituto de Astrofísica de Andalucía, Spain.
    Ortiz, A.
    Polito, V.
    Chromospheric Heating due to Cancellation of Quiet Sun Internetwork Fields2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 857, no 1, article id 48Article in journal (Refereed)
    Abstract [en]

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

  • 18. Henriques, V. M. J.
    et al.
    Mathioudakis, M.
    Socas-Navarro, H.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    A Hot Downflowing Model Atmosphere for Umbral Flashes and the Physical Properties of Their Dark Fibrils2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 845, no 2, article id 102Article in journal (Refereed)
    Abstract [en]

    We perform non-LTE inversions in a large set of umbral flashes, including the dark fibrils visible within them, and in the quiescent umbra by using the inversion code NICOLE on a set of full Stokes high-resolution Ca II lambda 8542 observations of a sunspot at disk center. We find that the dark structures have Stokes profiles that are distinct from those of the quiescent and flashed regions. They are best reproduced by atmospheres that are more similar to the flashed atmosphere in terms of velocities, even if with reduced amplitudes. We also find two sets of solutions that finely fit the flashed profiles: a set that is upflowing, featuring a transition region that is deeper than in the quiescent case and preceded by a slight dip in temperature, and a second solution with a hotter atmosphere in the chromosphere but featuring downflows close to the speed of sound at such heights. Such downflows may be related, or even dependent, on the presence of coronal loops, rooted in the umbra of sunspots, as is the case in the region analyzed. Similar loops have been recently observed to have supersonic downflows in the transition region and are consistent with the earlier sunspot plumes, which were invariably found to display strong downflows in sunspots. Finally, we find, on average, a magnetic field reduction in the flashed areas, suggesting that the shock pressure is moving field lines in the upper layers.

  • 19. Jafarzadeh, S.
    et al.
    Van Der Voort, L. Rouppe
    Rodriguez, Jaime De La Cruz
    Stockholm University, Faculty of Science, Department of Astronomy.
    MAGNETIC UPFLOW EVENTS IN THE QUIET-SUN PHOTOSPHERE. I. OBSERVATIONS2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 810, no 1Article in journal (Refereed)
    Abstract [en]

    Rapid magnetic upflows in the quiet-Sun photosphere were recently uncovered from both SUNRISE/IMaX and Hinode/SOT observations. Here, we study magnetic upflow events (MUEs) from high-quality, high- (spatial, temporal, and spectral) resolution, and full Stokes observations in four photospheric magnetically sensitive Fe I lines centered at 5250.21, 6173.34, 6301.51, and 6302.50 angstrom acquired with the Swedish Solar Telescope (SST)/CRISP. We detect MUEs by subtracting in-line Stokes V signals from those in the far blue wing whose signal-to-noise ratio (S/N) >= 7. We find a larger number of MUEs at any given time (2.0 x 10(-2) arcsec(-2)), larger by one to two orders of magnitude, than previously reported. The MUEs appear to fall into four classes presenting different shapes of Stokes V profiles with (I) asymmetric double lobes, (II) single lobes, (III) double-humped (two same-polarity lobes), and (IV) three lobes (an extra blueshifted bump in addition to double lobes), of which less than half are single-lobed. We also find that MUEs are almost equally distributed in network and internetwork areas and they appear in the interior or at the edge of granules in both regions. Distributions of physical properties, except for horizontal velocity, of the MUEs (namely, Stokes V signal, size, line-of-sight velocity, and lifetime) are almost identical for the different spectral lines in our data. A bisector analysis of our spectrally resolved observations shows that these events host modest upflows and do not show a direct indication of the presence of supersonic upflows reported earlier. Our findings reveal that the numbers, types (classes), and properties determined for MUEs can strongly depend on the detection techniques used and the properties of the employed data, namely, S/Ns, resolutions, and wavelengths.

