Author name code: vannoort ADS astronomy entries on 2022-09-14 author:"van Noort, Michiel J." ------------------------------------------------------------------------ Title: Similarities of magnetoconvection in the umbra and in the penumbra of sunspots Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2021A&A...655A..61L Altcode: 2021arXiv211001352L Context. It is unclear why there is a rather sharp boundary in sunspots between the umbra and the penumbra. Both regions exhibit magnetoconvection, which manifests in penumbral filaments in the penumbra and in umbral dots in the umbra.
Aims: Here we compare the physical properties of umbral dots and penumbral filaments. Our goal is to understand how the properties of these convective features change across the boundary between the umbra and the penumbra and how this is related to the rapid increase in brightness at the umbra-penumbra boundary.
Methods: We derived ensemble averages of the physical properties of different types of convective features based on observations of two sunspots with Hinode.
Results: There are strong similarities between the convective features in the outer parts of the umbra and the ones in the penumbra, with most physical parameters being smooth and continuous functions of the length of the features.
Conclusions: Our results indicate that the transition in brightness from the umbra to the penumbra is solely caused by an increased effectiveness of magnetoconvection within individual convective cells. There is no significant difference in the number density of convective elements between the outer umbra and the inner penumbra. Penumbral filaments exhibit a larger area and a higher brightness compared to umbral dots. It is still unclear how exactly the underlying magnetic field causes the increase in the size and brightness of convective features in the penumbra. Title: Correction of atmospheric stray light in restored slit spectra Authors: Saranathan, S.; van Noort, M.; Solanki, S. K. Bibcode: 2021A&A...653A..17S Altcode: 2021arXiv211202648S Context. A long-standing issue in solar ground-based observations has been the contamination of data due to stray light, which is particularly relevant in inversions of spectropolarimetric data.
Aims: We aim to build on a statistical method of correcting stray-light contamination due to residual high-order aberrations and apply it to ground-based slit spectra.
Methods: The observations were obtained at the Swedish Solar Telescope, and restored using the multi-frame blind deconvolution restoration procedure. Using the statistical properties of seeing, we created artificially degraded synthetic images generated from magneto-hydrodynamic simulations. We then compared the synthetic data with the observations to derive estimates of the amount of the residual stray light in the observations. In the final step, the slit spectra were deconvolved with a stray-light point spread function to remove the residual stray light from the observations.
Results: The RMS granulation contrasts of the deconvolved spectra were found to increase to approximately 12.5%, from 9%. Spectral lines, on average, were found to become deeper in the granules and shallower in the inter-granular lanes, indicating systematic changes to gradients in temperature. The deconvolution was also found to increase the redshifts and blueshifts of spectral lines, suggesting that the velocities of granulation in the solar photosphere are higher than had previously been observed. Title: How wrong are the results of inverting Fe I lines when NLTE and 3D radiative transfer effects are ignored? Authors: Narayanamurthy, S.; Holzreuter, R.; van Noort, M.; Solanki, S. Bibcode: 2021AAS...23811305N Altcode: The Fe I 6301.5 Å and 6302.5 Å lines are widely used to probe the solar photosphere. They are known to be affected by the non-local thermodynamic equilibrium (NLTE) conditions due to the ultraviolet overionisation of iron atoms in the solar atmosphere. This leads to deviations in their level populations based on Saha-Boltzmann statistics. When inverting their Stokes profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE. In this work, we discuss how the routinely employed LTE inversion introduces errors in the derived atmospheric quantities. We show that when the NLTE effects are neglected, these errors can be as high as 13% in temperature, and in line-of-sight velocity and magnetic field strength the errors can even exceed 50%. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region with strong vertical gradients in the atmosphere. Similarly, strong horizontal gradients in temperature introduce 3D effects in these lines. We find that errors due to neglecting the 3D effects are more localised and are lower than 5% in temperature, and lower than 20% in both velocity and magnetic field strength. The NLTE and 3D effects are found to persist when the Stokes profiles are spatially and spectrally degraded to the resolution of the Swedish Solar Telescope (SST) or Daniel K. Inouye Solar Telescope (DKIST). Our findings have wide-ranging consequences since many results derived in solar physics are based on inversions of these Fe I lines carried out in LTE. Title: Nonequilibrium Equation of State in Stellar Atmospheres Authors: Anusha, L. S.; van Noort, M.; Cameron, R. H. Bibcode: 2021ApJ...911...71A Altcode: 2021arXiv210413650A In the stellar chromospheres, radiative energy transport is dominated by only the strongest spectral lines. For these lines, the approximation of local thermodynamic equilibrium (LTE) is known to be very inaccurate, and a state of equilibrium cannot be assumed in general. To calculate the radiative energy transport under these conditions, the population evolution equation must be evaluated explicitly, including all time-dependent terms. We develop a numerical method to solve the evolution equation for the atomic-level populations in a time-implicit way, keeping all time-dependent terms to first order. We show that the linear approximation of the time dependence of the populations can handle very large time steps without losing accuracy. We reproduce the benchmark solutions from earlier, well-established works in terms of non-LTE kinetic equilibrium solutions and typical ionization/recombination timescales in the solar chromosphere. Title: The influence of NLTE effects in Fe I lines on an inverted atmosphere. II. 6301 Å and 6302 Å lines formed in 3D NLTE Authors: Smitha, H. N.; Holzreuter, R.; van Noort, M.; Solanki, S. K. Bibcode: 2021A&A...647A..46S Altcode: 2021arXiv210100506S Context. This paper forms the second part of our study of how neglecting non-local thermodynamic equilibrium (NLTE) conditions in the formation of Fe I 6301.5 Å and the 6302.5 Å lines affects the atmosphere that is obtained by inverting the Stokes profiles of these lines in LTE. The main cause of NLTE effects in these lines is the line opacity deficit that is due to the excess ionisation of Fe I atoms by ultraviolet (UV) photons in the Sun.
Aims: In the first paper, these photospheric lines were assumed to have formed in 1D NLTE and the effects of horizontal radiation transfer (RT) were neglected. In the present paper, the iron lines are computed by solving the RT in 3D. We investigate the effect of horizontal RT on the inverted atmosphere and how it can enhance or reduce the errors that are due to neglecting 1D NLTE effects.
Methods: The Stokes profiles of the iron lines were computed in LTE, 1D NLTE, and 3D NLTE. They were all inverted using an LTE inversion code. The atmosphere from the inversion of LTE profiles was taken as the reference model. The atmospheres from the inversion of 1D NLTE profiles (testmodel-1D) and 3D NLTE profiles (testmodel-3D) were compared with it. Differences between reference and testmodels were analysed and correspondingly attributed to NLTE and 3D effects.
Results: The effects of horizontal RT are evident in regions surrounded by strong horizontal temperature gradients. That is, along the granule boundaries, regions surrounding magnetic elements, and its boundaries with intergranular lanes. In some regions, the 3D effects enhance the 1D NLTE effects, and in some, they weaken these effects. In the small region analysed in this paper, the errors due to neglecting the 3D effects are lower than 5% in temperature. In most of the pixels, the errors are lower than 20% in both velocity and magnetic field strength. These errors also persist when the Stokes profiles are spatially and spectrally degraded to the resolution of the Swedish Solar Telescope (SST) or Daniel K. Inouye Solar Telescope (DKIST).
Conclusions: Neglecting horizontal RT introduces errors not only in the derived temperature, but also in other atmospheric parameters. The error sizes depend on the strength of the local horizontal temperature gradients. Compared to the 1D NLTE effect, the 3D effects are more localised in specific regions in the atmosphere and are weaker overall. Title: Non-equilibrium equation-of-state in stellar atmospheres Authors: Lokanathapura Seetharamabhasari, Anusha; Cameron, Robert; Van Noort, Michiel Bibcode: 2021cosp...43E.985L Altcode: In the stellar atmospheres, radiative energy transport is dominated by only the strongest spectral lines. For these lines, the approximation of local thermo-dynamic equilibrium (LTE) is known to be very inaccurate, and a state of equilibrium cannot be assumed in general. Therefore to understand the structure and dynamics of stellar atmospheres through evolving magneto-hydro-dynamic equations, one needs a non-equilibrium equation of state. To calculate the radiative energy transport under these conditions, the population evolution equation must be evaluated including all time dependent terms. To this end, we have developed a new numerical method to solve the non-LTE non-equilibrium radiative transfer problem. We solve evolution equation for the atomic level populations in a time-implicit way, keeping all time dependent terms to first order. We have tested our method by reproducing earlier works, namely, a) determining chromosperic time-scales of ionization/recombination, b) showing that our non-equilibrium solver evolves to the statistical equilibrium solution obtained from an independent non-LTE spectral synthesis code. In this presentation, I will describe the method, and discuss equilibrium solutions. Title: Power spectrum of turbulent convection in the solar photosphere Authors: Yelles Chaouche, L.; Cameron, R. H.; Solanki, S. K.; Riethmüller, T. L.; Anusha, L. S.; Witzke, V.; Shapiro, A. I.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2020A&A...644A..44Y Altcode: 2020arXiv201009037Y The solar photosphere provides us with a laboratory for understanding turbulence in a layer where the fundamental processes of transport vary rapidly and a strongly superadiabatic region lies very closely to a subadiabatic layer. Our tools for probing the turbulence are high-resolution spectropolarimetric observations such as have recently been obtained with the two balloon-borne SUNRISE missions, and numerical simulations. Our aim is to study photospheric turbulence with the help of Fourier power spectra that we compute from observations and simulations. We also attempt to explain some properties of the photospheric overshooting flow with the help of its governing equations and simulations. We find that quiet-Sun observations and smeared simulations are consistent with each other and exhibit a power-law behavior in the subgranular range of their Doppler velocity power spectra with a power-law index of ≈ - 2. The unsmeared simulations exhibit a power law that extends over the full range between the integral and Taylor scales with a power-law index of ≈ - 2.25. The smearing, reminiscent of observational conditions, considerably reduces the extent of the power-law-like portion of the power spectra. This suggests that the limited spatial resolution in some observations might eventually result in larger uncertainties in the estimation of the power-law indices. The simulated vertical velocity power spectra as a function of height show a rapid change in the power-law index (at the subgranular range) from roughly the optical depth unity layer, that is, the solar surface, to 300 km above it. We propose that the cause of the steepening of the power-law index is the transition from a super- to a subadiabatic region, in which the dominant source of motions is overshooting convection. A scale-dependent transport of the vertical momentum occurs. At smaller scales, the vertical momentum is more efficiently transported sideways than at larger scales. This results in less vertical velocity power transported upward at small scales than at larger scales and produces a progressively steeper vertical velocity power law below 180 km. Above this height, the gravity work progressively gains importance at all relevant scales, making the atmosphere progressively more hydrostatic and resulting in a gradually less steep power law. Radiative heating and cooling of the plasma is shown to play a dominant role in the plasma energetics in this region, which is important in terms of nonadiabatic damping of the convective motions. Title: No universal connection between the vertical magnetic field and the umbra-penumbra boundary in sunspots Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2020A&A...639A.106L Altcode: 2020arXiv200602346L Context. It has been reported that the boundary between the umbra and the penumbra of sunspots occurs at a canonical value of the strength of the vertical magnetic field, independently of the size of the spot. This critical field strength is interpreted to be the threshold for the onset of magnetoconvection.
Aims: Here we investigate the reasons why this criterion, also called the Jurčák criterion in the literature, does not always identify the boundary between the umbra and the penumbra.
Methods: We performed a statistical analysis of 23 sunspots observed with Hinode/SOT. We compared the properties of the continuum intensity and the vertical magnetic field between filaments and spines and how they vary between spots of different sizes.
Results: We find that the inner boundary of the penumbra is not related to a universal value of the vertical magnetic field. The properties of spines and filaments vary between spots of different sizes. Both components are darker in larger spots and the spines exhibit a stronger vertical magnetic field. These variations of the properties of filaments and spines with the spot size are also the reason for the reported invariance in the averaged vertical magnetic field at 50% of the mean continuum intensity.
