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  • 1. Amarsi, A. M.
    et al.
    Asplund, M.
    Collet, R.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Non-LTE oxygen line formation in 3D hydrodynamic model stellar atmospheres2016Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 455, nr 4, s. 3735-3751Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The O (Iota) 777 nm lines are among the most commonly used diagnostics for the oxygen abundances in the atmospheres of FGK-type stars. However, they form in conditions that are far from local thermodynamic equilibrium (LTE). We explore the departures from LTE of atomic oxygen, and their impact on O I lines, across the STAGGER-grid of three-dimensional hydrodynamic model atmospheres. For the O (Iota) 777 nm triplet, we find significant departures from LTE. These departures are larger in stars with larger effective temperatures, smaller surface gravities, and larger oxygen abundances. We present grids of predicted 3D non-LTE based equivalent widths for the O (Iota) 616 nm, [O (Iota)] 630 nm, [O (I)] 636 nm, and O (Iota) 777 nm lines, as well as abundance corrections to 1D LTE based results.

  • 2. Amarsi, A. M.
    et al.
    Asplund, M.
    Collet, R.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    The Galactic chemical evolution of oxygen inferred from 3D non-LTE spectral-line-formation calculations2015Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 454, nr 1, s. L11-L15Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We revisit the Galactic chemical evolution of oxygen, addressing the systematic errors inherent in classical determinations of the oxygen abundance that arise from the use of one-dimensional (1D) hydrostatic model atmospheres and from the assumption of local thermodynamic equilibrium (LTE). We perform detailed 3D non-LTE radiative-transfer calculations for atomic oxygen lines across a grid of 3D hydrodynamic STAGGER model atmospheres for dwarfs and subgiants. We apply our grid of predicted line strengths of the [O I] 630 nm and O I 777 nm lines using accurate stellar parameters from the literature. We infer a steep decay in [O/Fe] for [Fe/H] greater than or similar to -1.0, a plateau [O/Fe] approximate to 0.5 down to [Fe/H] approximate to -2.5, and an increasing trend for [Fe/H] less than or similar to -2.5. Our 3D non-LTE calculations yield overall concordant results from the two oxygen abundance diagnostics.

  • 3. Bastian, T. S.
    et al.
    Chintzoglou, G.
    De Pontieu, B.
    Shimojo, M.
    Schmit, D.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Loukitcheva, M.
    A First Comparison of Millimeter Continuum and MgII Ultraviolet Line Emission from the Solar Chromosphere2017Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 845, nr 2, artikel-id L19Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present joint observations of the Sun by the Atacama Large Millimeter/submillimeter Array (ALMA) and the Interface Region Imaging Spectrograph (IRIS). Both millimeter/submillimeter-lambda continuum emission and ultraviolet (UV) line emission originate from the solar chromosphere and both have the potential to serve as powerful and complementary diagnostics of physical conditions in this enigmatic region of the solar atmosphere. The observations were made of a solar active region on 2015 December 18 as part of the ALMA science verification effort. A map of the Sun's continuum emission was obtained by ALMA at a wavelength of 1.25 mm (239 GHz). A contemporaneous map was obtained by IRIS in the Mg II h doublet line at 2803.5 angstrom. While a clear correlation between the 1.25 mm brightness temperature T-B and the Mg II h line radiation temperature T-rad is observed, the slope is <1, perhaps as a result of the fact that these diagnostics are sensitive to different parts of the chromosphere and that the Mg II h line source function includes a scattering component. There is a significant difference (35%) between the mean TB (1.25 mm) and mean T-rad (Mg II). Partitioning the maps into sunspot, quiet areas, and plage regions we find the relation between the IRIS Mg II h line Trad and the ALMA TB region-dependent. We suggest this may be the result of regional dependences of the formation heights of the IRIS and ALMA diagnostics and/or the increased degree of coupling between the UV source function and the local gas temperature in the hotter, denser gas in plage regions.

  • 4.
    Bjørgen, Johan P.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Numerical non-LTE 3D radiative transfer using a multigrid method2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, artikel-id A118Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. 3D non-LTE radiative transfer problems are computationally demanding, and this sets limits on the size of the problems that can be solved. So far, multilevel accelerated lambda iteration (MALI) has been the method of choice to perform high-resolution computations in multidimensional problems. The disadvantage of MALI is that its computing time scales as O(n(2)), with n the number of grid points. When the grid becomes finer, the computational cost increases quadratically. Aims. We aim to develop a 3D non-LTE radiative transfer code that is more efficient than MALI. Methods. We implement a non-linear multigrid, fast approximation storage scheme, into the existing Multi3D radiative transfer code. We verify our multigrid implementation by comparing with MALI computations. We show that multigrid can be employed in realistic problems with snapshots from 3D radiative magnetohydrodynamics (MHD) simulations as input atmospheres. Results. With multigrid, we obtain a factor 3.3-4.5 speed-up compared to MALI. With full-multigrid, the speed-up increases to a factor 6. The speed-up is expected to increase for input atmospheres with more grid points and finer grid spacing. Conclusions. Solving 3D non-LTE radiative transfer problems using non-linear multigrid methods can be applied to realistic atmospheres with a substantial increase in speed.