  • 20.
    Leenaarts, Jorrit
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. University of Oslo, Norway.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kochukhov, Oleg
    Carlsson, Mats
    THE EFFECT OF ISOTOPIC SPLITTING ON THE BISECTOR AND INVERSIONS OF THE SOLAR Ca II 854.2 nm LINE2014In: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 784, no 1, p. L17-Article in journal (Refereed)
    Abstract [en]

    The Ca II 854.2 nm spectral line is a common diagnostic of the solar chromosphere. The average line profile shows an asymmetric core, and its bisector shows a characteristic inverse-C shape. The line actually consists of six components with slightly different wavelengths depending on the isotope of calcium. This isotopic splitting of the line has been taken into account in studies of non-solar stars, but never for the Sun. We performed non-LTE radiative transfer computations from three models of the solar atmosphere and show that the line-core asymmetry and inverse C-shape of the bisector of the 854.2 nm line can be explained by isotopic splitting. We confirm this finding by analyzing observations and showing that the line asymmetry is present irrespective of conditions in the solar atmosphere. Finally, we show that inversions based on the Ca II 854.2 nm line should take the isotopic splitting into account, otherwise the inferred atmospheres will contain erroneous velocity gradients and temperatures.

  • 21.
    Leenaarts, Jorrit
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Danilovic, Sanja
    Stockholm University, Faculty of Science, Department of Astronomy.
    Scharmer, Göran
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, Mats
    Chromospheric heating during flux emergence in the solar atmosphere2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A28Article in journal (Refereed)
    Abstract [en]

    Context. The radiative losses in the solar chromosphere vary from 4 kW m(-2) in the quiet Sun, to 20 kW m(-2) in active regions. The mechanisms that transport non-thermal energy to and deposit it in the chromosphere are still not understood. Aims. We aim to investigate the atmospheric structure and heating of the solar chromosphere in an emerging flux region. Methods. We have used observations taken with the CHROMIS and CRISP instruments on the Swedish 1-m Solar Telescope in the Ca II K, Ca II 854.2 nm, H alpha, and Fe I 630.1 nm and 630.2 nm lines. We analysed the various line profiles and in addition perform multi-line, multi-species, non-local thermodynamic equilibrium (non-LTE) inversions to estimate the spatial and temporal variation of the chromospheric structure. Results. We investigate which spectral features of Ca II K contribute to the frequency-integrated Ca II K brightness, which we use as a tracer of chromospheric radiative losses. The majority of the radiative losses are not associated with localised high-Ca II K-brightness events, but instead with a more gentle, spatially extended, and persistent heating. The frequency-integrated Ca II K brightness correlates strongly with the total linear polarization in the Ca II 854.2 nm, while the Ca II K profile shapes indicate that the bulk of the radiative losses occur in the lower chromosphere. Non-LTE inversions indicate a transition from heating concentrated around photospheric magnetic elements below log tau(500) = -3 to a more space-filling and time-persistent heating above log tau(500) = -4. The inferred gas temperature at log tau(500) = -3.8 correlates strongly with the total linear polarization in the Ca II 854.2 nm line, suggesting that that the heating rate correlates with the strength of the horizontal magnetic field in the low chromosphere.

  • 22.
    Libbrecht, Tine
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Joshi, Jayant
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    Asensio Ramos, Andrés
    Observations of Ellerman bomb emission features in He I D-3 and He I 10 830 angstrom2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 598, article id A33Article in journal (Refereed)
    Abstract [en]

    Context. Ellerman bombs (EBs) are short-lived emission features, characterised by extended wing emission in hydrogen Balmer lines. Until now, no distinct signature of EBs has been found in the He I 10 830 angstrom line, and conclusive observations of EBs in He I D-3 have never been reported.

    Aims. We aim to study the signature of EBs in neutral helium triplet lines.

    Methods. The observations consisted of ten consecutive SST/TRIPPEL raster scans close to the limb, featuring the H beta, He I D-3 and He I 10 830 angstrom spectral regions. We also obtained raster scans with IRIS and made use of the SDO/AIA 1700 angstrom channel. We used HAZEL to invert the neutral helium triplet lines.