Conclusions: The formation of filaments and the onset of magnetoconvection are not related to a canonical value of the strength of the vertical magnetic field. The seemingly unique magnetic field strength is rather an effect of the filling factor of spines and penumbral filaments. Title: Detection of the Strongest Magnetic Field in a Sunspot Light Bridge Authors: Castellanos Durán, J. S.; Lagg, Andreas; Solanki, Sami K.; van Noort, Michiel Bibcode: 2020ApJ...895..129C Altcode: 2020arXiv200312078C; 2020ApJ...895..129D Traditionally, the strongest magnetic fields on the Sun have been measured in sunspot umbrae. More recently, however, much stronger fields have been measured at the ends of penumbral filaments carrying the Evershed and counter-Evershed flows. Superstrong fields have also been reported within a light bridge separating two umbrae of opposite polarities. We aim to accurately determine the strengths of the strongest fields in a light bridge using an advanced inversion technique and to investigate their detailed structure. We analyze observations from the spectropolarimeter on board the Hinode spacecraft of the active region AR 11967. The thermodynamic and magnetic configurations are obtained by inverting the Stokes profiles using an inversion scheme that allows multiple height nodes. Both the traditional 1D inversion technique and the so-called 2D coupled inversions, which take into account the point-spread function of the Hinode telescope, are used. We find a compact structure with an area of 32.7 arcsec2 within a bipolar light bridge with field strengths exceeding 5 kG, confirming the strong fields in this light bridge reported in the literature. Two regions associated with downflows of ∼5 km s-1 harbor field strengths larger than 6.5 kG, covering a total area of 2.97 arcsec2. The maximum field strength found is 8.2 kG, which is the largest ever observed field in a bipolar light bridge up to now. Title: Solar Disk Center Shows Scattering Polarization in the Sr I 4607 Å Line Authors: Zeuner, Franziska; Manso Sainz, Rafael; Feller, Alex; van Noort, Michiel; Solanki, Sami K.; Iglesias, Francisco A.; Reardon, Kevin; Martínez Pillet, Valentín Bibcode: 2020ApJ...893L..44Z Altcode: 2020arXiv200403679Z Magnetic fields in turbulent, convective high-β plasma naturally develop highly tangled and complex topologies - the solar photosphere being the paradigmatic example. These fields are mostly undetectable by standard diagnostic techniques with finite spatio-temporal resolution due to cancellations of Zeeman polarization signals. Observations of resonance scattering polarization have been considered to overcome these problems. But up to now, observations of scattering polarization lack the necessary combination of high sensitivity and high spatial resolution in order to directly infer the turbulent magnetic structure at the resolution limit of solar telescopes. Here, we report the detection of clear spatial structuring of scattering polarization in a magnetically quiet solar region at disk center in the Sr I 4607 Å spectral line on granular scales, confirming theoretical expectations. We find that the linear polarization presents a strong spatial correlation with the local quadrupole of the radiation field. The result indicates that polarization survives the dynamic and turbulent magnetic environment of the middle photosphere and is thereby usable for spatially resolved Hanle observations. This is an important step toward the long-sought goal of directly observing turbulent solar magnetic fields at the resolution limit and investigating their spatial structure. Title: Connecting the Wilson depression to the magnetic field of sunspots Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2020A&A...635A.202L Altcode: 2020arXiv200207484L Context. In sunspots, the geometric height of continuum optical depth unity is depressed compared to the quiet Sun. This so-called Wilson depression is caused by the Lorentz force of the strong magnetic field inside the spots. However, it is not understood in detail yet how the Wilson depression is related to the strength and geometry of the magnetic field or to other properties of the sunspot.
Aims: We aim to study the dependence of the Wilson depression on the properties of the magnetic field of the sunspots and how exactly the magnetic field contributes to balancing the Wilson depression with respect to the gas pressure of the surroundings of the spots.
Methods: Our study is based on 24 spectropolarimetric scans of 12 individual sunspots performed with Hinode. We derived the Wilson depression for each spot using both a recently developed method that is based on minimizing the divergence of the magnetic field and an approach that was developed earlier, which enforces an equilibrium between the gas pressure and the magnetic pressure inside the spot and the gas pressure in the quiet Sun, thus neglecting the influence of the curvature force. We then performed a statistical analysis by comparing the Wilson depression resulting from the two techniques with each other and by relating them to various parameters of the sunspots, such as their size or the strength of the magnetic field.
Results: We find that the Wilson depression becomes larger for spots with a stronger magnetic field, but not as much as one would expect from the increased magnetic pressure. This suggests that the curvature integral provides an important contribution to the Wilson depression, particularly for spots with a weak magnetic field. Our results indicate that the geometry of the magnetic field in the penumbra is different between spots with different strengths of the average umbral magnetic field. Title: The influence of NLTE effects in Fe I lines on an inverted atmosphere. I. 6301 Å and 6302 Å lines formed in 1D NLTE Authors: Smitha, H. N.; Holzreuter, R.; van Noort, M.; Solanki, S. K. Bibcode: 2020A&A...633A.157S Altcode: 2019arXiv191207007S Context. Ultraviolet overionisation of iron atoms in the solar atmosphere leads to deviations in their level populations based on Saha-Boltzmann statistics. This causes their line profiles to form in non-local thermodynamic equilibrium (NLTE) conditions. When inverting such profiles to determine atmospheric parameters, the NLTE effects are often neglected and other quantities are tweaked to compensate for deviations from the LTE.
Aims: We investigate how the routinely employed LTE inversion of iron lines formed in NLTE underestimates or overestimates atmospheric quantities, such as temperature (T), line-of-sight velocity (vLOS), magnetic field strength (B), and inclination (γ) while the earlier papers have focused mainly on T. Our findings has wide-ranging consequences since many results derived in solar physics are based on inversions of Fe I lines carried out in LTE.
Methods: We synthesized the Stokes profiles of Fe I 6301.5 Å and 6302.5 Å lines in both LTE and NLTE using a snapshot of a 3D magnetohydrodynamic simulation. The profiles were then inverted in LTE. We considered the atmosphere inferred from the inversion of LTE profiles as the fiducial model and compared it to the atmosphere resulting from the inversion of NLTE profiles. The observed differences have been attributed to NLTE effects.
Results: Neglecting the NLTE effects introduces errors in the inverted atmosphere. While the errors in T can go up to 13%, in vLOS and B, the errors can go as high as 50% or above. We find these errors to be present at all three inversion nodes. Importantly, they survive degradation from the spatial averaging of the profiles.
Conclusions: We provide an overview of how neglecting NLTE effects influences the values of T, vLOS, B, and γ that are determined by inverting the Fe I 6300 Å line pair, as observed, for example, by Hinode/SOT/SP. Errors are found at the sites of granules, intergranular lanes, magnetic elements, and basically in every region susceptible to NLTE effects. For an accurate determination of the atmospheric quantities and their stratification, it is, therefore, important to take the NLTE effects into account. Title: Superstrong photospheric magnetic fields in sunspot penumbrae Authors: Siu-Tapia, A.; Lagg, A.; van Noort, M.; Rempel, M.; Solanki, S. K. Bibcode: 2019A&A...631A..99S Altcode: 2019arXiv190913619S Context. Recently, there have been some reports of unusually strong photospheric magnetic fields (which can reach values of over 7 kG) inferred from Hinode SOT/SP sunspot observations within penumbral regions. These superstrong penumbral fields are even larger than the strongest umbral fields on record and appear to be associated with supersonic downflows. The finding of such fields has been controversial since they seem to show up only when spatially coupled inversions are performed.
Aims: Here, we investigate and discuss the reliability of those findings by studying in detail observed spectra associated with particularly strong magnetic fields at the inner edge of the penumbra of active region 10930.
Methods: We applied classical diagnostic methods and various inversions with different model atmospheres to the observed Stokes profiles in two selected pixels with superstrong magnetic fields, and compared the results with a magnetohydrodynamic simulation of a sunspot whose penumbra contains localized regions with strong fields (nearly 5 kG at τ = 1) associated with supersonic downflows.
Results: The different inversions provide different results: while the SPINOR 2D inversions consider a height-dependent single-component model and return B > 7 kG and supersonic positive vLOS (corresponding to a counter-Evershed flow), height-dependent two-component inversions suggest the presence of an umbral component (almost at rest) with field strengths ∼4 - 4.2 kG and a penumbral component with vLOS ∼ 16 - 18 km s-1 and field strengths up to ∼5.8 kG. Likewise, height-independent two-component inversions find a solution for an umbral component and a strongly redshifted (vLOS ∼ 15 - 17 km s-1) penumbral component with B ∼ 4 kG. According to a Bayesian information criterion, the inversions providing a better balance between the quality of the fits and the number of free parameters considered by the models are the height-independent two-component inversions, but they lie only slightly above the SPINOR 2D inversions. Since it is expected that the physical parameters all display considerable gradients with height, as supported by magnetohydrodynamic (MHD) sunspot simulations, the SPINOR 2D inversions are the preferred ones.
Conclusions: According to the MHD sunspot simulation analyzed here, the presence of counter-Evershed flows in the photospheric penumbra can lead to the necessary conditions for the observation of ∼5 kG fields at the inner penumbra. Although a definite conclusion about the potential existence of fields in excess of 7 kG cannot be given, their nature could be explained (based on the simulation results) as the consequence of the extreme dynamical effects introduced by highly supersonic counter-Evershed flows (vLOS > 10 km s-1 and up to ∼30 km s-1 according to SPINOR 2D). The latter are much faster and more compressive downflows than those found in the MHD simulations and therefore could lead to field intensification up to considerably stronger fields. Also, a lower gas density would lead to a deeper depression of the τ = 1 surface, making possible the observation of deeper-lying stronger fields. The superstrong magnetic fields are expected to be nearly force-free, meaning that they can attain much larger strengths than expected when considering only balance between magnetic pressure and the local gas pressure. Title: A comparison between solar plage and network properties Authors: Buehler, D.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2019A&A...630A..86B Altcode: 2019arXiv190807464B
Aims: We compare the properties of kG magnetic structures in the solar network and in active region plage at high spatial resolution.
Methods: Our analysis used six SP scans of the solar disc centre aboard Hinode SOT and inverted the obtained spectra of the photospheric 6302 Å line pair using the 2D SPINOR code.
Results: Photospheric magnetic field concentrations in network and plage areas are on average 1.5 kG strong with inclinations of 10° -20°, and have < 400 m s-1 internal and 2-3 km s-1 external downflows. At the disc centre, the continuum intensity of magnetic field concentrations in the network are on average 10% brighter than the mean quiet Sun, whilst their plage counterparts are 3% darker. A more detailed analysis revealed that all sizes of individual kG patches in the network have 150 G higher field strengths on average, 5% higher continuum contrasts, and 800 m s-1 faster surrounding downflows than similarly sized patches in the plage. The speed of the surrounding downflows also correlates with the patch area, and patches containing pores can produce supersonic flows exceeding 11 km s-1 in individual pixels. Furthermore, the magnetic canopies of kG patches are on average 9° more horizontal in the plage compared to the network.
Conclusions: Most of the differences between the network and plage are due to their different patch size distributions, but the intrinsic differences between similarly sized patches likely results from the modification of the convection photospheric convection with increasing amounts of magnetic flux. Title: Response Functions for NLTE Lines Authors: Milić, I.; van Noort, M. Bibcode: 2019ASPC..526..179M Altcode: Response functions quantify the sensitivity of the emergent polarized spectrum to perturbations in the atmospheric quantities. They are an important diagnostics tool and an essential ingredient of the so-called inversion codes, widely used in solar spectropolarimetry. The computation of response functions for spectral lines formed out of local thermodynamic equilibrium is complicated because of strong spatial and non-linear couplings of the atomic populations. We have recently proposed a novel, analytic approach for the computation of NLTE response functions, and in this short contribution we discuss the possibilities of computing response functions for scattering polarization. We explicitly show the procedure for a two level atom line (normal Zeeman triplet), and discuss the "formation heights" of intensity and scattering polarization for a prototype line. Title: Recent advancements in the EST project Authors: Jurčák, Jan; Collados, Manuel; Leenaarts, Jorrit; van Noort, Michiel; Schlichenmaier, Rolf Bibcode: 2019AdSpR..63.1389J Altcode: 2018arXiv181100851J The European Solar Telescope (EST) is a project of a new-generation solar telescope. It has a large aperture of 4 m, which is necessary for achieving high spatial and temporal resolution. The high polarimetric sensitivity of the EST will allow to measure the magnetic field in the solar atmosphere with unprecedented precision. Here, we summarise the recent advancements in the realisation of the EST project regarding the hardware development and the refinement of the science requirements. Title: Measuring the Wilson depression of sunspots using the divergence-free condition of the magnetic field vector Authors: Löptien, B.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2018A&A...619A..42L Altcode: 2018arXiv180806867L Context. The Wilson depression is the difference in geometric height of unit continuum optical depth between the sunspot umbra and the quiet Sun. Measuring the Wilson depression is important for understanding the geometry of sunspots. Current methods suffer from systematic effects or need to make assumptions on the geometry of the magnetic field. This leads to large systematic uncertainties of the derived Wilson depressions.
Aims: We aim to develop a robust method for deriving the Wilson depression that only requires the information about the magnetic field that is accessible from spectropolarimetry, and that does not rely on assumptions on the geometry of sunspots or on their magnetic field.
Methods: Our method is based on minimizing the divergence of the magnetic field vector derived from spectropolarimetric observations. We have focused on large spatial scales only in order to reduce the number of free parameters.
Results: We tested the performance of our method using synthetic Hinode data derived from two sunspot simulations. We find that the maximum and the umbral averaged Wilson depression for both spots determined with our method typically lies within 100 km of the true value obtained from the simulations. In addition, we applied the method to Hinode observations of a sunspot. The derived Wilson depression (∼600 km) is consistent with results typically obtained from the Wilson effect. We also find that the Wilson depression obtained from using horizontal force balance gives 110-180 km smaller Wilson depressions than both, what we find and what we deduce directly from the simulations. This suggests that the magnetic pressure and the magnetic curvature force contribute to the Wilson depression by a similar amount. Title: Spectropolarimetric NLTE inversion code SNAPI Authors: Milić, I.; van Noort, M. Bibcode: 2018A&A...617A..24M Altcode: 2018arXiv180608134M Context. Inversion codes are computer programs that fit a model atmosphere to the observed Stokes spectra, thus retrieving the relevant atmospheric parameters. The rising interest in the solar chromosphere, where spectral lines are formed by scattering, requires developing, testing, and comparing new non-local thermal equilibrium (NLTE) inversion codes.