  • 5.
    Bjørgen, Johan P.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Sukhorukov, Andrii V.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. National Academy of Sciences of Ukraine, Ukraine.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Carlsson, Mats
    de la Cruz Rodríguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Scharmer, Göran B.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Hansteen, Viggo H.
    Three-dimensional modeling of the Ca II H and K lines in the solar atmosphere2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 611, artikel-id A62Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 6. Carlsson, Mats
    et al.
    Hansteen, Viggo H.
    Gudiksen, Boris V.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    De Pontieu, Bart
    A publicly available simulation of an enhanced network region of the Sun2016Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 585, artikel-id A4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The solar chromosphere is the interface between the solar surface and the solar corona. Modelling of this region is difficult because it represents the transition from optically thick to thin radiation escape, from gas-pressure domination to magnetic-pressure domination, from a neutral to an ionised state, from MHD to plasma physics, and from near-equilibrium (LTE) to non-equilibrium conditions. Aims. Our aim is to provide the community with realistic simulations of the magnetic solar outer atmosphere. This will enable detailed comparison of existing and upcoming observations with synthetic observables from the simulations, thereby elucidating the complex interactions of magnetic fields and plasma that are crucial for our understanding of the dynamic outer atmosphere. Methods. We used the radiation magnetohydrodynamics code Bifrost to perform simulations of a computational volume with a magnetic field topology similar to an enhanced network area on the Sun. Results. The full simulation cubes are made available from the Hinode Science Data Centre Europe. The general properties of the simulation are discussed, and limitations are discussed.

  • 7. Carlsson, Mats
    et al.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    De Pontieu, Bart
    WHAT DO IRIS OBSERVATIONS OF Mg II k TELL US ABOUT THE SOLAR PLAGE CHROMOSPHERE?2015Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 809, nr 2, artikel-id L30Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We analyze observations from the Interface Region Imaging Spectrograph of the Mg II k line, the Mg II UV subordinate lines, and the O I 135.6 nm line to better understand the solar plage chromosphere. We also make comparisons with observations from the Swedish 1-m Solar Telescope of the H alpha line, the Ca II 8542 line, and Solar Dynamics Observatory/Atmospheric Imaging Assembly observations of the coronal 19.3 nm line. To understand the observed Mg II profiles, we compare these observations to the results of numerical experiments. The single-peaked or flat-topped Mg II k profiles found in plage imply a transition region at a high column mass and a hot and dense chromosphere of about 6500 K. This scenario is supported by the observed large-scale correlation between moss brightness and filled-in profiles with very little or absent self-reversal. The large wing width found in plage also implies a hot and dense chromosphere with a steep chromospheric temperature rise. The absence of emission in the Mg II subordinate lines constrain the chromospheric temperature and the height of the temperature rise while the width of the O I 135.6 nm line sets a limit to the non-thermal velocities to around 7 km s(-1).

  • 8.
    da Silva Santos, Joäo Manuel
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    de la Cruz Rodriguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Temperature constraints from inversions of synthetic solar optical, UV, and radio spectra2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, artikel-id A124Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 9.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Asensio Ramos, Andrés
    NON-LTE INVERSIONS OF THE Mg II h & k AND UV TRIPLET LINES2016Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 830, nr 2, artikel-id L30Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 10.
    de la Cruz Rodriguez, Jaime
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Danilovic, Sanja
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Uitenbroek, H.
    STiC: A multiatom non-LTE PRD inversion code for full-Stokes solar observations2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, artikel-id A74Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The inference of the underlying state of the plasma in the solar chromosphere remains extremely challenging because of the nonlocal character of the observed radiation and plasma conditions in this layer. Inversion methods allow us to derive a model atmosphere that can reproduce the observed spectra by undertaking several physical assumptions. The most advanced approaches involve a depth-stratified model atmosphere described by temperature, line-of-sight velocity, turbulent velocity, the three components of the magntic field vector, and gas and electron pressure. The parameters of the radiative transfer equation are computed from a solid ground of physical principles. In order to apply these techniques to spectral lines that sample the chromosphere, nonlocal thermodynamical equilibrium effects must be included in the calculations. We developed a new inversion code STiC (STockholm inversion Code) to study spectral lines that sample the upper chromosphere. The code is based on the RH forward synthesis code, which we modified to make the inversions faster and more stable. For the first time, STiC facilitates the processing of lines from multiple atoms in non-LTE, also including partial redistribution effects (PRD) in angle and frequency of scattered photons. Furthermore, we include a regularization strategy that allows for model atmospheres with a complex depth stratification, without introducing artifacts in the reconstructed physical parameters, which are usually manifested in the form of oscillatory behavior. This approach takes steps toward a node-less inversion, in which the value of the physical parameters at each grid point can be considered a free parameter. In this paper we discuss the implementation of the aforementioned techniques, the description of the model atmosphere, and the optimizations that we applied to the code. We carry out some numerical experiments to show the performance of the code and the regularization techniques that we implemented. We made STiC publicly available to the community.

