Mikolaj Szydlarski

• Enerhaug, Elisabeth; Carlsson, M.; Szydlarski, Mikolaj; Gudiksen, Boris Vilhelm & Moortel, Ineke de (2025). Identifying magnetohydrodynamic wave modes in the solar atmosphere: Experiments in 2D. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 701. doi: 10.1051/0004-6361/202554210. Fulltekst i vitenarkiv Vis sammendrag

  Context. Magnetohydrodynamic (MHD) waves are ubiquitous in the solar atmosphere. They are thought to play an important role in maintaining the temperature of the solar corona and are important components of coronal seismology. However, research into reliable wave mode identification schemes is still in its early stages. Such a scheme would be a valuable tool for understanding the role of waves in the coronal heating problem. A widely renowned 2003 study in 2D on magnetoacoustic gravity (MAG) waves crossing the β  = 1 layer in the atmosphere provided valuable insights into wave propagation, from the lower to the upper atmosphere. The in-depth analyses of wave propagation through the solar atmosphere offer a valuable reference, which can be used to test other wave mode identifier schemes that could subsequently be expanded to 3D. Aims. We aim to analyse a set of wave mode identification components designed to isolate properties of fast and slow MAG waves. Methods. We recreated the 2003 experiment using the numerical code Bifrost with simplified boundary conditions. We then compared the existing wave analysis to our own scheme. Results. We show that our wave mode identification scheme is equivalent to that of the scheme used in the 2003 study. We show how physical properties such as steepening, maximum, and minimum can be deduced from the scheme. As these wave mode identifiers can be expanded to 3D, the recreation of the piston experiment along with the careful analysis of the components opens the door for further studies into how MAG waves are transmitted across the β  = 1 layer.

• Saberi, Maryam; Jafarzadeh, S.; Wedemeyer, Sven; Gafeira, R.; Szydlarski, Mikolaj & Jess, D. B. [Vis alle 7 forfattere av denne artikkelen] (2025). The Sun at millimeter wavelengths: V. Magnetohydrodynamic waves in a fibrillar structure. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202451833. Fulltekst i vitenarkiv Vis sammendrag

  Magnetohydrodynamic (MHD) waves, playing a crucial role in transporting energy through the solar atmosphere, manifest in various chromospheric structures. Here, we investigated MHD waves in a long-lasting dark fibril using high-temporal-resolution (2 s cadence) Atacama Large Millimeter/submillimeter Array (ALMA) observations in Band 6 (centered at 1.25 mm). We detected oscillations in brightness temperature, horizontal displacement, and width at multiple locations along the fibril, with median periods and standard deviations of 240 ± 114 s, 225 ± 102 s, and 272 ± 118 s, respectively. Wavelet analysis revealed a combination of standing and propagating waves, suggesting the presence of both MHD kink and sausage modes. Less dominant than standing waves, oppositely propagating waves exhibit phase speeds (median and standard deviation of distributions) of 74 ± 204 km/s, 52 ± 197 km/s, and 28 ± 254 km/s for the three observables, respectively. This work demonstrates ALMA’s capability to effectively sample dynamic fibrillar structures, despite previous doubts. This provides valuable insights into wave dynamics in the upper chromosphere.

• Cherry, George; Szydlarski, Mikolaj & Gudiksen, Boris Vilhelm (2024). Implementation of thermal conduction energy transfer models in the Bifrost solar atmosphere MHD code. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 692. doi: 10.1051/0004-6361/202452012. Fulltekst i vitenarkiv Vis sammendrag

  Context. Thermal conductivity provides important contributions to the energy evolution of the upper solar atmosphere, behaving as a non-linear concentration-dependent diffusion equation. Recently, different methods have been offered as best-fit solutions to these problems in specific situations, but their effectiveness and limitations are rarely discussed. Aims. We have rigorously tested the different implementations of solving the conductivity flux, in the massively parallel magnetohydrodynamics code, Bifrost, with the aim of specifying the best scenarios for the use of each method. Methods. We compared the differences and limitations of explicit versus implicit methods, and analyse the convergence of a hyperbolic approximation. Among the tests, we used a newly derived first-order self-similar approximation to compare the efficacy of each method analytically in a 1D pure-thermal test scenario. Results. We find that although the hyperbolic approximation proves the most accurate and the fastest to compute in long-running simulations, there is no optimal method to calculate the mid-term conductivity with both accuracy and efficiency. We also find that the solution of this approximation is sensitive to the initial conditions, and can lead to faster convergence if used correctly. Hyperdiffusivity is particularly useful in aiding the methods to perform optimally. Conclusions. We discuss recommendations for the use of each method within more complex simulations, whilst acknowledging the areas where there are opportunities for new methods to be developed.