    Results. Three EBs in our data show distinct emission signatures in neutral helium triplet lines, most prominently visible in the He I D-3 line. The helium lines have two components: a broad and blueshifted emission component associated with the EB, and a narrower absorption component formed in the overlying chromosphere. One of the EBs in our data shows evidence of strong velocity gradients in its emission component. The emission component of the other two EBs could be fitted using a constant slab. Our analysis hints towards thermal Doppler motions having a large contribution to the broadening for helium and IRIS lines. We conclude that the EBs must have high temperatures to exhibit emission signals in neutral helium triplet lines. An order of magnitude estimate places our observed EBs in the range of T similar to 2 x 10(4) 10(5) K.

  • 23. Louis, Rohan E.
    et al.
    Bellot Rubio, Luis R.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Socas-Navarro, Héctor
    Ortiz, Ada
    Small-scale magnetic flux emergence in a sunspot light bridge2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 584, article id A1Article in journal (Refereed)
    Abstract [en]

    Context: Light bridges are convective intrusions in sunspots that often show enhanced chromospheric activity.

    Aims: We seek to determine the nature of flux emergence in a light bridge and the processes related to its evolution in the solar atmosphere.

    Methods: We analyse a sequence of high-resolution spectropolarimetric observations of a sunspot taken at the Swedish 1-m Solar Telescope. The data consist of spectral scans of the photospheric Fe I line pair at 630 nm and the chromospheric Ca II 854.2 nm line. Bisectors were used to construct Dopplergrams from the Fe I 630.15 nm measurements. We employed LTE and non-LTE inversions to derive maps of physical parameters in the photosphere and chromosphere, respectively.

    Results: We observe the onset of blueshifts of about 2 km s(-1) near the entrance of a granular light bridge on the limbward side of the spot. The blueshifts lie immediately next to a strongly redshifted patch that appeared six minutes earlier. Both patches can be seen for 25 min until the end of the sequence. The blueshifts coincide with an elongated emerging granule, while the redshifts appear at the end of the granule. In the photosphere, the development of the blueshifts is accompanied by a simultaneous increase in field strength of about 400 G. The field inclination increases by some 25 degrees, becoming nearly horizontal. At the position of the redshifts, the magnetic field is equally horizontal but of opposite polarity. An intense brightening is seen in the Ca II filtergrams over the blueshifts and redshifts, about 17 min after their detection in the photosphere. The brightening is due to emission in the blue wing of the Ca II 854.2 nm line, close to its knee. Non-LTE inversions reveal that this kind of asymmetric emission is caused by a temperature enhancement of similar to 700 K between -5.0 <= log tau <= -3.0 and a blueshift of 3 km s(-1) at log tau = - 2.3 that decreases to zero at log tau = - 6.0

    Conclusions: The photospheric blueshifts and redshifts observed in a granular light bridge seem to be caused by the emergence of a small-scale, flat Omega-loop with highly inclined footpoints of opposite polarity that brings new magnetic field to the surface. The gas motions detected in the two footpoints are reminiscent of a siphon flow. The rising loop is probably confined to the lower atmosphere by the overlying sunspot magnetic field and the interaction between the two flux systems may be responsible for temperature enhancements in the upper photosphere/lower chromosphere. This is the first time that magnetic flux is observed to emerge in the strongly magnetised environment of sunspots, pushed upwards by the convective flows of a granular light bridge.

  • 24. Martinez Gonzalez, M. J.
    et al.
    Asensio Ramos, A.
    Arregui, I.
    Collados, M.
    Beck, C.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    ON THE MAGNETISM AND DYNAMICS OF PROMINENCE LEGS HOSTING TORNADOES2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 825, no 2, article id 119Article in journal (Refereed)
    Abstract [en]

    Solar tornadoes are dark vertical filamentary structures observed in the extreme ultraviolet associated with prominence legs and filament barbs. Their true nature and relationship to prominences requires an understanding of their magnetic structure and dynamic properties. Recently, a controversy has arisen: is the magnetic field organized forming vertical, helical structures or is it dominantly horizontal? And concerning their dynamics, are tornadoes really rotating or is it just a visual illusion? Here we analyze four consecutive spectro-polarimetric scans of a prominence hosting tornadoes on its legs, which helps us shed some light on their magnetic and dynamical properties. We show that the magnetic field is very smooth in all the prominence, which is probably an intrinsic property of the coronal field. The prominence legs have vertical helical fields that show slow temporal variation that is probably related to the motion of the fibrils. Concerning the dynamics, we argue that (1) if rotation exists, it is intermittent, lasting no more than one hour, and (2) the observed velocity pattern is also consistent with an oscillatory velocity pattern (waves).