  • 11. Golding, T. P.
    et al.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Carlsson, M.
    Formation of the helium extreme-UV resonance lines2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 597, artikel-id A102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. While classical models successfully reproduce intensities of many transition region lines, they predict helium extreme-UV (EUV) line intensities roughly an order of magnitude lower than the observed value. Aims. Our aim is to determine the relevant formation mechanism(s) of the helium EUV resonance lines capable of explaining the high intensities under quiet Sun conditions. Methods. We synthesised and studied the emergent spectra from a 3D radiation-magnetohydrodynamics simulation model. The effects of coronal illumination and non-equilibrium ionisation of hydrogen and helium are included self-consistently in the numerical simulation. Results. Radiative transfer calculations result in helium EUV line intensities that are an order of magnitude larger than the intensities calculated under the classical assumptions. The enhanced intensity of HeI lambda 584 is primarily caused by He II recombination cascades. The enhanced intensity of He II lambda 304 and He II lambda 256 is caused primarily by non-equilibrium helium ionisation. Conclusions. The analysis shows that the long standing problem of the high helium EUV line intensities disappears when taking into account optically thick radiative transfer and non-equilibrium ionisation effects.

  • 12. Golding, Thomas Peter
    et al.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Carlsson, Mats
    NON-EQUILIBRIUM HELIUM IONIZATION IN AN MHD SIMULATION OF THE SOLAR ATMOSPHERE2016Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 817, nr 2, artikel-id 125Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The ionization state of the gas in the dynamic solar chromosphere can depart strongly from the instantaneous statistical equilibrium commonly assumed in numerical modeling. We improve on earlier simulations of the solar atmosphere that only included non-equilibrium hydrogen ionization by performing a 2D radiation-magnetohydrodynamics simulation featuring non-equilibrium ionization of both hydrogen and helium. The simulation includes the effect of hydrogen Lya and the EUV radiation from the corona on the ionization and heating of the atmosphere. Details on code implementation are given. We obtain helium ion fractions that are far from their equilibrium values. Comparison with models with local thermodynamic equilibrium (LTE) ionization shows that non-equilibrium helium ionization leads to higher temperatures in wavefronts and lower temperatures in the gas between shocks. Assuming LTE ionization results in a thermostat-like behavior with matter accumulating around the temperatures where the LTE ionization fractions change rapidly. Comparison of DEM curves computed from our models shows that non-equilibrium ionization leads to more radiating material in the temperature range 11-18 kK, compared to models with LTE helium ionization. We conclude that non-equilibrium helium ionization is important for the dynamics and thermal structure of the upper chromosphere and transition region. It might also help resolve the problem that intensities of chromospheric lines computed from current models are smaller than those observed.

  • 13. Hansteen, V. H.
    et al.
    Archontis, V.
    Pereira, T. M. D.
    Carlsson, M.
    Rouppe van der Voort, L.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Bombs and Flares at the Surface and Lower Atmosphere of the Sun2017Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 839, nr 1, artikel-id 22Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A spectacular manifestation of solar activity is the appearance of transient brightenings in the far wings of the Ha line, known as Ellerman bombs (EBs). Recent observations obtained by the Interface Region Imaging Spectrograph have revealed another type of plasma bombs (UV bursts) with high temperatures of perhaps up to 8 x 10(4) K within the cooler lower solar atmosphere. Realistic numerical modeling showing such events is needed to explain their nature. Here, we report on 3D radiative magnetohydrodynamic simulations of magnetic flux emergence in the solar atmosphere. We find that ubiquitous reconnection between emerging bipolar magnetic fields can trigger EBs in the photosphere, UV bursts in the mid/low chromosphere and small (nano-/micro-) flares (10(6) K) in the upper chromosphere. These results provide new insights into. the emergence and build up of the coronal magnetic field and the dynamics and heating of the solar surface and lower atmosphere.