Aims: We present a new NLTE inversion code that is based on the analytical computation of the response functions. We named the code SNAPI, which is short for spectropolarimetic NLTE analytically powered inversion.
Methods: SNAPI inverts full Stokes spectrum in order to obtain a depth-dependent stratification of the temperature, velocity, and the magnetic field vector. It is based on the so-called node approach, where atmospheric parameters are free to vary in several fixed points in the atmosphere, and are assumed to behave as splines in between. We describe the inversion approach in general and the specific choices we have made in the implementation.
Results: We test the performance on one academic problem and on two interesting NLTE examples, the Ca II 8542 and Na I D spectral lines. The code is found to have excellent convergence properties and outperforms a finite-difference based code in this specific implementation by at least a factor of three. We invert synthetic observations of Na lines from a small part of a simulated solar atmosphere and conclude that the Na lines reliably retrieve the magnetic field and velocity in the range -3 < logτ < -0.5. Title: Radiative Diagnostics in the Solar Photosphere and Chromosphere Authors: de la Cruz Rodríguez, J.; van Noort, M. Bibcode: 2018smf..book..109D Altcode: No abstract at ADS Title: Image restoration of solar spectra Authors: van Noort, M. Bibcode: 2017A&A...608A..76V Altcode: 2017arXiv171109629V Context. When recording spectra from the ground, atmospheric turbulence causes degradation of the spatial resolution.
Aims: We present a data reduction method that restores the spatial resolution of the spectra to their undegraded state.
Methods: By assuming that the point spread function (PSF) estimated from a strictly synchronized, broadband slit-jaw camera is the same as the PSF that spatially degraded the spectra, we can quantify what linear combination of undegraded spectra is present in each degraded data point.
Results: The set of equations obtained in this way is found to be generally well-conditioned and sufficiently diagonal to be solved using an iterative linear solver. The resulting solution has regained a spatial resolution comparable to that of the restored slit-jaw images.
Conclusions: We have developed a new image restoration method for the restoration of ground-based spectral data over a large field of view. The method builds on the PSF information recovered by the MOMFBD code and typically reaches a spatial resolution comparable to that of the broadband slit-jaw images used to recover the PSF. Title: Normal and counter Evershed flows in the photospheric penumbra of a sunspot. SPINOR 2D inversions of Hinode-SOT/SP observations Authors: Siu-Tapia, A.; Lagg, A.; Solanki, S. K.; van Noort, M.; Jurčák, J. Bibcode: 2017A&A...607A..36S Altcode: 2017arXiv170907386S Context. The Evershed effect, a nearly horizontal outflow of material seen in the penumbrae of sunspots in the photospheric layers, is a common characteristic of well-developed penumbrae, but is still not well understood. Even less is known about photospheric horizontal inflows in the penumbra, also known as counter Evershed flows.
Aims: Here we present a rare feature observed in the penumbra of the main sunspot of AR NOAA 10930. This spot displays the normal Evershed outflow in most of the penumbra, but harbors a fast photospheric inflow of material over a large sector of the disk-center penumbra. We investigate the driving forces of both, the normal and the counter Evershed flows.
Methods: We invert the spectropolarimetric data from Hinode SOT/SP using the spatially coupled version of the SPINOR inversion code, which allows us to derive height-dependent maps of the relevant physical parameters in the sunspot. These maps show considerable fine structure. Similarities and differences between the normal Evershed outflow and the counter Evershed flow are investigated.
Results: In both the normal and the counter Evershed flows, the material flows from regions with field strengths of the order of 1.5-2 kG to regions with stronger fields. The sources and sinks of both penumbral flows display opposite field polarities, with the sinks (tails of filaments) harboring local enhancements in temperature, which are nonetheless colder than their sources (heads of filaments).
Conclusions: The anti-correlation of the gradients in the temperature and magnetic pressure between the endpoints of the filaments from the two distinct penumbral regions is compatible with both the convective driver and the siphon flow scenarios. A geometrical scale of the parameters is necessary to determine which is the dominant force driving the flows. Title: The Maximum Entropy Limit of Small-scale Magnetic Field Fluctuations in the Quiet Sun Authors: Gorobets, A. Y.; Berdyugina, S. V.; Riethmüller, T. L.; Blanco Rodríguez, J.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..233....5G Altcode: 2017arXiv171008361G The observed magnetic field on the solar surface is characterized by a very complex spatial and temporal behavior. Although feature-tracking algorithms have allowed us to deepen our understanding of this behavior, subjectivity plays an important role in the identification and tracking of such features. In this paper, we continue studies of the temporal stochasticity of the magnetic field on the solar surface without relying either on the concept of magnetic features or on subjective assumptions about their identification and interaction. We propose a data analysis method to quantify fluctuations of the line-of-sight magnetic field by means of reducing the temporal field’s evolution to the regular Markov process. We build a representative model of fluctuations converging to the unique stationary (equilibrium) distribution in the long time limit with maximum entropy. We obtained different rates of convergence to the equilibrium at fixed noise cutoff for two sets of data. This indicates a strong influence of the data spatial resolution and mixing-polarity fluctuations on the relaxation process. The analysis is applied to observations of magnetic fields of the relatively quiet areas around an active region carried out during the second flight of the Sunrise/IMaX and quiet Sun areas at the disk center from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory satellite. Title: Granular cells in the presence of magnetic field Authors: Jurčák, J.; Lemmerer, B.; van Noort, M. Bibcode: 2017IAUS..327...34J Altcode: We present a statistical study of the dependencies of the shapes and sizes of the photospheric convective cells on the magnetic field properties. This analysis is based on a 2.5 hour long SST observations of active region NOAA 11768. We have blue continuum images taken with a cadence of 5.6 sec that are used for segmentation of individual granules and 270 maps of spectropolarimetric CRISP data allowing us to determine the properties of the magnetic field along with the line-of-sight velocities. The sizes and shapes of the granular cells are dependent on the the magnetic field strength, where the granules tend to be smaller in regions with stronger magnetic field. In the presence of highly inclined magnetic fields, the eccentricity of granules is high and we do not observe symmetric granules in these regions. The mean up-flow velocities in granules as well as the granules intensities decrease with increasing magnetic field strength. Title: Radiative Diagnostics in the Solar Photosphere and Chromosphere Authors: de la Cruz Rodríguez, J.; van Noort, M. Bibcode: 2017SSRv..210..109D Altcode: 2016arXiv160908324D; 2016SSRv..tmp...73D 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. Title: Line response functions in nonlocal thermodynamic equilibrium. Isotropic case Authors: Milić, I.; van Noort, M. Bibcode: 2017A&A...601A.100M Altcode: Context. Response functions provide us with a quantitative measure of sensitivity of the emergent spectrum to perturbations in the solar atmosphere and are thus the method of choice for interpreting spectropolarimetric observations. For the lines formed in the solar chromosphere, it is necessary to compute these responses taking into account nonlocal thermodynamic equilibrium (NLTE) effects.
Aims: We show how to analytically compute the response of the level populations in NLTE to a change of a given physical quantity at a given depth in the atmosphere. These responses are then used to compute opacity and emissivity responses, which are then propagated to obtain the response of the emergent intensity.
Methods: Our method is based on the derivative of the rate equations, where we explicitly incorporate spatial coupling in the radiative rate terms. After considering and collecting all interdependencies, the problem reduces to a linear system of equations with a dimension equal to the product of the number of spatial points and the number of energy levels.
Results: We compare analytically computed response functions with those obtained using a finite difference approach and find very good agreement. In addition, a more accurate way of propagating opacity and emissivity perturbations through the numerical solution of the radiative transfer equation was developed.
Conclusions: This method allows for the fast evaluation of the response of the emergent spectrum to perturbations of a given quantity at a given depth, and thus is a significant step towards more efficient NLTE inversions. Title: Erratum: Morphological Properties of Slender CaII H Fibrils Observed by sunrise II (ApJS 229, 1, 6) Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.; van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W. Bibcode: 2017ApJS..230...11G Altcode: No abstract at ADS Title: Slender Ca II H Fibrils Mapping Magnetic Fields in the Low Solar Chromosphere Authors: Jafarzadeh, S.; Rutten, R. J.; Solanki, S. K.; Wiegelmann, T.; Riethmüller, T. L.; van Noort, M.; Szydlarski, M.; Blanco Rodríguez, J.; Barthol, P.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez, D.; Schmidt, W. Bibcode: 2017ApJS..229...11J Altcode: 2016arXiv161003104J A dense forest of slender bright fibrils near a small solar active region is seen in high-quality narrowband Ca II H images from the SuFI instrument onboard the Sunrise balloon-borne solar observatory. The orientation of these slender Ca II H fibrils (SCF) overlaps with the magnetic field configuration in the low solar chromosphere derived by magnetostatic extrapolation of the photospheric field observed with Sunrise/IMaX and SDO/HMI. In addition, many observed SCFs are qualitatively aligned with small-scale loops computed from a novel inversion approach based on best-fit numerical MHD simulation. Such loops are organized in canopy-like arches over quiet areas that differ in height depending on the field strength near their roots. Title: Magneto-static Modeling from Sunrise/IMaX: Application to an Active Region Observed with Sunrise II Authors: Wiegelmann, T.; Neukirch, T.; Nickeler, D. H.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229...18W Altcode: 2017arXiv170101458N; 2017arXiv170101458W Magneto-static models may overcome some of the issues facing force-free magnetic field extrapolations. So far they have seen limited use and have faced problems when applied to quiet-Sun data. Here we present a first application to an active region. We use solar vector magnetic field measurements gathered by the IMaX polarimeter during the flight of the Sunrise balloon-borne solar observatory in 2013 June as boundary conditions for a magneto-static model of the higher solar atmosphere above an active region. The IMaX data are embedded in active region vector magnetograms observed with SDO/HMI. This work continues our magneto-static extrapolation approach, which was applied earlier to a quiet-Sun region observed with Sunrise I. In an active region the signal-to-noise-ratio in the measured Stokes parameters is considerably higher than in the quiet-Sun and consequently the IMaX measurements of the horizontal photospheric magnetic field allow us to specify the free parameters of the model in a special class of linear magneto-static equilibria. The high spatial resolution of IMaX (110-130 km, pixel size 40 km) enables us to model the non-force-free layer between the photosphere and the mid-chromosphere vertically by about 50 grid points. In our approach we can incorporate some aspects of the mixed beta layer of photosphere and chromosphere, e.g., taking a finite Lorentz force into account, which was not possible with lower-resolution photospheric measurements in the past. The linear model does not, however, permit us to model intrinsic nonlinear structures like strongly localized electric currents. Title: The Second Flight of the Sunrise Balloon-borne Solar Observatory: Overview of Instrument Updates, the Flight, the Data, and First Results Authors: Solanki, S. K.; Riethmüller, T. L.; Barthol, P.; Danilovic, S.; Deutsch, W.; Doerr, H. -P.; Feller, A.; Gandorfer, A.; Germerott, D.; Gizon, L.; Grauf, B.; Heerlein, K.; Hirzberger, J.; Kolleck, M.; Lagg, A.; Meller, R.; Tomasch, G.; van Noort, M.; Blanco Rodríguez, J.; Gasent Blesa, J. L.; Balaguer Jiménez, M.; Del Toro Iniesta, J. C.; López Jiménez, A. C.; Orozco Suarez, D.; Berkefeld, T.; Halbgewachs, C.; Schmidt, W.; Álvarez-Herrero, A.; Sabau-Graziati, L.; Pérez Grande, I.; Martínez Pillet, V.; Card, G.; Centeno, R.; Knölker, M.; Lecinski, A. Bibcode: 2017ApJS..229....2S Altcode: 2017arXiv170101555S The Sunrise balloon-borne solar observatory, consisting of a 1 m aperture telescope that provides a stabilized image to a UV filter imager and an imaging vector polarimeter, carried out its second science flight in 2013 June. It provided observations of parts of active regions at high spatial resolution, including the first high-resolution images in the Mg II k line. The obtained data are of very high quality, with the best UV images reaching the diffraction limit of the telescope at 3000 Å after Multi-Frame Blind Deconvolution reconstruction accounting for phase-diversity information. Here a brief update is given of the instruments and the data reduction techniques, which includes an inversion of the polarimetric data. Mainly those aspects that evolved compared with the first flight are described. A tabular overview of the observations is given. In addition, an example time series of a part of the emerging active region NOAA AR 11768 observed relatively close to disk center is described and discussed in some detail. The observations cover the pores in the trailing polarity of the active region, as well as the polarity inversion line where flux emergence was ongoing and a small flare-like brightening occurred in the course of the time series. The pores are found to contain magnetic field strengths ranging up to 2500 G, and while large pores are clearly darker and cooler than the quiet Sun in all layers of the photosphere, the temperature and brightness of small pores approach or even exceed those of the quiet Sun in the upper photosphere. Title: A Tale of Two Emergences: Sunrise II Observations of Emergence Sites in a Solar Active Region Authors: Centeno, R.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Orozco Suárez, D.; Berkefeld, T.