  • 25. Martinez Gonzalez, M. J.
    et al.
    Sainz, R. Manso
    Ramos, A. Asensio
    Beck, C.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Diaz, A. J.
    SPECTRO-POLARIMETRIC IMAGING REVEALS HELICAL MAGNETIC FIELDS IN SOLAR PROMINENCE FEET2015In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 802, no 1, article id 3Article in journal (Refereed)
    Abstract [en]

    Solar prominences are clouds of cool plasma levitating above the solar surface and insulated from the million-degree corona by magnetic fields. They form in regions of complex magnetic topology, characterized by non-potential fields, which can evolve abruptly, disintegrating the prominence and ejecting magnetized material into the heliosphere. However, their physics is not yet fully understood because mapping such complex magnetic configurations and their evolution is extremely challenging, and must often be guessed by proxy from photometric observations. Using state-of-the-art spectro-polarimetric data, we reconstruct the structure of the magnetic field in a prominence. We find that prominence feet harbor helical magnetic fields connecting the prominence to the solar surface below.

  • 26. Noda, C. Quintero
    et al.
    Kato, Y.
    Katsukawa, Y.
    Oba, T.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, M.
    Shimizu, T.
    Orozco Suarez, D.
    Ruiz Cobo, B.
    Kubo, M.
    Anan, T.
    Ichimoto, K.
    Suematsu, Y.
    Chromospheric polarimetry through multiline observations of the 850-nm spectral region - II. A magnetic flux tube scenario2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 472, no 1, p. 727-737Article in journal (Refereed)
    Abstract [en]

    In this publication, we continue the work started in Quintero Noda et al., examining this time a numerical simulation of a magnetic flux tube concentration. Our goal is to study if the physical phenomena that take place in it, in particular, the magnetic pumping, leaves a specific imprint on the examined spectral lines. We find that the profiles from the interior of the flux tube are periodically doppler shifted following an oscillation pattern that is also reflected in the amplitude of the circular polarization signals. In addition, we analyse the properties of the Stokes profiles at the edges of the flux tube discovering the presence of linear polarization signals for the Ca II lines, although they are weak with an amplitude around 0.5 per cent of the continuum intensity. Finally, we compute the response functions to perturbations in the longitudinal field, and we estimate the field strength using the weak-field approximation. Our results indicate that the height of formation of the spectral lines changes during the magnetic pumping process, which makes the interpretation of the inferred magnetic field strength and its evolution more difficult. These results complement those from previous works, demonstrating the capabilities and limitations of the 850-nm spectrum for chromospheric Zeeman polarimetry in a very dynamic and complex atmosphere.

  • 27. Noda, C. Quintero
    et al.
    Shimizu, T.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Katsukawa, Y.
    Ichimoto, K.
    Anan, T.
    Suematsu, Y.
    Spectropolarimetric capabilities of Ca II 8542 angstrom line2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 459, no 3, p. 3363-3376Article in journal (Refereed)
    Abstract [en]

    The next generation of space-and ground-based solar missions aim to study the magnetic properties of the solar chromosphere using the infrared Ca II lines and the He I 10830 angstrom line. The former seem to be the best candidates to study the stratification of magnetic fields in the solar chromosphere and their relation to the other thermodynamical properties underlying the chromospheric plasma. The purpose of this work is to provide a detailed analysis of the diagnostic capabilities of the Ca II 8542 angstrom line, anticipating forthcoming observational facilities. We study the sensitivity of the Ca II 8542 angstrom line to perturbations applied to the physical parameters of reference semi-empirical 1D model atmospheres using response functions and we make use of 3D magnetohydrodynamics simulations to examine the expected polarization signals for moderate magnetic field strengths. Our results indicate that the Ca II 8542 angstrom line is mostly sensitive to the layers enclosed in the range log tau = [0, -5.5], under the physical conditions that are present in our model atmospheres. In addition, the simulated magnetic flux tube generates strong longitudinal signals in its centre and moderate transversal signals, due to the vertical expansion of magnetic field lines, in its edge. Thus, observing the Ca II 8542 angstrom line we will be able to infer the 3D geometry of moderate magnetic field regions.