  • 14. Jurčák, Jan
    et al.
    Collados, Manuel
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    van Noort, Michiel
    Schlichenmaier, Rolf
    Recent advancements in the EST project2019Ingår i: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 63, nr 4, s. 1389-1395Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 15.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Tracing the evolution of radiation-MHD simulations of solar and stellar atmospheres in the Lagrangian frame2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, artikel-id A136Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Radiation magnetohydrodynamics (radiation-MHD) simulations have become a standard tool for investigating the physics of solar and stellar atmospheres. Aims. The aim of this paper is to present a method that allows the efficient and accurate analysis of flows in such simulations in the Lagrangian frame. Methods. This paper presents a method that allows the construction of pathlines given a seed point that can be chosen freely at any location and at any time during the simulation where the simulation state is stored. The method is based on passive tracer particles. Through injection of particles in expanding regions the occurrence of particle-free volumes is avoided, even in the case of strongly compressive flows. Results. The method was implemented in the solar and stellar atmosphere simulation code Bifrost. It is efficient and accurate. As examples I present an analysis of a gas parcel in the convection zone and a particle in the solar transition region.

  • 16.
    Leenaarts, Jorrit
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Carlsson, Mats
    van der Voort, Luc Rouppe
    On Fibrils and Field Lines: the Nature of Hα Fibrils in the Solar Chromosphere2015Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 802, nr 2, artikel-id 136Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Observations of the solar chromosphere in the line. core of the H alpha line show dark elongated structures called fibrils that show swaying motion. We performed a three-dimensional radiation-MHD simulation of a network region. and computed synthetic Ha images from this simulation to investigate the relation between fibrils and the magnetic field lines in the chromosphere. The periods, amplitudes, and phase. speeds of the simulated fibrils are consistent with observations. We find that some fibrils trace out the same field line along the fibril's length, while other fibrils sample different field lines at different locations along their length. Fibrils sample the same field lines on a timescale of similar to 200 s. This is shorter than their own lifetime. Fibril-threading field lines carry slow-mode waves, as well as transverse waves propagating with the Alfven speed. Transverse waves propagating in opposite directions cause an interference pattern with complex apparent phase speeds. The relationship between fibrils and field lines is governed by constant migration and swaying of the field lines, their mass loading and draining, and their visibility in Ha. Field lines are visible where they lie close to the optical depth unity surface. The location of the latter is at a height at which the column mass reaches a certain fixed value. The visibility of the field line is thus determined by its own mass density and by the mass density of the material above it. Using the swaying motion of fibrils as a tracer of chromospheric transverse oscillations must be done with caution.

  • 17.
    Leenaarts, Jorrit
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. University of Oslo, Norway.
    de la Cruz Rodriguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Kochukhov, Oleg
    Carlsson, Mats
    THE EFFECT OF ISOTOPIC SPLITTING ON THE BISECTOR AND INVERSIONS OF THE SOLAR Ca II 854.2 nm LINE2014Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 784, nr 1, s. L17-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 18.
    Leenaarts, Jorrit
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    de la Cruz Rodríguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Danilovic, Sanja
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Scharmer, Göran
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Carlsson, Mats
    Chromospheric heating during flux emergence in the solar atmosphere2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, artikel-id A28Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 19.
    Leenaarts, Jorrit
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Golding, Thomas
    Carlsson, Mats
    Libbrecht, Tine
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Joshi, Jayant
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    The cause of spatial structure in solar He I 1083 nm multiplet images2016Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 594, artikel-id A104Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The HeI 1083 nm is a powerful diagnostic for inferring properties of the upper solar chromosphere, in particular for the magnetic field. The basic formation of the line in one-dimensional models is well understood, but the influence of the complex three-dimensional structure of the chromosphere and corona has however never been investigated. This structure must play an essential role because images taken in HeI 1083 nm show structures with widths down to 100 km. Aims. We aim to understand the effect of the three-dimensional temperature and density structure in the solar atmosphere on the formation of the HeI 1083 nm line. Methods. We solved the non-LTE radiative transfer problem assuming statistical equilibrium for a simple nine-level helium atom that nevertheless captures all essential physics. As a model atmosphere we used a snapshot from a 3D radiation-MHD simulation computed with the Bifrost code. Ionising radiation from the corona was self-consistently taken into account. Results. The emergent intensity in the HeI 1083 nm is set by the source function and the opacity in the upper chromosphere. The former is dominated by scattering of photospheric radiation and does not vary much with spatial location. The latter is determined by the photonionisation rate in the HeI ground state continuum, as well as the electron density in the chromosphere. The spatial variation of the flux of ionising radiation is caused by the spatially-structured emissivity of the ionising photons from material at T approximate to 100 kK in the transition region. The hotter coronal material produces more ionising photons, but the resulting radiation field is smooth and does not lead to small-scale variation of the UV flux. The corrugation of the transition region further increases the spatial variation of the amount of UV radiation in the chromosphere. Finally we find that variations in the chromospheric electron density also cause strong variation in Het 1083 nm opacity. We compare our findings to observations using SST, IRIS and SDO/AIA data.