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229....3C Altcode: 2016arXiv161003531C In 2013 June, the two scientific instruments on board the second Sunrise mission witnessed, in detail, a small-scale magnetic flux emergence event as part of the birth of an active region. The Imaging Magnetograph Experiment (IMaX) recorded two small (∼ 5\prime\prime ) emerging flux patches in the polarized filtergrams of a photospheric Fe I spectral line. Meanwhile, the Sunrise Filter Imager (SuFI) captured the highly dynamic chromospheric response to the magnetic fields pushing their way through the lower solar atmosphere. The serendipitous capture of this event offers a closer look at the inner workings of active region emergence sites. In particular, it reveals in meticulous detail how the rising magnetic fields interact with the granulation as they push through the Sun’s surface, dragging photospheric plasma in their upward travel. The plasma that is burdening the rising field slides along the field lines, creating fast downflowing channels at the footpoints. The weight of this material anchors this field to the surface at semi-regular spatial intervals, shaping it in an undulatory fashion. Finally, magnetic reconnection enables the field to release itself from its photospheric anchors, allowing it to continue its voyage up to higher layers. This process releases energy that lights up the arch-filament systems and heats the surrounding chromosphere. Title: Photospheric Response to an Ellerman Bomb-like Event—An Analogy of Sunrise/IMaX Observations and MHD Simulations Authors: Danilovic, S.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229....5D Altcode: 2016arXiv160903817D Ellerman Bombs are signatures of magnetic reconnection, which is an important physical process in the solar atmosphere. How and where they occur is a subject of debate. In this paper, we analyze Sunrise/IMaX data, along with 3D MHD simulations that aim to reproduce the exact scenario proposed for the formation of these features. Although the observed event seems to be more dynamic and violent than the simulated one, simulations clearly confirm the basic scenario for the production of EBs. The simulations also reveal the full complexity of the underlying process. The simulated observations show that the Fe I 525.02 nm line gives no information on the height where reconnection takes place. It can only give clues about the heating in the aftermath of the reconnection. However, the information on the magnetic field vector and velocity at this spatial resolution is extremely valuable because it shows what numerical models miss and how they can be improved. Title: Transverse Oscillations in Slender Ca II H Fibrils Observed with Sunrise/SuFI Authors: Jafarzadeh, S.; Solanki, S. K.; Gafeira, R.; van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W. Bibcode: 2017ApJS..229....9J Altcode: 2016arXiv161007449J We present observations of transverse oscillations in slender Ca II H fibrils (SCFs) in the lower solar chromosphere. We use a 1 hr long time series of high- (spatial and temporal-) resolution seeing-free observations in a 1.1 Å wide passband covering the line core of Ca II H 3969 Å from the second flight of the Sunrise balloon-borne solar observatory. The entire field of view, spanning the polarity inversion line of an active region close to the solar disk center, is covered with bright, thin, and very dynamic fine structures. Our analysis reveals the prevalence of transverse waves in SCFs with median amplitudes and periods on the order of 2.4 ± 0.8 km s-1 and 83 ± 29 s, respectively (with standard deviations given as uncertainties). We find that the transverse waves often propagate along (parts of) the SCFs with median phase speeds of 9 ± 14 km s-1. While the propagation is only in one direction along the axis in some of the SCFs, propagating waves in both directions, as well as standing waves are also observed. The transverse oscillations are likely Alfvénic and are thought to be representative of magnetohydrodynamic kink waves. The wave propagation suggests that the rapid high-frequency transverse waves, often produced in the lower photosphere, can penetrate into the chromosphere with an estimated energy flux of ≈15 kW m-2. Characteristics of these waves differ from those reported for other fibrillar structures, which, however, were observed mainly in the upper solar chromosphere. Title: Kinematics of Magnetic Bright Features in the Solar Photosphere Authors: Jafarzadeh, S.; Solanki, S. K.; Cameron, R. H.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Martínez Pillet, V.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W.; van Noort, M. Bibcode: 2017ApJS..229....8J Altcode: 2016arXiv161007634J Convective flows are known as the prime means of transporting magnetic fields on the solar surface. Thus, small magnetic structures are good tracers of turbulent flows. We study the migration and dispersal of magnetic bright features (MBFs) in intergranular areas observed at high spatial resolution with Sunrise/IMaX. We describe the flux dispersal of individual MBFs as a diffusion process whose parameters are computed for various areas in the quiet-Sun and the vicinity of active regions from seeing-free data. We find that magnetic concentrations are best described as random walkers close to network areas (diffusion index, γ =1.0), travelers with constant speeds over a supergranule (γ =1.9{--}2.0), and decelerating movers in the vicinity of flux emergence and/or within active regions (γ =1.4{--}1.5). The three types of regions host MBFs with mean diffusion coefficients of 130 km2 s-1, 80-90 km2 s-1, and 25-70 km2 s-1, respectively. The MBFs in these three types of regions are found to display a distinct kinematic behavior at a confidence level in excess of 95%. Title: Spectropolarimetric Evidence for a Siphon Flow along an Emerging Magnetic Flux Tube Authors: Requerey, Iker S.; Ruiz Cobo, B.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Blanco Rodríguez, J.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229...15R Altcode: 2016arXiv161106732R We study the dynamics and topology of an emerging magnetic flux concentration using high spatial resolution spectropolarimetric data acquired with the Imaging Magnetograph eXperiment on board the sunrise balloon-borne solar observatory. We obtain the full vector magnetic field and the line of sight (LOS) velocity through inversions of the Fe I line at 525.02 nm with the SPINOR code. The derived vector magnetic field is used to trace magnetic field lines. Two magnetic flux concentrations with different polarities and LOS velocities are found to be connected by a group of arch-shaped magnetic field lines. The positive polarity footpoint is weaker (1100 G) and displays an upflow, while the negative polarity footpoint is stronger (2200 G) and shows a downflow. This configuration is naturally interpreted as a siphon flow along an arched magnetic flux tube. Title: Morphological Properties of Slender Ca II H Fibrils Observed by SUNRISE II Authors: Gafeira, R.; Lagg, A.; Solanki, S. K.; Jafarzadeh, S.; van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W. Bibcode: 2017ApJS..229....6G Altcode: 2016arXiv161200319G We use seeing-free high spatial resolution Ca II H data obtained by the SUNRISE observatory to determine properties of slender fibrils in the lower solar chromosphere. In this work we use intensity images taken with the SuFI instrument in the Ca II H line during the second scientific flight of the SUNRISE observatory to identify and track elongated bright structures. After identification, we analyze theses structures to extract their morphological properties. We identify 598 slender Ca II H fibrils (SCFs) with an average width of around 180 km, length between 500 and 4000 km, average lifetime of ≈400 s, and average curvature of 0.002 arcsec-1. The maximum lifetime of the SCFs within our time series of 57 minutes is ≈2000 s. We discuss similarities and differences of the SCFs with other small-scale, chromospheric structures such as spicules of type I and II, or Ca II K fibrils. Title: A New MHD-assisted Stokes Inversion Technique Authors: Riethmüller, T. L.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229...16R Altcode: 2016arXiv161105175R We present a new method of Stokes inversion of spectropolarimetric data and evaluate it by taking the example of a Sunrise/IMaX observation. An archive of synthetic Stokes profiles is obtained by the spectral synthesis of state-of-the-art magnetohydrodynamics (MHD) simulations and a realistic degradation to the level of the observed data. The definition of a merit function allows the archive to be searched for the synthetic Stokes profiles that best match the observed profiles. In contrast to traditional Stokes inversion codes, which solve the Unno-Rachkovsky equations for the polarized radiative transfer numerically and fit the Stokes profiles iteratively, the new technique provides the full set of atmospheric parameters. This gives us the ability to start an MHD simulation that takes the inversion result as an initial condition. After a relaxation process of half an hour solar time we obtain physically consistent MHD data sets with a target similar to the observation. The new MHD simulation is used to repeat the method in a second iteration, which further improves the match between observation and simulation, resulting in a factor of 2.2 lower mean {χ }2 value. One advantage of the new technique is that it provides the physical parameters on a geometrical height scale. It constitutes a first step toward inversions that give results consistent with the MHD equations. Title: Oscillations on Width and Intensity of Slender Ca II H Fibrils from Sunrise/SuFI Authors: Gafeira, R.; Jafarzadeh, S.; Solanki, S. K.; Lagg, A.; van Noort, M.; Barthol, P.; Blanco Rodríguez, J.; del Toro Iniesta, J. C.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Knölker, M.; Orozco Suárez, D.; Riethmüller, T. L.; Schmidt, W. Bibcode: 2017ApJS..229....7G Altcode: 2017arXiv170102801G We report the detection of oscillations in slender Ca II H fibrils (SCFs) from high-resolution observations acquired with the Sunrise balloon-borne solar observatory. The SCFs show obvious oscillations in their intensity, but also their width. The oscillatory behaviors are investigated at several positions along the axes of the SCFs. A large majority of fibrils show signs of oscillations in intensity. Their periods and phase speeds are analyzed using a wavelet analysis. The width and intensity perturbations have overlapping distributions of the wave period. The obtained distributions have median values of the period of 32 ± 17 s and 36 ± 25 s, respectively. We find that the fluctuations of both parameters propagate in the SCFs with speeds of {11}-11+49 km s-1 and {15}-15+34 km s-1, respectively. Furthermore, the width and intensity oscillations have a strong tendency to be either in anti-phase or, to a smaller extent, in phase. This suggests that the oscillations of both parameters are caused by the same wave mode and that the waves are likely propagating. Taking all the evidence together, the most likely wave mode to explain all measurements and criteria is the fast sausage mode. Title: Solar Coronal Loops Associated with Small-scale Mixed Polarity Surface Magnetic Fields Authors: Chitta, L. P.; Peter, H.; Solanki, S. K.; Barthol, P.; Gandorfer, A.; Gizon, L.; Hirzberger, J.; Riethmüller, T. L.; van Noort, M.; Blanco Rodríguez, J.; Del Toro Iniesta, J. C.; Orozco Suárez, D.; Schmidt, W.; Martínez Pillet, V.; Knölker, M. Bibcode: 2017ApJS..229....4C Altcode: 2016arXiv161007484C How and where are coronal loops rooted in the solar lower atmosphere? The details of the magnetic environment and its evolution at the footpoints of coronal loops are crucial to understanding the processes of mass and energy supply to the solar corona. To address the above question, we use high-resolution line-of-sight magnetic field data from the Imaging Magnetograph eXperiment instrument on the Sunrise balloon-borne observatory and coronal observations from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory of an emerging active region. We find that the coronal loops are often rooted at the locations with minor small-scale but persistent opposite-polarity magnetic elements very close to the larger dominant polarity. These opposite-polarity small-scale elements continually interact with the dominant polarity underlying the coronal loop through flux cancellation. At these locations we detect small inverse Y-shaped jets in chromospheric Ca II H images obtained from the Sunrise Filter Imager during the flux cancellation. Our results indicate that magnetic flux cancellation and reconnection at the base of coronal loops due to mixed polarity fields might be a crucial feature for the supply of mass and energy into the corona. Title: Image restoration of polarimetric slit spectra Authors: van Noort, Michiel Bibcode: 2017psio.confE..90V Altcode: No abstract at ADS Title: Observed and simulated power spectra of kinetic and magnetic energy retrieved with 2D inversions Authors: Danilovic, S.; Rempel, M.; van Noort, M.; Cameron, R. Bibcode: 2016A&A...594A.103D Altcode: 2016arXiv160706242D Context. Information on the origin of internetwork magnetic field is hidden at the smallest spatial scales.
Aims: We try to retrieve the power spectra with certainty to the highest spatial frequencies allowed by current instrumentation.
Methods: To accomplish this, we use a 2D inversion code that is able to recover information up to the instrumental diffraction limit.
Results: The retrieved power spectra have shallow slopes that extend further down to much smaller scales than has been found before. They do not seem to show any power law. The observed slopes at subgranular scales agree with those obtained from recent local dynamo simulations. Small differences are found for the vertical component of kinetic energy that suggest that observations suffer from an instrumental effect that is not taken into account.
Conclusions: Local dynamo simulations quantitatively reproduce the observed magnetic energy power spectra on the scales of granulation down to the resolution limit of Hinode/SP, within the error bars inflicted by the method used and the instrumental effects replicated. Title: Internetwork magnetic field as revealed by two-dimensional inversions Authors: Danilovic, S.; van Noort, M.; Rempel, M. Bibcode: 2016A&A...593A..93D Altcode: 2016arXiv160700772D Context. Properties of magnetic field in the internetwork regions are still fairly unknown because of rather weak spectropolarimetric signals.
Aims: We address the matter by using the two-dimensional (2D) inversion code, which is able to retrieve the information on smallest spatial scales up to the diffraction limit, while being less susceptible to noise than most of the previous methods used.
Methods: Performance of the code and the impact of various effects on the retrieved field distribution is tested first on the realistic magneto-hydrodynamic (MHD) simulations. The best inversion scenario is then applied to the real data obtained by Spectropolarimeter (SP) on board Hinode.
Results: Tests on simulations show that: (1) the best choice of node position ensures a decent retrieval of all parameters; (2) the code performs well for different configurations of magnetic field; (3) slightly different noise levels or slightly different defocus included in the spatial point spread function (PSF) produces no significant effect on the results; and (4) temporal integration shifts the field distribution to a stronger, more horizontally inclined field.