  • 28. Noda, C. Quintero
    et al.
    Shimizu, T.
    Katsukawa, Y.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Carlsson, M.
    Anan, T.
    Oba, T.
    Ichimoto, K.
    Suematsu, Y.
    Chromospheric polarimetry through multiline observations of the 850-nm spectral region2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 464, no 4, p. 4534-4543Article in journal (Refereed)
    Abstract [en]

    Future solar missions and ground-based telescopes aim to understand the magnetism of the solar chromosphere. We performed a supporting study in Quintero Noda et al. focused on the infrared Ca (II) 8542 angstrom line and we concluded that it is one of the best candidates because it is sensitive to a large range of atmospheric heights, from the photosphere to the middle chromosphere. However, we believe that it is worth trying to improve the results produced by this line observing additional spectral lines. In that regard, we examined the neighbourhood solar spectrum looking for spectral lines which could increase the sensitivity to the atmospheric parameters. Interestingly, we discovered several photospheric lines which greatly improve the photospheric sensitivity to the magnetic field vector. Moreover, they are located close to a second chromospheric line which also belongs to the Ca (II) infrared triplet, i.e. the Ca (II) 8498 angstrom line, and enhances the sensitivity to the atmospheric parameters at chromospheric layers. We conclude that the lines in the vicinity of the Ca (II) 8542 A line not only increase its sensitivity to the atmospheric parameters at all layers, but also they constitute an excellent spectral window for chromospheric polarimetry.

  • 29. Ortiz, Ada
    et al.
    Bellot Rubio, Luis R.
    Hansteen, Viggo H.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy. Uppsala University, Sweden.
    van der Voort, Luc Rouppe
    EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. I. SPECTROPOLARIMETRIC OBSERVATIONS AND SIMULATIONS2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 781, no 2Article in journal (Refereed)
    Abstract [en]

    We study a granular-sized magnetic flux emergence event that occurred in NOAA 11024 in 2009 July. The observations were made with the CRISP spectropolarimeter at the Swedish 1 m Solar Telescope achieving a spatial resolution of 0.14. Simultaneous full Stokes observations of the two photospheric Fe i lines at 630.2 nm and the chromospheric Ca ii 854.2 nm line allow us to describe in detail the emergence process across the solar atmosphere. We report here on three-dimensional (3D) semi-spherical bubble events, where instead of simple magnetic footpoints, we observe complex semi-circular feet straddling a few granules. Several phenomena occur simultaneously, namely, abnormal granulation, separation of opposite-polarity legs, and brightenings at chromospheric heights. However, the most characteristic signature in these events is the observation of a dark bubble in filtergrams taken in the wings of the Ca ii 854.2 nm line. There is a clear coincidence between the emergence of horizontal magnetic field patches and the formation of the dark bubble. We can infer how the bubble rises through the solar atmosphere as we see it progressing from the wings to the core of Ca ii 854.2 nm. In the photosphere, the magnetic bubble shows mean upward Doppler velocities of 2 km s-1 and expands at a horizontal speed of 4 kms-1. In about 3.5minutes it travels some 1100 km to reach themid chromosphere, implying an average ascent speed of 5.2 km s-1. The maximum separation attained by the magnetic legs is 6.6. From an inversion of the observed Stokes spectra with the SIR code, we find maximum photospheric field strengths of 480 G and inclinations of nearly 90. in the magnetic bubble interior, along with temperature deficits of up to 250 K at log t = -2 and above. To aid the interpretation of the observations, we carry out 3D numerical simulations of the evolution of a horizontal, untwisted magnetic flux sheet injected in the convection zone, using the Bifrost code. The computational domain spans from the upper convection zone to the lower corona. In the modeled chromosphere, the rising flux sheet produces a large, cool, magnetized bubble. We compare this bubble with the observed ones and find excellent agreement, including similar field strengths and velocity signals in the photosphere and chromosphere, temperature deficits, ascent speeds, expansion velocities, and lifetimes.