  • 20.
    Libbrecht, Tine
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    de la Cruz Rodríguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Danilovic, Sanja
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Pazira, Hiva
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Chromospheric condensations and magnetic field in a C3.6-class flare studied via He I D-3 spectro-polarimetry2019Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, artikel-id A35Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Magnetic reconnection during flares takes place in the corona, but a substantial part of flare energy is deposited in the chromosphere. However, high-resolution spectro-polarimetric chromospheric observations of flares are very rare. The most used observables are Ca II 8542 angstrom and He I 10830 angstrom.

    Aims. We aim to study the chromosphere during a C3.6 class flare via spectro-polarimetric observations of the He I D-3 line.

    Methods. We present the first SST/CRISP spectro-polarimetric observations of He I D3. We analyzed the data using the inversion code HAZEL, and estimate the line-of-sight velocity and the magnetic field vector.

    Results. Strong He I D-3 emission at the flare footpoints, as well as strong He I D(3 )absorption profiles tracing the flaring loops are observed during the flare. The He I D-3 traveling emission kernels at the flare footpoints exhibit strong chromospheric condensations of up to similar to 60 km s(-1) at their leading edge. Our observations suggest that such condensations result in shocking the deep chromosphere, causing broad and modestly blueshifted He I D-3 profiles indicating subsequent upflows. A strong and rather vertical magnetic field of up to similar to 2500 G is measured in the flare footpoints, confirming that the He I D-3 line is likely formed in the deep chromosphere at those locations. We provide chromospheric line-of-sight velocity and magnetic field maps obtained via He I D-3 inversions. We propose a fan-spine configuration as the flare magnetic field topology.

    Conclusions. The He I D-3 line is an excellent diagnostic to study the chromosphere during flares. The impact of strong condensations on the deep chromosphere has been observed. Detailed maps of the flare dynamics and the magnetic field are obtained.

  • 21.
    Libbrecht, Tine
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Joshi, Jayant
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    de la Cruz Rodríguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Asensio Ramos, Andrés
    Observations of Ellerman bomb emission features in He I D-3 and He I 10 830 angstrom2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 598, artikel-id A33Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

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

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

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

  • 22.
    Libbrecht, Tine
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Pires Bjørgen, Johan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    de la Cruz Rodríguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Hansteen, Viggo
    Joshi, Jayant
    Line formation of He I D3 and He I 10830 Å in a small-scale reconnection eventManuskript (preprint) (Övrigt vetenskapligt)
  • 23. Lin, Hsiao-Hsuan
    et al.
    Carlsson, Mats
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. University of Oslo, Norway.
    The Formation of IRIS Diagnostics. IX. The Formation of the CI 135.58 NM Line in the Solar Atmosphere2017Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 846, nr 1, artikel-id 40Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The C I 135.58 nm line is located in the wavelength range of NASA's Interface Region Imaging Spectrograph (IRIS) small explorer mission. We study the formation and diagnostic potential of this line by means of non local-thermodynamic-equilibrium modeling, employing both 1D and 3D radiation-magnetohydrodynamic models. The C I/C II ionization balance is strongly influenced by photoionization by Ly alpha emission. The emission in the C I 135.58 nm line is dominated by a recombination cascade and the line forming region is optically thick. The Doppler shift of the line correlates strongly with the vertical velocity in its line forming region, which is typically located at 1.5 Mm height. With IRIS, the C I 135.58 nm line is usually observed together with the O I 135.56 nm line, and from the Doppler shift of both lines, we obtain the velocity difference between the line forming regions of the two lines. From the ratio of the C I/O I line core intensity, we can determine the distance between the C I and the O I forming layers. Combined with the velocity difference, the velocity gradient at mid-chromospheric heights can be derived. The C I/O I total intensity line ratio is correlated with the inverse of the electron density in the mid-chromosphere. We conclude that the C I 135.58 nm line is an excellent probe of the middle chromosphere by itself, and together with the O I 135.56 nm line the two lines provide even more information, which complements other powerful chromospheric diagnostics of IRIS such as the Mg II h and k lines and the C II lines around 133.5 nm.

  • 24. Lind, K.
    et al.
    Amarsi, A. M.
    Asplund, M.
    Barklem, P. S.
    Bautista, M.
    Bergemann, M.
    Collet, R.
    Kiselman, Dan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Pereira, T. M. D.
    Non-LTE line formation of Fe in late-type stars - IV. Modelling of the solar centre-to-limb variation in 3D2017Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, nr 4, s. 4311-4322Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Our ability to model the shapes and strengths of iron lines in the solar spectrum is a critical test of the accuracy of the solar iron abundance, which sets the absolute zero-point of all stellar metallicities. We use an extensive 463-level Fe atom with new photoionization cross-sections for Fe I and quantum mechanical calculations of collisional excitation and charge transfer with neutral hydrogen; the latter effectively remove a free parameter that has hampered all previous line formation studies of Fe in non-local thermodynamic equilibrium (NLTE). For the first time, we use realistic 3D NLTE calculations of Fe for a quantitative comparison to solar observations. We confront our theoretical line profiles with observations taken at different viewing angles across the solar disc with the Swedish 1-m Solar Telescope. We find that 3D modelling well reproduces the observed centre-to-limb behaviour of spectral lines overall, but highlight aspects that may require further work, especially cross-sections for inelastic collisions with electrons. Our inferred solar iron abundance is log(epsilon(Fe)) = 7.48 +/- 0.04 dex.