Conclusions: Although the contribution of the weak field is slightly overestimated owing to noise, 2D inversions are able to recover well the overall distribution of the magnetic field strength. Application of the 2D inversion code on the Hinode SP internetwork observations reveals a monotonic field strength distribution. The mean field strength at optical depth unity is ~ 130 G. At higher layers, field strength drops as the field becomes more horizontal. Regarding the distribution of the field inclination, tests show that we cannot directly retrieve it with the observations and tools at hand, however, the obtained distributions are consistent with those expected from simulations with a quasi-isotropic field inclination after accounting for observational effects. Title: Formation of a solar Hα filament from orphan penumbrae Authors: Buehler, D.; Lagg, A.; van Noort, M.; Solanki, S. K. Bibcode: 2016A&A...589A..31B Altcode: 2016arXiv160305899B
Aims: The formation and evolution of an Hα filament in active region (AR) 10953 is described.
Methods: Observations from the Solar Optical Telescope (SOT) aboard the Hinode satellite starting from UT 18:09 on 27th April 2007 until UT 06:08 on 1st May 2007 were analysed. 20 scans of the 6302 Å Fe I line pair recorded by SOT/SP were inverted using the spatially coupled version of the SPINOR code. The inversions were analysed together with co-spatial SOT/BFI G-band and Ca II H and SOT/NFI Hα observations.
Results: Following the disappearance of an initial Hα filament aligned along the polarity inversion line (PIL) of the AR, a new Hα filament formed in its place some 20 h later, which remained stable for, at least, another 1.5 days. The creation of the new Hα filament was driven by the ascent of horizontal magnetic fields from the photosphere into the chromosphere at three separate locations along the PIL. The magnetic fields at two of these locations were situated directly underneath the initial Hα filament and formed orphan penumbrae already aligned along the Hα filament channel. The 700 G orphan penumbrae were stable and trapped in the photosphere until the disappearance of the overlying initial Hα filament, after which they started to ascend into the chromosphere at 10 ± 5 m/s. Each ascent was associated with a simultaneous magnetic flux reduction of up to 50% in the photosphere. The ascended orphan penumbrae formed dark seed structures in Hα in parallel with the PIL, which elongated and merged to form an Hα filament. The filament channel featured horizontal magnetic fields of on average 260 G at log (τ) = -2 suspended above the nearly field-free lower photosphere. The fields took on an overall inverse configuration at log (τ) = -2 suggesting a flux rope topology for the new Hα filament. The destruction of the initial Hα filament was likely caused by the flux emergence at the third location along the PIL.
Conclusions: We present a new interpretation of the Hα filament formation in AR 10953 whereby the mainly horizontal fields of orphan penumbrae, aligned along the Hα filament channel, ascend into the chromosphere, forming seed fragments for a new, second Hα filament. The orphan penumbral fields ascend into the chromosphere ~9-24 h before the Hα filament is fully formed. Title: Depth-dependent global properties of a sunspot observed by Hinode using the Solar Optical Telescope/Spectropolarimeter Authors: Tiwari, Sanjiv K.; van Noort, Michiel; Solanki, Sami K.; Lagg, Andreas Bibcode: 2015A&A...583A.119T Altcode: 2015arXiv150804830T Context. For the past two decades, the three-dimensional structure of sunspots has been studied extensively. A recent improvement in the Stokes inversion technique prompts us to revisit the depth-dependent properties of sunspots.
Aims: In the present work, we aim to investigate the global depth-dependent thermal, velocity, and magnetic properties of a sunspot, as well as the interconnection between various local properties.
Methods: We analysed high-quality Stokes profiles of the disk-centred, regular, leading sunspot of NOAA AR 10933, acquired by the Solar Optical Telescope/Spectropolarimeter (SOT/SP) on board the Hinode spacecraft. To obtain depth-dependent stratification of the physical parameters, we used the recently developed, spatially coupled version of the SPINOR inversion code.
Results: First, we study the azimuthally averaged physical parameters of the sunspot. We find that the vertical temperature gradient in the lower- to mid-photosphere is at its weakest in the umbra, while it is considerably stronger in the penumbra, and stronger still in the spot's surroundings. The azimuthally averaged field becomes more horizontal with radial distance from the centre of the spot, but more vertical with height. At continuum optical depth unity, the line-of-sight velocity shows an average upflow of ~300 ms-1 in the inner penumbra and an average downflow of ~1300 ms-1 in the outer penumbra. The downflow continues outside the visible penumbral boundary. The sunspot shows, at most, a moderate negative twist of <5° at log (τ) = 0, which increases with height. The sunspot umbra and the spines of the penumbra show considerable similarity with regard to their physical properties, albeit with some quantitative differences (weaker, somewhat more horizontal fields in spines, commensurate with their location being further away from the sunspot's core). The temperature shows a general anti-correlation with the field strength, with the exception of the heads of penumbral filaments, where a weak positive correlation is found. The dependence of the physical parameters on each other over the full sunspot shows a qualitative similarity to that of a standard penumbral filament and its surrounding spines.
Conclusions: The large-scale variation in the physical parameters of a sunspot at various optical depths is presented. Our results suggest that the spines in the penumbra are basically the outward extension of the umbra. The spines and the penumbral filaments, together, are the basic elements that form a sunspot penumbra. Title: Statistical analysis of supersonic downflows in sunspot penumbrae. Authors: Kim, Hyunnam; Lagg, Andreas; Solanki, Sami K.; Narayan, Gautam; van Noort, Michiel; Kim, Kap-Sung Bibcode: 2015IAUGA..2254868K Altcode: Supersonic downflow patches was found in the outer edge of sunspot penumbra. These patches are believed to be the return channels of the Evershed flow. There was previous study to investigate their structure in detail using Hinode SOT/SP observations (M. van Noort et al. 2013) but their data sample was only two sunspots. To make general description it needs to check more sunspot data sample.We selected 242 downflow patches of 16 sunspots using Hinode SOT/SP observations from 2006 to 2012. Height-dependent maps of atmospheric parameters of these downflows was produced by using HeLix which was height dependent LTE inversion code of Stokes profiles.Statistical analysis of magnetic field strength, inclination angle of field line, temperature and line-of-sight velocity are presented. The recovered atmospheric data tell us that downflow patches have different physical signatures comparing normal penumbra properties. Furthermore, our results of three height-dependent layer support that heating process should occur on the downflow patches in the middle of layer. Title: Properties of solar plage from a spatially coupled inversion of Hinode SP data Authors: Buehler, D.; Lagg, A.; Solanki, S. K.; van Noort, M. Bibcode: 2015A&A...576A..27B Altcode: 2015arXiv150101151B
Aims: The properties of magnetic fields forming an extended plage region in AR 10953 were investigated.
Methods: Stokes spectra of the Fe I line pair at 6302 Å recorded by the spectropolarimeter aboard the Hinode satellite were inverted using the SPINOR code. The code performed a 2D spatially coupled inversion on the Stokes spectra, allowing the retrieval of gradients in optical depth within the atmosphere of each pixel, whilst accounting for the effects of the instrument's PSF. Consequently, no magnetic filling factor was needed.
Results: The inversion results reveal that plage is composed of magnetic flux concentrations (MFCs) with typical field strengths of 1520 G at log (τ) = -0.9 and inclinations of 10°-15°. The MFCs expand by forming magnetic canopies composed of weaker and more inclined magnetic fields. The expansion and average temperature stratification of isolated MFCs can be approximated well with an empirical plage thin flux tube model. The highest temperatures of MFCs are located at their edges in all log (τ) layers. Whilst the plasma inside MFCs is nearly at rest, each is surrounded by a ring of downflows of on average 2.4 km s-1 at log (τ) = 0 and peak velocities of up to 10 km s-1, which are supersonic. The downflow ring of an MFC weakens and shifts outwards with height, tracing the MFC's expansion. Such downflow rings often harbour magnetic patches of opposite polarity to that of the main MFC with typical field strengths below 300 G at log (τ) = 0. These opposite polarity patches are situated beneath the canopy of their main MFC. We found evidence of a strong broadening of the Stokes profiles in MFCs and particularly in the downflow rings surrounding MFCs (expressed by a microturbulence in the inversion). This indicates the presence of strong unresolved velocities. Larger magnetic structures such as sunspots cause the field of nearby MFCs to be more inclined. Title: Inclinations of small quiet-Sun magnetic features based on a new geometric approach Authors: Jafarzadeh, S.; Solanki, S. K.; Lagg, A.; Bellot Rubio, L. R.; van Noort, M.; Feller, A.; Danilovic, S. Bibcode: 2014A&A...569A.105J Altcode: 2014arXiv1408.2443J Context. High levels of horizontal magnetic flux have been reported in the quiet-Sun internetwork, often based on Stokes profile inversions.
Aims: Here we introduce a new method for deducing the inclination of magnetic elements and use it to test magnetic field inclinations from inversions.
Methods: We determine accurate positions of a set of small, bright magnetic elements in high spatial resolution images sampling different photospheric heights obtained by the Sunrise balloon-borne solar observatory. Together with estimates of the formation heights of the employed spectral bands, these provide us with the inclinations of the magnetic features. We also compute the magnetic inclination angle of the same magnetic features from the inversion of simultaneously recorded Stokes parameters.
Results: Our new, geometric method returns nearly vertical fields (average inclination of around 14° with a relatively narrow distribution having a standard deviation of 6°). In strong contrast to this, the traditionally used inversions give almost horizontal fields (average inclination of 75 ± 8°) for the same small magnetic features, whose linearly polarised Stokes profiles are adversely affected by noise. We show that for such magnetic features inversions overestimate the flux in horizontal magnetic fields by an order of magnitude.
Conclusions: The almost vertical field of bright magnetic features from our geometric method is clearly incompatible with the nearly horizontal magnetic fields obtained from the inversions. This indicates that the amount of magnetic flux in horizontal fields deduced from inversions is overestimated in the presence of weak Stokes signals, in particular if Stokes Q and U are close to or under the noise level. Inversions should be used with great caution when applied to data with no clear Stokes Q and no U signal. By combining the proposed method with inversions we are not just improving the inclination, but also the field strength. This technique allows us to analyse features that are not reliably treated by inversions, thus greatly extending our capability to study the complete magnetic field of the quiet Sun. Title: Vigorous convection in a sunspot granular light bridge Authors: Lagg, Andreas; Solanki, Sami K.; van Noort, Michiel; Danilovic, Sanja Bibcode: 2014A&A...568A..60L Altcode: 2014arXiv1407.1202L Context. Light bridges are the most prominent manifestation of convection in sunspots. The brightest representatives are granular light bridges composed of features that appear to be similar to granules.
Aims: An in-depth study of the convective motions, temperature stratification, and magnetic field vector in and around light bridge granules is presented with the aim of identifying similarities and differences to typical quiet-Sun granules.
Methods: Spectropolarimetric data from the Hinode Solar Optical Telescope were analyzed using a spatially coupled inversion technique to retrieve the stratified atmospheric parameters of light bridge and quiet-Sun granules.
Results: Central hot upflows surrounded by cooler fast downflows reaching 10 km s-1 clearly establish the convective nature of the light bridge granules. The inner part of these granules in the near surface layers is field free and is covered by a cusp-like magnetic field configuration. We observe hints of field reversals at the location of the fast downflows. The quiet-Sun granules in the vicinity of the sunspot are covered by a low-lying canopy field extending radially outward from the spot.
Conclusions: The similarities between quiet-Sun and light bridge granules point to the deep anchoring of granular light bridges in the underlying convection zone. The fast, supersonic downflows are most likely a result of a combination of invigorated convection in the light bridge granule due to radiative cooling into the neighboring umbra and the fact that we sample deeper layers, since the downflows are immediately adjacent to the slanted walls of the Wilson depression.

The two movies are available in electronic form at http://www.aanda.org Title: Analysis of magnetic fields in a plage region using a spatially coupled 2D inversion technique Authors: Buehler, David; Lagg, Andreas; Solanki, Sami K.; Van Noort, Michiel Bibcode: 2014cosp...40E.427B Altcode: The properties of magnetic features (MFCs) within a plage region in the vicinity of a sunspot were investigated at high spatial resolution. Stokes spectra of the 630nm line pair recorded by the spectropolarimeter aboard Hinode were inverted using an extended version of the SPINOR code. The code preformed a spatially coupled inversion of the Stokes spectra using three log(tau) nodes in optical depth. No magnetic filling factors was employed. The analysis of the inversion results reveals that the MFCs have typical field strengths of 1500G at log(tau)=-0.9 and inclinations between 10-15 degrees in all three log(tau) nodes. The MFCs expand by forming magnetic canopies composed of weaker and more inclined magnetic fields. The expansion of the magnetic field and temperature stratification of MFCs with optical depth is in good agreement with a thin flux tube model. Whilst the gas inside magnetic flux concentrations is typically at rest, the majority of MFCs were surrounded by a ring of downflows with an average value of 2.5km/s at log(tau)=0. The ring gradually shifts outwards following the expansion of the MFC. Within the downflow rings of MFCs small magnetic patches of opposite polarity to that of the main MFC were identified, which are predominantly situated beneath the canopy of its main MFC. We found evidence for a strong broadening of the Stokes profiles within MFCs and their surrounding downflow rings (expressed by a microturbulence in the inversion). This indicates the presence of strong unresolved velocities. Title: Peripheral downflows in sunspot penumbrae Authors: van Noort, M.; Lagg, A.; Tiwari, S. K.; Solanki, S. K. Bibcode: 2013A&A...557A..24V Altcode: 2013arXiv1308.0466V Context. Sunspot penumbrae show high-velocity patches along the periphery.