  • 30. Ortiz, Ada
    et al.
    Hansteen, Viggo H.
    Bellot Rubio, Luis Ramon
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    De Pontieu, Bart
    Carlsson, Mats
    van der Voort, Luc Rouppe
    EMERGENCE OF GRANULAR-SIZED MAGNETIC BUBBLES THROUGH THE SOLAR ATMOSPHERE. III. THE PATH TO THE TRANSITION REGION2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 825, no 2, article id 93Article in journal (Refereed)
    Abstract [en]

    We study, for the first time, the ascent of granular-sized magnetic bubbles from the solar photosphere through the chromosphere into the transition region and above. Such events occurred in a flux emerging region in NOAA 11850 on 2013 September 25. During that time, the first co-observing campaign between the Swedish 1-m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS) spacecraft was carried out. Simultaneous observations of the chromospheric Ha 656.28 nm and Ca II 854.2 nm lines, plus the photospheric Fe I 630.25 nm line, were made with the CRISP spectropolarimeter at the Spitzer Space Telescope (SST) reaching a spatial resolution of 0 ''.14. At the same time, IRIS was performing a four-step dense raster of the emerging flux region, taking slit jaw images at 133 (C II, transition region), 140 (Si IV, transition region), 279.6 (Mg II k, core, upper chromosphere), and 283.2 nm (Mg II k, wing, photosphere). Spectroscopy of several lines was performed by the IRIS spectrograph in the far-and near-ultraviolet, of which we have used the Si IV 140.3 and the Mg II k 279.6 nm lines. Coronal images from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory were used to investigate the possible coronal signatures of the flux emergence events. The photospheric and chromospheric properties of small-scale emerging magnetic bubbles have been described in detail in Ortiz et al. Here we are able to follow such structures up to the transition region. We describe the properties, including temporal delays, of the observed flux emergence in all layers. We believe this may be an important mechanism of transporting energy and magnetic flux from subsurface layers to the transition region and corona.

  • 31.
    Robustini, Carolina
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    van der Voort, Luc Rouppe
    Fan-shaped jets above the light bridge of a sunspot driven by reconnection2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 590, article id A57Article in journal (Refereed)
    Abstract [en]

    We report on a fan-shaped set of high-speed jets above a strongly magnetized light bridge (LB) of a sunspot observed in the H alpha line. We study the origin, dynamics, and thermal properties of the jets using high-resolution imaging spectroscopy in H alpha from the Swedish 1m Solar Telescope and data from the Solar Dynamics Observatory and Hinode. The H alpha jets have lengths of 7-38 Mm, are impulsively accelerated to a speed of similar to 100 km s(-1) close to photospheric footpoints in the LB, and exhibit a constant deceleration consistent with solar effective gravity. They are predominantly launched from one edge of the light bridge, and their footpoints appear bright in the H alpha wings. Atmospheric Imaging Assembly data indicates elongated brightenings that are nearly co-spatial with the H alpha jets. We interpret them as jets of transition region temperatures. The magnetic field in the light bridge has a strength of 0.8-2 kG and it is nearly horizontal. All jet properties are consistent with magnetic reconnection as the driver.