  • 25. Nordlander, T.
    et al.
    Amarsi, A. M.
    Lind, K.
    Asplund, M.
    Barklem, P. S.
    Casey, A. R.
    Collet, R.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    3D NLTE analysis of the most iron-deficient star, SMSS0313-67082017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 597, artikel-id A6Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Models of star formation in the early universe require a detailed understanding of accretion, fragmentation and radiative feedback in metal-free molecular clouds. Different simulations predict different initial mass functions of the first stars, ranging from predominantly low-mass (0.1-10 M-circle dot), to massive (10-100 M-circle dot), or even supermassive (100-1000 M-circle dot). The mass distribution of the first stars should lead to unique chemical imprints on the low-mass second and later generation metal-poor stars still in existence. The chemical composition of SMSS0313-6708, which has the lowest abundances of Ca and Fe of any star known, indicates it was enriched by a single massive supernova. Aims. The photospheres of metal-poor stars are relatively transparent in the UV, which may lead to large three-dimensional (3D) effects as well as departures from local thermodynamical equilibrium (LTE), even for weak spectral lines. If 3D effects and departures from LTE (NLTE)are ignored or treated incorrectly, errors in the inferred abundances may significantly bias the inferred properties of the polluting supernovae. We redetermine the chemical composition of SMSS0313-6708 by means of the most realistic methods available, and compare the results to predicted supernova yields. Methods. A 3D hydrodynamical Stagger model atmosphere and 3D NLTE radiative transfer were applied to obtain accurate abundances for Li, Na, Mg, Al, Ca and Fe. The model atoms employ realistic collisional rates, with no calibrated free parameters. Results. We find significantly higher abundances in 3D NLTE than 1D LTE by 0.8 dex for Fe, and 0.5 dex for Mg, Al and Ca, while Li and Na are unaffected to within 0.03 dex. In particular, our upper limit for [Fe/H] is now a factor ten larger, at [Fe/H] < -6.53 (3 sigma), than previous estimates based on < 3D > NLTE (i.e., using averaged 3D models). This higher estimate is due to a conservative upper limit estimation, updated NLTE data, and 3D-< 3D > NLTE differences, all of which lead to a higher abundance determination. Conclusions. We find that supernova yields for models in a wide range of progenitor masses reproduce the revised chemical composition. In addition to massive progenitors of 20-60 M-circle dot exploding with low energies (1-2 B, where 1 B = 10(51) erg), we also find good fits for progenitors of 10 M-circle dot, with very low explosion energies (< 1 B). We cannot reconcile the new abundances with supernovae or hypernovae with explosion energies above 2.5 B, nor with pair-instability supernovae.

  • 26.
    Pazira, Hiva
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Kiselman, Dan
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Solar off-limb emission of the O I 7772 angstrom line2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 604, artikel-id A49Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. The aim of this paper is to understand the formation of the O I line at 7772 angstrom in the solar chromosphere. Methods. We used SST/CRISP observations to observe O I 7772 angstrom in several places around the solar limb. We compared the observations with synthetic spectra calculated with the RH code in the one-dimension spherical geometry mode. New accurate hydrogen collisional rates were included for the RH calculations. Results. The observations reveal a dark gap in the lower chromosphere, which is caused by variations in the line opacity as shown by our models. The lower level of the 7772 angstrom transition is populated by a downward cascade from the continuum. We study the effect of Lyman-beta pumping and hydrogen collisions between the triplet and quintet system in O I. Both have a small but non-negligible influence on the line intensity.