Aims: The high-velocity downflow patches are believed to be the return channels of the Evershed flow. We aim to investigate their structure in detail using Hinode SOT/SP observations.
Methods: We employ Fourier interpolation in combination with spatially coupled height dependent LTE inversions of Stokes profiles to produce high-resolution, height-dependent maps of atmospheric parameters of these downflows and investigate their properties.
Results: High-speed downflows are observed over a wide range of viewing angles. They have supersonic line-of-sight velocities, some in excess of 20 km s-1, and very high magnetic field strengths, reaching values of over 7 kG. A relation between the downflow velocities and the magnetic field strength is found, in good agreement with MHD simulations.
Conclusions: The coupled inversion at high resolution allows for the accurate determination of small-scale structures. The recovered atmospheric structure indicates that regions with very high downflow velocities contain some of the strongest magnetic fields that have ever been measured on the Sun.

Two movies are available in electronic form at http://www.aanda.org Title: Structure of sunspot penumbral filaments: a remarkable uniformity of properties Authors: Tiwari, Sanjiv Kumar; van Noort, Michiel; Lagg, Andreas; Solanki, Sami K. Bibcode: 2013A&A...557A..25T Altcode: 2013arXiv1307.3668T Context. The sunspot penumbra comprises numerous thin, radially elongated filaments that are central for heat transport within the penumbra, but whose structure is still not clear.
Aims: We aim to investigate the fine-scale structure of these penumbral filaments.
Methods: We perform a depth-dependent inversion of spectropolarimetric data of a sunspot very close to solar disk center obtained by Solar Optical Telescope/Spectropolarimeter onboard the Hinode spacecraft. We have used a recently developed, spatially coupled 2D inversion scheme, which allows us to analyze the fine structure of individual penumbral filaments up to the diffraction limit of the telescope.
Results: Filaments of different sizes in all parts of the penumbra display very similar magnetic field strengths, inclinations, and velocity patterns. The temperature structure is also similar, although the filaments in the inner penumbra have cooler tails than those in the outer penumbra. The similarities allowed us to average all these filaments and to subsequently extract the physical properties common to all of them. This average filament shows upflows associated with an upward-pointing field at its inner, umbral end (head) and along its axis, as well as downflows along the lateral edge and strong downflows in the outer end (tail) associated with a nearly vertical, strong, and downward-pointing field. The upflowing plasma is significantly, i.e., up to 800 K, hotter than the downflowing plasma. The hot, tear-shaped head of the averaged filament can be associated with a penumbral grain. The central part of the filament shows nearly horizontal fields with strengths in the range of 1 kG. The field above the filament converges, whereas a diverging trend is seen in the deepest layers near the head of the filament. The fluctuations in the physical parameters along and across the filament increase rapidly with depth.
Conclusions: We put forward a unified observational picture of a sunspot penumbral filament. It is consistent with such a filament being a magneto-convective cell, in line with recent magnetohydrodynamic simulations. The uniformity of its properties over the penumbra sets constraints on penumbral models and simulations. The complex and inhomogeneous structure of the filament provides a natural explanation for a number of long-running controversies in the literature. Title: Physical properties of a sunspot chromosphere with umbral flashes Authors: de la Cruz Rodríguez, J.; Rouppe van der Voort, L.; Socas-Navarro, H.; van Noort, M. Bibcode: 2013A&A...556A.115D Altcode: 2013arXiv1304.0752D We present new high-resolution spectro-polarimetric Ca IIλ8542 observations of umbral flashes in sunspots. At nearly 0.18 arcsec, and spanning about one hour of continuous observation, this is the most detailed dataset published thus far. Our study involves both LTE and non-LTE inversions (but includes also a weak field analysis as a sanity check) to quantify temperatures, mass flows and the full magnetic field vector geometry. We confirm earlier reports that UFs have very fine structure with hot and cool material intermixed at sub-arcsecond scales. The shock front is roughly 1000 K hotter than the surrounding material. We do not observe significant fluctuations of the field in the umbra. In the penumbra, however, the passage of the running penumbral waves alter the magnetic field strength by some 200 G (peak-to-peak amplitude) but it does not change the field orientation (at least not significantly within our sensitivity of a few degrees). From a fast Fourier transform analysis, we find a trend of decreasing power at high temporal frequencies at those locations with more horizontal magnetic fields, for the line-of-sight velocity and magnetic field strength. In the outer penumbra we find an absence of high frequency power while there is increasingly more power at high frequencies towards the umbra.

Movie and Appendices A and B are available in electronic form at http://www.aanda.org Title: Vertical flows and mass flux balance of sunspot umbral dots Authors: Riethmüller, T. L.; Solanki, S. K.; van Noort, M.; Tiwari, S. K. Bibcode: 2013A&A...554A..53R Altcode: 2013arXiv1305.1164R A new Stokes inversion technique that greatly reduces the effect of the spatial point spread function of the telescope is used to constrain the physical properties of umbral dots (UDs). The depth-dependent inversion of the Stokes parameters from a sunspot umbra recorded with Hinode SOT/SP revealed significant temperature enhancements and magnetic field weakenings in the core of the UDs in deep photospheric layers. Additionally, we found upflows of around 960 m/s in peripheral UDs (i.e., UDs close to the penumbra) and ≈600 m/s in central UDs. For the first time, we also detected systematic downflows for distances larger than 200 km from the UD center that balance the upflowing mass flux. In the upper photosphere, we found almost no difference between the UDs and their diffuse umbral background. Title: Spatially coupled inversion of spectro-polarimetric image data. I. Method and first results Authors: van Noort, M. Bibcode: 2012A&A...548A...5V Altcode: 2012arXiv1210.4636V Context. When inverting solar spectra, image degradation effects that are present in the data are usually approximated or not considered.
Aims: We develop a data reduction method that takes these issues into account and minimizes the resulting errors.
Methods: By accounting for the diffraction PSF of the telescope during the inversions, we can produce a self-consistent solution that best fits the observed data, while simultaneously requiring fewer free parameters than conventional approaches.
Results: Simulations using realistic MHD data indicate that the method is stable for all resolutions, including those with pixel scales well beyond those that can be resolved with a 0.5 m telescope, such as the Hinode SOT. Application of the presented method to reduce full Stokes data from the Hinode spectro-polarimeter results in dramatically increased image contrast and an increase in the resolution of the data to the diffraction limit of the telescope in almost all Stokes and fit parameters. The resulting data allow for detecting and interpreting solar features that have so far only been observed with 1m class ground-based telescopes.
Conclusions: A new inversion method was developed that allows for accurate fitting of solar spectro-polarimetric imaging data over a large field of view, while simultaneously improving the noise statistics and spatial resolution of the results significantly. Title: 2D Inversions Authors: van Noort, Michiel Bibcode: 2012decs.confE..37V Altcode: A new approach to inversion of high-resolution spectro-polarimetric solar image data is presented that explicitly takes the effects of telescope diffraction and other optical abberations in the observed data into account. The 2-dimensional solution can reliably reproduce the atmospheres from simulated data cubes and significantly improves the accuracy of profiles fitted to Hinode-SP data, compared to an equivalent 1-dimensional solution, without needing a more complex atmospheric model. Title: Stokes imaging polarimetry using image restoration: a calibration strategy for Fabry-Pérot based instruments Authors: Schnerr, R. S.; de La Cruz Rodríguez, J.; van Noort, M. Bibcode: 2011A&A...534A..45S Altcode: 2010arXiv1012.1225S Context. The combination of image restoration and a Fabry-Pérot interferometer (FPI) based instrument in solar observations results in specific calibration issues. FPIs generally show variations over the field-of-view, while in the image restoration process, the 1-to-1 relation between pixel space and image space is lost, thus complicating any correcting for such variations.
Aims: We develop a data reduction method that takes these issues into account and minimizes the resulting errors.
Methods: By accounting for the time variations in the telescope's Mueller matrix and using separate calibration data optimized for the wavefront sensing in the MOMFBD image restoration process and for the final deconvolution of the data, we have removed most of the calibration artifacts from the resulting data.
Results: Using this method to reduce full Stokes data from CRISP at the SST, we find that it drastically reduces the instrumental and image restoration artifacts resulting from cavity errors, reflectivity variations, and the polarization dependence of flatfields. The results allow for useful scientific interpretation. Inversions of restored data from the δ sunspot AR11029 using the Nicole inversion code, reveal strong (~10 km s-1) downflows near the disk center side of the umbra.
Conclusions: The use of image restoration in combination with an FPI-based instrument leads to complications in the calibrations and intrinsic limitations to the accuracy that can be achieved. We find that for CRISP, the resulting errors can be kept mostly below the polarimetric accuracy of ~10-3. Similar instruments aiming for higher polarimetric and high spectroscopic accuracy, will, however, need to take these problems into account. Title: Observation and analysis of chromospheric magnetic fields . Authors: de la Cruz Rodríguez, J.; Socas-Navarro, H.; van Noort, M.; Rouppe van der Voort, L. Bibcode: 2010MmSAI..81..716D Altcode: 2010arXiv1004.0698D The solar chromosphere is a vigorously dynamic region of the sun, where waves and magnetic fields play an important role. To improve chromospheric diagnostics, we present new observations in Ca II 8542 carried out with the SST/CRISP on La Palma, working in full-Stokes mode. We measured Stokes line profiles in active regions. The line profiles observed close to the solar limb show signals in all four Stokes parameters, while profiles observed close to disk center only show signals above the noise level in Stokes I and V. We used the NLTE inversion code 'NICOLE' to derive atmospheric parameters in umbral flashes present in a small round sunspot without penumbra. Title: Photospheric Temperatures from Ca IIH Authors: Henriques, V. M. J.; Kiselman, D.; van Noort, M. Bibcode: 2010ASSP...19..511H Altcode: 2010mcia.conf..511H The temperature stratification in the upper photosphere can be extracted from Ca II H&K spectrograms following Shine and Linsky (1974) by assuming LTE, the Eddington-Barbier approximation, hydrostatic equilibrium, and that Ca II is mostly in the ground state. Rouppe van der Voort (2002) confirmed that these assumptions were solid for a wide range in the Ca IIK wings and further developed the method including forward computation using MULTI (Carlsson 1986). Title: Spectropolarimetric Diagnostics at the Solar Photosphere near the Limb Authors: Yelles Chaouche, L.; Solanki, S. K.; Rouppe van der Voort, L.; van Noort, M. Bibcode: 2009ASPC..405..189Y Altcode: 2009arXiv0910.2885Y In the present work, we investigate the formation of Stokes profiles and spectro-polarimetric diagnostics in an active region plage near the limb. We use 3-D radiation-MHD simulations with unipolar fields of an average strength of 400 G, which is largely concentrated in flux tubes in which the field reaches typical kilo-Gauss values. We generate synthetic Stokes spectra by radiative transfer calculations, then we degrade the simulated Stokes signal to account for observational conditions. The synthetic data treated in this manner are compared with and found to roughly reproduce spectro-polarimetric high-resolution observations at μ=0.39 obtained by the SOUP instrument with the Swedish 1-m Solar Telescope at the beginning of 2006. Title: CRISP Spectropolarimetric Imaging of Penumbral Fine Structure Authors: Scharmer, G. B.; Narayan, G.; Hillberg, T.; de la Cruz Rodriguez, J.; Löfdahl, M. G.; Kiselman, D.; Sütterlin, P.; van Noort, M.; Lagg, A. Bibcode: 2008ApJ...689L..69S Altcode: 2008arXiv0806.1638S 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. Title: Stokes imaging polarimetry using image restoration at the Swedish 1-m solar telescope Authors: van Noort, M. J.; Rouppe van der Voort, L. H. M. Bibcode: 2008A&A...489..429V Altcode: 2008arXiv0805.4296V Aims: We aim to achieve both high spatial resolution and high polarimetric sensitivity, using an earth-based 1m-class solar telescope, for the study of magnetic fine structure on the surface of the Sun.
Methods: We use a setup with 3 high-speed, low-noise cameras to construct datasets with interleaved polarimetric states, particularly suitable for Multi-Object Multi-Frame Blind Deconvolution image restorations. We discuss the polarimetric calibration routine and various potential sources of error in the results.
Results: We obtained near diffraction limited images, which have a noise level of ≈ 10-3 I_cont. We confirm that dark cores have a weaker magnetic field and a lower inclination angle with respect to the solar surface than the edges of the penumbral filament. We demonstrate that the magnetic field strength in faculae-striations is significantly lower than in other nearby parts of the faculae. Title: Spectropolarimetry of Sunspots at 0.16 ARCSEC resolution Authors: Scharmer, G.; Henriques, V.; Hillberg, T.; Kiselman, D.; Löfdahl, M.; Narayan, G.; Sütterlin, P.; van Noort, M.; de la Cruz Rodríguez, J. Bibcode: 2008ESPM...12..2.5S Altcode: We present first observations of sunspots with the imaging spectropolarimeter CRISP, recently installed at the Swedish 1-m Solar Telescope (SST) on La Palma. This spectropolarimeter is based on a high-fidelity dual Fabry-Perot filter system.