  • 32.
    Robustini, Carolina
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Leenaarts, Jorrit
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    The chromosphere above a delta-sunspot in the presence of fan-shaped jets2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 609, article id A14Article in journal (Refereed)
    Abstract [en]

    Context. Delta-sunspots are known to be favourable locations for fast and energetic events like flares and coronal mass ejections. The photosphere of this sunspot type has been thoroughly investigated in the past three decades. The atmospheric conditions in the chromosphere are not as well known, however. Aims. This study is focused on the chromosphere of a delta-sunspot that harbours a series of fan-shaped jets in its penumbra. The aim of this study is to establish the magnetic field topology and the temperature distribution in the presence of jets in the photosphere and the chromosphere. Methods. We use data from the Swedish 1m Solar Telescope (SST) and the Solar Dynamics Observatory. We invert the spectropolarimetric Fe I 6302 angstrom and Ca II 8542 angstrom data from the SST using the non-LTE inversion code NICOLE to estimate the magnetic field configuration, temperature, and velocity structure in the chromosphere. Results. A loop-like magnetic structure is observed to emerge in the penumbra of the sunspot. The jets are launched from this structure. Magnetic reconnection between this emerging field and the pre-existing vertical field is suggested by hot plasma patches on the interface between the two fields. The height at which the reconnection takes place is located between log tau(500) = 2 and log tau(500) = -3. The magnetic field vector and the atmospheric temperature maps show a stationary configuration during the whole observation.

  • 33.
    Scharmer, Göran B.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Werkhoven, T. I. M.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    High-order aberration compensation with multi-frame blind deconvolution and phase diversity image restoration techniques2010In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 521, p. A68-Article in journal (Refereed)
    Abstract [en]

    Context. For accurately measuring intensities and determining magnetic field strengths of small-scale solar (magnetic) structure, knowledge of and compensation for the point spread function is crucial. For images recorded with the Swedish 1-meter Solar Telescope (SST), restoration with multi-frame blind deconvolution (MFBD) and joint phase diverse speckle (JPDS) methods lead to remarkable improvements in image quality but granulation contrasts that are too low, indicating additional stray light. Aims. We propose a method to compensate for stray light from high-order atmospheric aberrations not included in MFBD and JPDS processing. Methods. To compensate for uncorrected aberrations, a reformulation of the image restoration process is proposed that allows the average effect of hundreds of high-order modes to be compensated for by relying on Kolmogorov statistics for these modes. The applicability of the method requires simultaneous measurements of Fried's parameter r(0). The method is tested with simulations as well as real data and extended to include compensation for conventional stray light. Results. We find that only part of the reduction of granulation contrast in SST images is due to uncompensated high-order aberrations. The remainder is still unaccounted for and attributed to stray light from the atmosphere, the telescope with its re-imaging system and to various high-altitude seeing effects. Conclusions. We conclude that statistical compensation of high-order modes is a viable method to reduce the loss of contrast occurring when a limited number of aberrations is explicitly compensated for with MFBD and JPDS processing. We show that good such compensation is possible with only 10 recorded frames. The main limitation of the method is that already MFBD and JPDS processing introduces high-order compensation that, if not taken into account, can lead to over-compensation.

  • 34.
    Scharmer, Göran B.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Narayan, Gautam
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hillberg, Tomas
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats G.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sütterlin, Peter
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    Lagg, Andreas
    CRISP spectropolarimetric imaging of penumbral fine structure2008In: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 689, no 1, p. L69-Article in journal (Refereed)
    Abstract [en]

    We discuss penumbral fine structure in a small part of a pore, observed with the CRISP imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST), close to its diffraction limit of 0.16 . Milne-Eddington inversions applied to these Stokes data reveal large variations of field strength and inclination angle over dark-cored penumbral intrusions and a dark-cored light bridge. The mid-outer part of this penumbra structure shows 0.3 wide spines, separated by 1.6 (1200 km) and associated with 30° inclination variations. Between these spines, there are no small-scale magnetic structures that easily can be identified with individual flux tubes. A structure with nearly 10° more vertical and weaker magnetic field is seen midway between two spines. This structure is cospatial with the brightest penumbral filament, possibly indicating the location of a convective upflow from below.