  • 27.
    Robustini, Carolina
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Esteban Pozuelo, Sara
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    de la Cruz Rodríguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Chromospheric observations and magnetic configuration of a supergranular structure2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, artikel-id A1Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Unipolar magnetic regions are often associated with supergranular cells. The chromosphere above these regions is regulated by the magnetic field, but the field structure is poorly known. In unipolar regions, the fibrillar arrangement does not always coincide with magnetic field lines, and polarimetric observations are needed to establish the chromospheric magnetic topology. Aims. In an active region close to the limb, we observed a unipolar annular network of supergranular size. This supergranular structure harbours a radial distribution of the fibrils converging towards its centre. We aim to improve the description of this structure by determining the magnetic field configuration and the line-of-sight velocity distribution in both the photosphere and the chromosphere. Methods. We observed the supergranular structure at different heights by taking data in the Fe I 6301-6302 angstrom, H alpha, Ca II 8542 angstrom, and the Ca II H&K spectral lines with the CRisp Imaging SpectroPolarimeter (CRISP) and CHROMospheric Imaging Spectrometer (CHROMIS) at the Swedish 1-m Solar Telescope. We performed Milne-Eddington inversions of the spectropolarimetric data of Fe I 6301-6302 angstrom and applied the weak field approximation to Ca II 8542 angstrom data to retrieve the magnetic field in the photosphere and chromosphere. We used photospheric magnetograms of CRISP, Hinode Solar Optical Telescope spectropolarimeter, and Helioseismic and Magnetic Imager to calculate the magnetic flux. We investigated the velocity distribution using the line-of-sight velocities computed from the Milne-Eddington inversion and from the Doppler shift of the K-3 feature in the Ca II K spectral line. To describe the typical spectral profiles characterising the chromosphere above the inner region of the supergranular structure, we performed a K-mean clustering of the spectra in Ca II K. Results. The photospheric magnetic flux shows that the supergranular boundary has an excess of positive polarity and the whole structure is not balanced. The magnetic field vector at chromospheric heights, retrieved by the weak field approximation, indicates that the field lines within the supergranular cell tend to point inwards, and might form a canopy above the unipolar region. In the centre of the supergranular cell hosting the unipolar region, we observe a persistent chromospheric brightening coinciding with a strong gradient in the line-of-sight velocity.

  • 28.
    Robustini, Carolina
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    de la Cruz Rodriguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    van der Voort, Luc Rouppe
    Fan-shaped jets above the light bridge of a sunspot driven by reconnection2016Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 590, artikel-id A57Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 29.
    Robustini, Carolina
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    de la Cruz Rodríguez, Jaime
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    The chromosphere above a delta-sunspot in the presence of fan-shaped jets2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 609, artikel-id A14Artikel i tidskrift (Refereegranskat)
    Abstract [en]

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

  • 30. Schmit, D.
    et al.
    Bryans, P.
    De Pontieu, B.
    McIntosh, S.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Carlsson, M.
    OBSERVED VARIABILITY OF THE SOLAR Mg II h SPECTRAL LINE2015Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 811, nr 2, artikel-id 127Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Mg II h&k doublet are two of the primary spectral lines observed by the Sun-pointing Interface Region Imaging Spectrograph (IRIS). These lines are tracers of the magnetic and thermal environment that spans from the photosphere to the upper chromosphere. We use a double-Gaussian model to fit the Mg II h profile for a full-Sun mosaic data set taken on 2014 August 24. We use the ensemble of high-quality profile fits to conduct a statistical study on the variability of the line profile as it relates the magnetic structure, dynamics, and center-to-limb viewing angle. The average internetwork profile contains a deeply reversed core and is weakly asymmetric at h2. In the internetwork, we find a strong correlation between h3 wavelength and profile asymmetry as well as h1 width and h2 width. The average reversal depth of the h3 core is inversely related to the magnetic field. Plage and sunspots exhibit many profiles that do not contain a reversal. These profiles also occur infrequently in the internetwork. We see indications of magnetically aligned structures in plage and network in statistics associated with the line core, but these structures are not clear or extended in the internetwork. The center-to-limb variations are compared to predictions of semi-empirical model atmospheres. We measure a pronounced limb darkening in the line core that is not predicted by the model. The aim of this work is to provide a comprehensive measurement baseline and preliminary analysis on the observed structure and formation of the Mg II profiles observed by IRIS.

  • 31. Stepan, J.
    et al.
    Trujillo Bueno, J.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Carlsson, M.
    THREE-DIMENSIONAL RADIATIVE TRANSFER SIMULATIONS OF THE SCATTERING POLARIZATION OF THE HYDROGEN LY alpha LINE IN A MAGNETOHYDRODYNAMIC MODEL OF THE CHROMOSPHERE-CORONA TRANSITION REGION2015Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 803, nr 2, artikel-id 65Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Probing the magnetism of the upper solar chromosphere requires measuring and modeling the scattering polarization produced by anisotropic radiation pumping in UV spectral lines. Here we apply PORTA (a novel radiative transfer code) to investigate the hydrogen Ly alpha line in a three-dimensional model of the solar atmosphere resulting from a state of the art magnetohydrodynamic (MHD) simulation. At full spatial resolution the linear polarization signals are very significant all over the solar disk, with a large fraction of the field of view (FOV) showing line-center amplitudes well above the 1% level. Via the Hanle effect the line-center polarization signals are sensitive to the magnetic field of the model's transition region, even when its mean field strength is only 15 G. The breaking of the axial symmetry of the radiation field produces significant forward-scattering polarization in Ly alpha, without the need of an inclined magnetic field. Interestingly, the Hanle effect tends to decrease such forward-scattering polarization signals in most of the points of the FOV. When the spatial resolution is degraded, the line-center polarization of Ly alpha drops below the 1% level, reaching values similar to those previously found in one-dimensional (1D) semi-empirical models (i.e., up to about 0.5%). The center to limb variation (CLV) of the spatially averaged polarization signals is qualitatively similar to that found in 1D models, with the largest line-center amplitudes at mu = cos theta approximate to 0.4 (theta being the heliocentric angle). These results are important, both for designing the needed space-based instrumentation and for a reliable interpretation of future observations of the Ly alpha polarization.