Two liquid crystals and a polarizing beam splitter are used to reduce seeing induced I,Q,U,V crosstalk by simultaneously recording images with two 1kx1k back-illuminated Sarnoff CCD's. A third CCD simultaneously records broadband images through the pre-filter of the FPI filter system, allowing image reconstruction and co-alignment of images of different polarization states and at different wavelengths in Zeeman sensitive spectral lines.

The first data, recorded in April 2008, demonstrate the capability of this system to record high cadence, high S/N polarimetric data with a spatial resolution at or close to the diffraction limit of the SST at 630 nm, 0.16 arcsec. We discuss the analysis of first spectropolarimetric data for sunspots, based on Milne-Eddington inversion techniques. Title: On small active region filaments, fibrils and surges Authors: Lin, Y.; Martin, S. F.; Engvold, O.; Rouppe van der Voort, L. H. M.; van Noort, M. Bibcode: 2008AdSpR..42..803L Altcode: High resolution Hα images and magnetograms (0.2 arc s) of an active region were obtained in alternating time series at 42 s cadences using the Swedish 1-m Solar Telescope on 2004 August 21. The Hα filtergrams reveal an active region filament and surges consisting of thread-like structures which have widths similar to the widths of chromospheric fibrils, both recorded down to the resolution limit in the best images. All observed structures in the active region appear highly dynamic. Fibrils show counterstreaming strongly resembling the counterstreaming threads in filaments. Streaming, along the threads of surges extending more than 10 arc s, is higher in speed (∼20 km s-1) than in the filament and fibrils and appears to flow independently over and above the chromospheric fibrils. Blue shifts seen in the Hα Dopplergrams confirm the outward mass motion of the surges. However, in at least one case, we also see simultaneous downflows from the same site but in the opposite direction and downward toward the chromosphere. We suggest that the site between these two outward and downward flows identifies the place where magnetic reconnection could occur and thereby cause of the surge. This appears to imply that the reconnection site is in the high chromosphere or low corona. Title: SST/CRISP Magnetometry with Fe I 630.2 nm Authors: Narayan, G.; Scharmer, G. B.; Hillberg, T.; Lofdahl, M.; van Noort, M.; Sutterlin, P.; Lagg, A. Bibcode: 2008ESPM...122.120N Altcode: We present recent full Stokes observations in the Fe I 630.2 nm line with CRISP, an imaging spectropolarimeter at the Swedish 1-m Solar Telescope (SST). The observations reach a spatial resolution of 0".16, close to the diffraction limit of the SST, representing a major improvement over any past ground based or space based spectropolarimetric data. We describe the data acquisition and reduction methods and present results of Milne-Eddington(ME) inversions applied on observations of plage. Title: SST/CRISP observations of Ca II 854.2 nm Authors: de la Cruz Rodríguez, J.; van Noort, M. Bibcode: 2008ESPM...12.2.77D Altcode: We present observations of the Ca II 854.2 nm line using the new Crisp Imaging Spectropolarimeter (CRISP) at the SST. CRISP allows high-cadence, high-spatial resolution scans through the line profile with full Stokes polarimetry. We discuss the performance of the instrument and the diagnostic potential of the Ca II IR triplet lines. Title: Moat Flow in the Vicinity of Sunspots for Various Penumbral Configurations Authors: Vargas Domínguez, S.; Rouppe van der Voort, L.; Bonet, J. A.; Martínez Pillet, V.; Van Noort, M.; Katsukawa, Y. Bibcode: 2008ApJ...679..900V Altcode: 2008arXiv0802.1457V High-resolution time series of sunspots have been obtained with the Swedish 1 m Solar Telescope between 2003 and 2006 at different locations on the solar disk. Proper motions in seven different active regions have been studied. The analysis was performed by applying local correlation tracking to every series of sunspots, each of them more than 40 minutes long. The sunspots' shapes include a different variety of penumbral configurations. We report on the systematic behavior of the large-scale outflows surrounding the sunspots, commonly known as moat flows, that are essentially present only when preceded by a penumbra not tangential but perpendicular to the sunspot border. We present one case for which this rule appears not to be confirmed. We speculate that the magnetic neutral line, which is located in the vicinity of the anomalous region, might be responsible for blocking the outflow. These new results confirm the systematic and strong relation between the moat flows and the existence of penumbrae. A comparative statistical study between moats and standard granulation is also performed. Title: Evidence of Traveling Waves in Filament Threads Authors: Lin, Y.; Engvold, O.; Rouppe van der Voort, L. H. M.; van Noort, M. Bibcode: 2007SoPh..246...65L Altcode: High-resolution Hα filtergrams (0.2″) obtained with the Swedish 1-m Solar Telescope resolve numerous very thin, thread-like structures in solar filaments. The threads are believed to represent thin magnetic flux tubes that must be longer than the observable threads. We report on evidence for small-amplitude (1 - 2 km s−1) waves propagating along a number of threads with an average phase velocity of 12 km s−1 and a wavelength of 4″. The oscillatory period of individual threads vary from 3 to 9 minutes. Temporal variation of the Doppler velocities averaged over a small area containing a number of individual threads shows a short-period (3.6 minutes) wave pattern. These short-period oscillations could possibly represent fast modes in accordance with numerical fibril models proposed by Díaz et al. (Astron. Astrophys.379, 1083, 2001). In some cases, it is clear that the propagating waves are moving in the same direction as the mass flows. Title: High-Resolution Observations and Numerical Simulations of Chromospheric Fibrils and Mottles Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2007ASPC..368...65D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. In this paper, we briefly summarize results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in Hα) above and about active region plage. Using extremely high-resolution observations obtained at the SST, and advanced numerical 2D radiative MHD simulations, we show that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations.

In addition, we present some preliminary observations of quiet Sun jets or mottles. We find that the mechanism that produces fibrils in active regions is most likely also at work in quiet Sun regions, although it is modified by the weaker magnetic field and the presence of more mixed-polarity. A comparison with numerical simulations suggests that the weaker magnetic field in quiet Sun allows for significantly stronger (than in active regions) transverse motions that are superposed on the field-aligned, shock-driven motions. This leads to a more dynamic, and much more complex environment than in active region plage. In addition, our observations of the mixed polarity environment in quiet Sun regions suggest that other mechanisms, such as reconnection, may well play a significant role in the formation of some quiet Sun jets. Simultaneous high-resolution magnetograms (such as those provided by Hinode), as well as numerical simulations that take into account a whole variety of different magnetic configurations, will be necessary to determine the relative importance in quiet Sun of, respectively, the fibril-mechanism and reconnection. Title: Magnetoacoustic Shocks as a Driver of Quiet-Sun Mottles Authors: Rouppe van der Voort, L. H. M.; De Pontieu, B.; Hansteen, V. H.; Carlsson, M.; van Noort, M. Bibcode: 2007ApJ...660L.169R Altcode: 2007astro.ph..3535R We present high spatial and high temporal resolution observations of the quiet Sun in Hα obtained with the Swedish 1-m Solar Telescope on La Palma. We observe that many mottles, jetlike features in the quiet Sun, display clear up- and downward motions along their main axis. In addition, many mottles show vigorous transverse displacements. Unique identification of the mottles throughout their lifetime is much harder than for their active region counterpart, dynamic fibrils. This is because many seem to lack a sharply defined edge at their top, and significant fading often occurs throughout their lifetime. For those mottles that can be reliably tracked, we find that the mottle tops often undergo parabolic paths. We find a linear correlation between the deceleration these mottles undergo and the maximum velocity they reach, similar to what was found earlier for dynamic fibrils. Combined with an analysis of oscillatory properties, we conclude that at least part of the quiet-Sun mottles are driven by magnetoacoustic shocks. In addition, the mixed-polarity environment and vigorous dynamics suggest that reconnection may play a significant role in the formation of some quiet-Sun jets. Title: Observations and Simulations of Fibrils and Mottles Authors: De Pontieu, Bart; Hansteen, Viggo H.; Rouppe van der Voort, Luc; van Noort, Michiel; Carlsson, Mats Bibcode: 2007astro.ph..2081D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. In this paper, we briefly summarize results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in H-alpha) above and about active region plage. Using extremely high-resolution observations obtained at the SST, and advanced numerical 2D radiative MHD simulations, we show that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. In addition, we present some preliminary observations of quiet Sun jets or mottles. We find that the mechanism that produces fibrils in active regions is most likely also at work in quiet Sun regions, although it is modified by the weaker magnetic field and the presence of more mixed-polarity. A comparison with numerical simulations suggests that the weaker magnetic field in quiet Sun allows for significantly stronger (than in active regions) transverse motions that are superposed on the field-aligned, shock-driven motions. This leads to a more dynamic, and much more complex environment than in active region plage. In addition, our observations of the mixed polarity environment in quiet Sun regions suggest that other mechanisms, such as reconnection, may well play a significant role in the formation of some quiet Sun jets. Title: Ca IIH line wing images of sunspot penumbrae recorded with the Swedish 1-m Solar Telescope Authors: Narayan, G.; van Noort, M. J.; Scharmer, G. Bibcode: 2007msfa.conf..213N Altcode: We present recent Ca IIH images of sunspot penumbrae taken with the Swedish 1m Solar Telescope (SST) during June-July 2006 and restored to a resolution close to 0".1. Images were recorded at different Ca IIH line positions using one tunable filter, one fixed wing filter and a wide band quasi-continuum filter. Apart from the images recorded at line center (and formed under non-LTE conditions), this provides temperature information from approximately the first 200-300 km above the photosphere. Title: Solar image restoration Authors: Löfdahl, M. G.; van Noort, M. J.; Denker, C. Bibcode: 2007msfa.conf..119L Altcode: Image restoration is used to repair solar images degraded by the turbulence in Earth's atmosphere. Restoration algorithms are based on models of the optical system that produce the images from the solar source of radiation, through Earth's atmosphere and telescope/instrument optics, to the detectors recording the data. In this review, these model components are discussed in the context of two very different classes of image restoration methods, i.e., Speckle Imaging and Phase Diversity/Multi-Frame Blind Deconvolution, which have been successfully used during the last two decades. The strengths and weaknesses of these two approaches are discussed, as well as some variants and recent progress. Title: High-Resolution Observations and Modeling of Dynamic Fibrils Authors: De Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2007ApJ...655..624D Altcode: 2007astro.ph..1786D We present unprecedented high-resolution Hα observations, obtained with the Swedish 1 m Solar Telescope, that, for the first time, spatially and temporally resolve dynamic fibrils in active regions on the Sun. These jetlike features are similar to mottles or spicules in quiet Sun. We find that most of these fibrils follow almost perfect parabolic paths in their ascent and descent. We measure the properties of the parabolic paths taken by 257 fibrils and present an overview of the deceleration, maximum velocity, maximum length, and duration, as well as their widths and the thickness of a bright ring that often occurs above dynamic fibrils. We find that the observed deceleration of the projected path is typically only a fraction of solar gravity and incompatible with a ballistic path at solar gravity. We report on significant differences of fibril properties between those occurring above a dense plage region and those above a less dense plage region where the magnetic field seems more inclined from the vertical. We compare these findings to advanced numerical two-dimensional radiative MHD simulations and find that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. Detailed comparison of observed and simulated fibril properties shows striking similarities of the values for deceleration, maximum velocity, maximum length, and duration. We compare our results with observations of mottles and find that a similar mechanism is most likely at work in the quiet Sun. Title: High Spatial Resolution Observations of Solar Magnetic Structures Authors: Rouppe van der Voort, L.; van Noort, M.; Carlsson, M.; Hansteen, V. Bibcode: 2006ASPC..354...37R Altcode: We present observations of the dynamic evolution of photospheric magnetic structures in the G-band, continuum, magnetograms and Dopplergrams. The observations were obtained with the Swedish one-m Solar Telescope on La Palma. Using adaptive optics and the Multi-Object Multi-Frame Blind Deconvolution image restoration technique, we obtained several datasets at close to the diffraction limit of the telescope (0.1 arcsec) over long periods of time. We show examples of the dynamical evolution of different magnetic structures: the advection of individual bright points by the granular flow, the formation and fragmentation of flux sheets, and the continuous transition between micro-pores, elongated ribbons and more circular ``flowers''. Narrow sheets with downdrafts are found right at the edges of magnetic field concentrations. Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks Authors: de Pontieu, B.; Hansteen, V. H.