  • 35.
    Scharmer, Göran
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    Henriques, Vasco
    Stockholm University, Faculty of Science, Department of Astronomy.
    Hillberg, Tomas
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kiselman, Dan
    Stockholm University, Faculty of Science, Department of Astronomy.
    Löfdahl, Mats
    Stockholm University, Faculty of Science, Department of Astronomy.
    Narayan, Gautam
    Stockholm University, Faculty of Science, Department of Astronomy.
    Sütterlin, Peter
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution2008In: 12th European Solar Physics Meeting, 2008Conference paper (Other academic)
  • 36.
    Schnerr, Roald
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    van Noort, Michiel
    Stockholm University, Faculty of Science, Department of Astronomy.
    Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope II: A calibration strategy for Fabry-Perot based instrumentsArticle in journal (Refereed)
  • 37. Socas-Navarro, H.
    et al.
    de la Cruz Rodriguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Asensio Ramos, A.
    Trujillo Bueno, J.
    Ruiz Cobo, B.
    An open-source, massively parallel code for non-LTE synthesis and inversion of spectral lines and Zeeman-induced Stokes profiles2015In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 577Article in journal (Refereed)
    Abstract [en]

    With the advent of a new generation of solar telescopes and instrumentation, interpreting chromospheric observations (in particular, spectropolarimetry) requires new, suitable diagnostic tools. This paper describes a new code, NICOLE, that has been designed for Stokes non-LTE radiative transfer, for synthesis and inversion of spectral lines and Zeeman-induced polarization profiles, spanning a wide range of atmospheric heights from the photosphere to the chromosphere. The code features a number of unique features and capabilities and has been built from scratch with a powerful parallelization scheme that makes it suitable for application on massive datasets using large supercomputers. The source code is written entirely in Fortran 90/2003 and complies strictly with the ANSI standards to ensure maximum compatibility and portability. It is being publicly released, with the idea of facilitating future branching by other groups to augment its capabilities.

  • 38. van der Voort, L. Rouppe
    et al.
    De Pontieu, B.
    Scharmer, Göran B.
    Stockholm University, Faculty of Science, Department of Astronomy. The Royal Swedish Academy of Sciences, Sweden.
    de la Cruz Rodríguez, Jaime
    Stockholm University, Faculty of Science, Department of Astronomy.
    Martínez-Sykora, J.
    Nóbrega-Siverio, D.
    Guo, L. J.
    Jafarzadeh, S.
    Pereira, T. M. D.
    Hansteen, V. H.
    Carlsson, M.
    Vissers, Gregal
    Stockholm University, Faculty of Science, Department of Astronomy.
    Intermittent Reconnection and Plasmoids in UV Bursts in the Low Solar Atmosphere2017In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 851, no 1, article id L6Article in journal (Refereed)
    Abstract [en]

    Magnetic reconnection is thought to drive a wide variety of dynamic phenomena in the solar atmosphere. Yet, the detailed physical mechanisms driving reconnection are difficult to discern in the remote sensing observations that are used to study the solar atmosphere. In this Letter, we exploit the high-resolution instruments Interface Region Imaging Spectrograph and the new CHROMIS Fabry Perot instrument at the Swedish 1-m Solar Telescope (SST) to identify the intermittency of magnetic reconnection and its association with the formation of plasmoids in socalled UV bursts in the low solar atmosphere. The Si IV 1403 angstrom UV burst spectra from the transition region show evidence of highly broadened line profiles with often non-Gaussian and triangular shapes, in addition to signatures of bidirectional flows. Such profiles had previously been linked, in idealized numerical simulations, to magnetic reconnection driven by the plasmoid instability. Simultaneous CHROMIS images in the chromospheric Ca 11 K 3934 angstrom line now provide compelling evidence for the presence of plasmoids by revealing highly dynamic and rapidly moving brightenings that are smaller than 0.12 and that evolve on timescales of the order of seconds. Our interpretation of the observations is supported by detailed comparisons with synthetic observables from advanced numerical simulations of magnetic reconnection and associated plasmoids in the chromosphere. Our results highlight how subarcsecond imaging spectroscopy sensitive to a wide range of temperatures combined with advanced numerical simulations that are realistic enough to compare with observations can directly reveal the small-scale physical processes that drive the wide range of phenomena in the solar atmosphere.

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