  • 32.
    Sukhorukov, Andrii V.
    et al.
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Main Astronomical Observatory, National Academy of Sciences of Ukraine, Ukraine.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Partial redistribution in 3D non-LTE radiative transfer in solar-atmosphere models2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 597, artikel-id A46Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Resonance spectral lines such as H I Ly alpha, Mg II h&k, and Ca II H& K that form in the solar chromosphere, are influenced by the effects of 3D radiative transfer as well as partial redistribution (PRD). So far no one has modeled these lines including both effects simultaneously owing to the high computing demands of existing algorithms. Such modeling is, however, indispensable for accurate diagnostics of the chromosphere. Aims. We present a computationally tractable method to treat PRD scattering in 3D model atmospheres using a 3D non-local thermodynamic equilibrium (non-LTE) radiative transfer code. Methods. To make the method memory-friendly, we use the hybrid approximation for the redistribution integral. To make the method fast, we use linear interpolation on equidistant frequency grids. We verify our algorithm against computations with the RH code and analyze it for stability, convergence, and usefulness of acceleration using model atoms of Mg i i with the h&k lines and H i with the Ly alpha line treated in PRD. Results. A typical 3D PRD solution can be obtained in a model atmosphere with 252 x 252 x 496 coordinate points in 50 000-200 000 CPU hours, which is a factor ten slower than computations assuming complete redistribution. We illustrate the importance of the joint action of PRD and 3D effects for the Mg II h&k lines for disk-center intensities, as well as the center-to-limb variation. Conclusions. The proposed method allows for the simulation of PRD lines in a time series of radiation-magnetohydrodynamic models, in order to interpret observations of chromospheric lines at high spatial resolution.

  • 33. Zacharias, P.
    et al.
    Hansteen, V. H.
    Leenaarts, Jorrit
    Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi.
    Carlsson, M.
    Gudiksen, B. V.
    Disentangling flows in the solar transition region2018Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 614, artikel-id A110Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The measured average velocities in solar and stellar spectral lines formed at transition region temperatures have been difficult to interpret. The dominant redshifts observed in the lower transition region naturally leads to the question of how the upper layers of the solar (and stellar) atmosphere can be maintained. Likewise, no ready explanation has been made for the average blueshifts often found in upper transition region lines. However, realistic three-dimensional radiation magnetohydrodynamics (3D rMHD) models of the solar atmosphere are able to reproduce the observed dominant line shifts and may thus hold the key to resolve these issues. Aims. These new 3D rMHD simulations aim to shed light on how mass flows between the chromosphere and corona and on how the coronal mass is maintained. These simulations give new insights into the coupling of various atmospheric layers and the origin of Doppler shifts in the solar transition region and corona. Methods. The passive tracer particles, so-called corks, allow the tracking of parcels of plasma over time and thus the study of changes in plasma temperature and velocity not only locally, but also in a co-moving frame. By following the trajectories of the corks, we can investigate mass and energy flows and understand the composition of the observed velocities. Results. Our findings show that most of the transition region mass is cooling. The preponderance of transition region redshifts in the model can be explained by the higher percentage of downflowing mass in the lower and middle transition region. The average upflows in the upper transition region can be explained by a combination of both stronger upflows than downflows and a higher percentage of upflowing mass. The most common combination at lower and middle transition region temperatures are corks that are cooling and traveling downward. For these corks, a strong correlation between the pressure gradient along the magnetic field line and the velocity along the magnetic field line has been observed, indicating a formation mechanism that is related to downward propagating pressure disturbances. Corks at upper transition region temperatures are subject to a rather slow and highly variable but continuous heating process. Conclusions. Corks are shown to be an essential tool in 3D rMHD models in order to study mass and energy flows. We have shown that most transition region plasma is cooling after having been heated slowly to upper transition region temperatures several minutes before. Downward propagating pressure disturbances are identified as one of the main mechanisms responsible for the observed redshifts at transition region temperatures.

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