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2006AGUFMSH23B0359D Altcode: With the recent advent of the Swedish 1-m Solar Telescope (SST), advanced image processing techniques, as well as numerical simulations that provide a more realistic view of the chromosphere, a comprehensive understanding of chromospheric jets such as spicules, mottles and fibrils is now within reach. We will present results from a recent analysis of dynamic fibrils, short-lived jet-like features that dominate the chromosphere (as imaged in Hα) above and about active region plage. These jets are similar to mottles and spicules in quiet Sun. Our analysis is based on a time series of extremely high-resolution (120 km) images taken in Hα linecenter at 1 second cadence, obtained by the Oslo group at the SST in October 2005. The 78 min long time series for the first time, spatially and temporally resolves dynamic fibrils in active regions. Our analysis shows that most of the fibrils follow almost perfect parabolic paths in their ascent and descent. We measure the properties of the parabolic paths taken by 257 different dynamic fibrils, and find that the observed deceleration of the projected path is typically only a fraction of solar gravity, and incompatible with a ballistic path at solar gravity. We report on significant differences of measured fibril properties between those occurring in association with a dense plage region, and those above a less dense plage region where the magnetic field seems more inclined away from the vertical. We compare these observational findings to advanced numerical 2D radiative MHD simulations, and find that fibrils are most likely formed by chromospheric shock waves that occur when convective flows and global oscillations leak into the chromosphere along the field lines of magnetic flux concentrations. Detailed comparison of the properties of fibrils found in our observations and those in our numerical simulations shows striking similarities of the values for deceleration, maximum velocity, maximum length and duration. The numerical simulations also reproduce the correlations we observe between various fibrils properties, as well as the regional differences, taking into account the different magnetic configuration for the various regions. We compare our results with observations of mottles and find that a similar mechanism is most likely at work in the quiet Sun. Title: Solar Image Restoration by use of Multi-Object Multi-Frame Blind Deconvolution Authors: van Noort, M.; Rouppe van der Voort, L.; Löfdahl, M. Bibcode: 2006ASPC..354...55V Altcode: We present examples of the application of the image restoration method of Multi-Object Multi-Frame Blind Deconvolution to observations obtained with the Swedish one-meter Solar Telescope on La Palma. This restoration method is an extension of Joint Phase Diverse Speckle image restoration. Multiple realizations of multiple objects can now be restored jointly, facilitating near-perfect alignment between different objects. This greatly reduces false signals in the determination of derived quantities, such as magnetograms, Dopplergrams and G-band-continuum difference images. Title: High-Resolution Observations of Fast Events in the Solar Chromosphere Authors: van Noort, M. J.; Rouppe van der Voort, L. H. M. Bibcode: 2006ApJ...648L..67V Altcode: We present new, high spatial and high temporal resolution observations of the Sun in Hα obtained with the Swedish 1 m Solar Telescope on La Palma. The combined use of adaptive optics and image restoration techniques yielded a near-diffraction-limited time series with a cadence of 3 frames s-1 of two different active regions. The unique combination of high temporal and spatial resolution reveals to us the existence of highly dynamic structures, moving at velocities of up to 240 km s-1, and high-velocity waves in the chromosphere. The rapid motions appear to be common, as they are observed in two data sets recorded in succession at different locations on the solar disk. The dynamic events are probably associated with reconfigurations of the magnetic field. Title: Rapid Temporal Variability of Faculae: High-Resolution Observations and Modeling Authors: De Pontieu, B.; Carlsson, M.; Stein, R.; Rouppe van der Voort, L.; Löfdahl, M.; van Noort, M.; Nordlund, Å.; Scharmer, G. Bibcode: 2006ApJ...646.1405D Altcode: We present high-resolution G-band observations (obtained with the Swedish 1 m Solar Telescope) of the rapid temporal variability of faculae, which occurs on granular timescales. By combining these observations with magnetoconvection simulations of a plage region, we show that much of this variability is not intrinsic to the magnetic field concentrations that are associated with faculae, but rather a phenomenon associated with the normal evolution and splitting of granules. We also show examples of facular variability caused by changes in the magnetic field, with movies of dynamic behavior of the striations that dominate much of the facular appearance at 0.1" resolution. Examples of these dynamics include merging, splitting, rapid motion, apparent fluting, and possibly swaying. Title: Dynamic Fibrils Are Driven by Magnetoacoustic Shocks Authors: Hansteen, V. H.; De Pontieu, B.; Rouppe van der Voort, L.; van Noort, M.; Carlsson, M. Bibcode: 2006ApJ...647L..73H Altcode: 2006astro.ph..7332H The formation of jets such as dynamic fibrils, mottles, and spicules in the solar chromosphere is one of the most important, but also most poorly understood, phenomena of the Sun's magnetized outer atmosphere. We use extremely high resolution observations from the Swedish 1 m Solar Telescope combined with advanced numerical modeling to show that in active regions these jets are a natural consequence of upwardly propagating slow-mode magnetoacoustic shocks. These shocks form when waves generated by convective flows and global p-mode oscillations in the lower lying photosphere leak upward into the magnetized chromosphere. We find excellent agreement between observed and simulated jet velocities, decelerations, lifetimes, and lengths. Our findings suggest that previous observations of quiet-Sun spicules and mottles may also be interpreted in light of a shock-driven mechanism. Title: High Resolution Spectropolarimetry of Penumbral Formation with IBIS Authors: Reardon, Kevin; Casini, R.; Cavallini, F.; Tomczyk, S.; Rouppe van der Voort, L.; Van Noort, M.; Woeger, F.; Socas Navarro, H.; IBIS Team Bibcode: 2006SPD....37.3503R Altcode: 2006BAAS...38..260R We present the results of first spectropolarimetric observations made with the Interferometric Bidimensional Spectrometer (IBIS) at the NSO/Dunn Solar Telescope. The use of narrowband imaging and post-facto reconstruction techniques allows for observations close to the diffraction limit of the vector magnetic field. We will show observations of the the formation of an individual penumbral filament around a small pore. We measure the magnetic field and velocity field of the forming penumbral filament. The spectropolarimetric mode of IBIS will be available to the community in the fall of 2006. Title: Dynamics of an active region filament, fibrils and surges in high resolution Authors: Lin, Y.; Martin, S. F.; Engvold, O.; Rouppe van der Voort, L. H. M.; van Noort, M. Bibcode: 2006cosp...36.3193L Altcode: 2006cosp.meet.3193L High resolution H alpha and magnetograms 0 2 arc sec of an active region were obtained in alternating time series at 42 sec cadences using the Swedish 1-m Solar Telescope on 2004 August 21 The 79 times 52 arcsec field of view was centered at N11 W5 The H alpha filtergrams reveal an active region filament and surges consisting of thread-like structures which have widths similar to the widths of chromospheric fibrils both recorded down to the resolution limit in the best images All observed structures in the active region are highly dynamic The flow speeds in some active filament threads 25 km s -1 are higher than typical speeds in quiescent filament threads Fibrils show counterstreaming strongly resembling the counterstreaming threads in filaments The transverse speeds of mass within fibrils are comparable to the typical speeds of mass flows in quiescent filament threads sim 10 km s -1 Streaming along the threads of surges extending more than 10 arc sec is higher in speed sim 20 km s -1 than in the filament and fibrils and appears to flow over the chromospheric fibrils Small surges near one end of the filament appear to flow into the filament thereby mass seems to be added to the filament Blue shifts seen in the H alpha Dopplergrams confirm the outward mass motion of the surges However in at least one case we also see simultaneous red shifts from the same site in the opposite direction toward the chromosphere We suggest that the site between these two opposite motions identifies the place where magnetic Title: Solar magnetic elements at 0.1 arcsec resolution. II. Dynamical evolution Authors: Rouppe van der Voort, L. H. M.; Hansteen, V. H.; Carlsson, M.; Fossum, A.; Marthinussen, E.; van Noort, M. J.; Berger, T. E. Bibcode: 2005A&A...435..327R Altcode: Small magnetic structures can be seen in G-band filtergrams as isolated bright points, strings of bright points and dark micro-pores. At a resolution of 0.1 arcsec, new forms of magnetic structures are found in strong field areas: elongated “ribbons” and more circular “flowers”. In this paper we study the temporal evolution of these small scale magnetic structures. In strong-field regions the time-evolution is more that of a magnetic fluid than that of collections of flux-tubes that keep their identity. We find that the granular flow concentrates the magnetic field into flux sheets that are visible as thin bright features in the filtergrams. Weak upflows are found in the flux sheets and downflows in the immediate surroundings. The flux sheets often become unstable to a fluting instability and the edges buckle. The sheets tend to break up into strings of bright points, still with weak upflows in the magnetic elements and zero velocity or downflows between them. Where there are larger flux concentrations we find ribbons, flowers and micro-pores. There is a continuous transition between these forms and they evolve from one form to another. The appearance is mostly determined by the horizontal size - larger structures are dark (micro-pores), narrower structures are ribbon shaped and the flowers are the smallest in extent. All these structures have darker inner parts and a bright edge. The plasma is found to be at rest in the ribbons, with small concentrations of weak upflow sites. Narrow sheets with downdrafts are found right at the edges of the magnetic field concentrations. Title: Solar Image Restoration By Use Of Multi-frame Blind De-convolution With Multiple Objects And Phase Diversity Authors: Van Noort, Michiel; Der Voort, Luc Rouppe Van; Löfdahl, Mats G. Bibcode: 2005SoPh..228..191V Altcode: 2005SoPh..228..191N An extension of Joint Phase Diverse Speckle image restoration is presented. Multiple realizations of multiple objects having known wavefront relations with each other can now be restored jointly. As the alignment of the imaging setup does not change, near-perfect alignment can be achieved between different objects, thus greatly reducing false signals in the determination of derived quantities, such as magnetograms, Dopplergrams, etc. The method was implemented in C++ as an image restoration server, to which worker clients can connect and disconnect randomly, so that a large number of CPUs can be used to speed up the restorations. We present a number of examples of applications of the restoration method to observations obtained with the Swedish 1-m Solar Telescope on La Palma. Title: Multidimensional ALI Radiative Transfer in Cartesian, Cylindrical and Spherical Coordinate Systems Authors: van Noort, M.; Hubeny, I.; Lanz, T. Bibcode: 2003ASPC..288..445V Altcode: 2003sam..conf..445V A new Radiative Transfer code that can calculate the non-LTE line transfer problem in a two-level atom formulation in Cartesian, cylindrical and spherical coordinate systems is presented. The transfer equation is solved using the ALI and the short characteristics methods, while allowing for an arbitrary 3-dimensional velocity field.

The code is modularised so that changing geometry can be accomplished by simply setting a switch, and parallelised for use on a networked PC cluster to increase computational speed. The spatial parallelization method is employed. It is found to be robust and efficient, while not relying heavily on fast communication.

The internal accuracy of the code is tested extensively in all three geometries and is shown to be in good agreement with appropriate 1-D solutions. Title: Multidimensional radiative transfer Authors: van Noort, Michiel Jan Bibcode: 2003PhDT........54V Altcode: No abstract at ADS Title: Multidimensional Non-LTE Radiative Transfer. I. A Universal Two-dimensional Short-Characteristics Scheme for Cartesian, Spherical, and Cylindrical Coordinate Systems Authors: van Noort, Michiel; Hubeny, Ivan; Lanz, Thierry Bibcode: 2002ApJ...568.1066V Altcode: We have developed an efficient and robust two-dimensional non-LTE radiation transfer solver appropriate for line transfer in the equivalent two-level atom formalism. The numerical method applies the accelerated lambda iteration technique together with the short-characteristics scheme. The code presented in this paper incorporates all three standard geometries (Cartesian, cylindrical, and spherical) in a transparent way while allowing for arbitrary (three-dimensional) velocity fields. The geometry-specific parts of the radiative transfer solver are modularized so that the change of geometry is accomplished by simply setting the appropriate switch. We have also developed a parallel version of the code, in which we use a parallelization in spatial subdomains, and showed that such a scheme is sufficiently robust. We have performed a number of tests of the performance of the solver in all three geometries. Finally, we discuss the internal accuracy of the transfer solutions depending on the number of spatial, angular, and frequency grid points. Title: Multidimensional, non-LTE Radiation Transport in Astrophysical Media Authors: Hubeny, I.; van Noort, M.; Lanz, T. Bibcode: 1998AAS...193.6918H Altcode: 1998BAAS...30.1352H We present first results of our long-term program whose ultimate goal is to develop multidimensional, self-consistent, non-LTE radiation hydrodynamic and magnetohydrodynamic spectroscopic diagnostics of astronomical objects. As the first step, we have developed an efficient and robust 2-D and 3-D radiation transfer solver, based on the short characteristics method. We assume the source function either to be fully specified (i.e., the so-called formal solution of the transfer equation), or to be given in terms of the equivalent-two-level atom formalism. In the latter case, we have adopted the idea of the Accelerated Lambda Iteration technique to treat iteratively the non-LTE coupling of radiation and matter. We compare the efficiency and numerical accuracy of several variants of the short-characteristics scheme. Finally, we present some illustrative examples of radiative transfer for 2-D and 3-D media with the specified temperature, density, and velocity structure. Title: Fe II emission lines in the UV spectrum of Sirius-A and VEGA Authors: van Noort, M.; Lanz, T.; Lamers, H. J. G. L. M.; Kurucz, R. L.; Ferlet, R.; Hebrard, G.; Vidal-Madjar, A. Bibcode: 1998A&A...334..633V Altcode: 1998astro.ph..3127V We present high-quality HST/GHRS spectra in the Hydrogen Lalpha spectral region of Vega and Sirius-A. Thanks to the signal-to-noise ratio achieved in these observations and to the similarity of the two spectra, we found clear evidence of emission features in the low flux region, lambda lambda 1190-1222 Angstroms. These emission lines can be attributed unambiguously to Fe Ii and Cr Ii transitions. In this spectral range, silicon lines are observed in absorption. We built a series of non-LTE model atmospheres with different, prescribed temperature stratification in the upper atmosphere and treating Fe Ii with various degrees of sophistication in non-LTE. Emission lines are produced by the combined effect of the Schuster mechanism and radiative interlocking, and can be explained without the presence of a chromosphere. Silicon absorption lines and the Lalpha profile set constraints on the presence of a chromosphere, excluding a strong temperature rise in layers deeper than tau_R ~ 10(-4) . Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.