• Kohutova, Petra; Antolin, Patrick; Szydlarski, Mikolaj & Poirier, Nicolas Aurelien Corentin (2024). Can we rely on EUV emission to identify coronal waveguides? Astronomy and Astrophysics (A & A). ISSN 0004-6361. 690. doi: 10.1051/0004-6361/202451196. Fulltekst i vitenarkiv Vis sammendrag

  Context. Traditional models of coronal oscillations rely on a modelling of the coronal structures that support them as compact cylindrical waveguides. An alternative model of the structure of the corona has recently been proposed, in which the thin strand-like coronal loops, that are observed in the extreme-UV (EUV) emission are the result of the line-of-sight integration of warps in more complex coronal structures. This is referred to as the coronal veil model. Aims. We extend the implications of the coronal veil model of the solar corona to models of coronal oscillations. Methods. Using convection-zone-to-corona simulations with the radiation-magnetohydrodynamics (rMHD) code Bifrost, we analysed the structure of the self-consistently formed simulated corona. We focused on the spatial variability of the volumetric emissivity of the Fe IX 171.073 Å EUV line and on the variability of the Alfvén speed, which captures the density and magnetic structuring of the simulated corona. We traced features associated with large magnitudes of the Alfvén speed gradient, which trap MHD waves and act as coronal waveguides. We searched for the correspondence with emitting regions, which appear as strand-like loops in the line-of-sight-integrated EUV emission. Results. We find that the cross sections of the waveguides bounded by large Alfvén speed gradients become less circular and more distorted with increasing height in the solar atmosphere. The waveguide filling factors corresponding to the fraction of the waveguides filled with plasma that emits in the given EUV wavelength range from 0.09–0.44. This suggests that we can only observe a small fraction of the waveguide. Similarly, the projected waveguide widths in the plane of the sky are several times larger than the widths of the apparent loops that are observed in the EUV. Conclusions. We conclude that the coronal veil structure is independent of the model. As a result, we find a lack of straightforward correspondence between peaks in the integrated emission profile that constitute apparent coronal loops and regions of plasma bound by a large Alfvén speed gradient that act as waveguides. Coronal waveguides cannot be reliably identified based on emission in a single EUV wavelength is not reliable in the simulated corona formed in convection-zone-to-corona models.

• Kohutova, Petra; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2023). Damping of coronal oscillations in self-consistent 3D radiative magnetohydrodynamics simulations of the solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 676. doi: 10.1051/0004-6361/202346671. Fulltekst i vitenarkiv Vis sammendrag

  Context. Oscillations are abundant in the solar corona. Coronal loop oscillations are typically studied using highly idealised models of magnetic flux tubes. In order to improve our understanding of coronal oscillations, it is necessary to consider the effect of a realistic magnetic field topology and the density structuring. Aims. We analyse the damping of coronal oscillations using a self-consistent 3D radiation-magnetohydrodynamics simulation of the solar atmosphere spanning from the convection zone into the corona, the associated oscillation dissipation and heating, and finally, the physical processes that cause the damping and dissipation. The simulated corona that forms in this model does not depend on any prior assumptions about the shape of the coronal loops. Methods. We analysed the evolution of a bundle of magnetic loops by tracing the magnetic field. Results. We find that the bundle of magnetic loops shows damped transverse oscillations in response to perturbations in two separate instances, with oscillation periods of 177 s and 191 s, velocity amplitudes of 10 km s −1 and 16 km s −1 , and damping times of 176 s and 198 s. The coronal oscillations lead to the development of velocity shear in the simulated corona, which results in the formation of vortices seen in the velocity field that are caused by the Kelvin-Helmholtz instability. This contributes to the damping and dissipation of the transverse oscillations. Conclusions. The oscillation parameters and evolution we observed are in line with the values that are typically seen in observations of coronal loop oscillations. The dynamic evolution of the coronal loop bundle suggests that the models of monolithic and static coronal loops with constant lengths might need to be re-evaluated by relaxing the assumption of highly idealised wave guides.

• Martinez-Sykora, Juan; Pontieu, Bart Walter De; Hansteen, Viggo Haraldson; Testa, Paola; Wargnier, Q.M. & Szydlarski, Mikolaj (2023). The Impact of Multifluid Effects in the Solar Chromosphere on the Ponderomotive Force under SE and NEQ Ionization Conditions. The Astrophysical Journal (ApJ). ISSN 0004-637X. 949(2). doi: 10.3847/1538-4357/acc465. Fulltekst i vitenarkiv Vis sammendrag

  Abstract The ponderomotive force has been suggested to be the main mechanism to produce the so-called first ionization potential (FIP) effect—the enrichment of low-FIP elements observed in the outer solar atmosphere, in the solar wind, and in solar energetic events. It is well known that the ionization of these elements occurs within the chromosphere. Therefore, this phenomenon is intimately tied to the plasma state in the chromosphere and the corona. For this study, we combine IRIS observations, a single-fluid 2.5D radiative magnetohydrodynamics (MHD) model of the solar atmosphere, including ion–neutral interaction effects and nonequilibrium (NEQ) ionization effects, and a novel multifluid multispecies numerical model (based on the Ebysus code). Nonthermal velocities of Si iv measured from IRIS spectra can provide an upper limit for the strength of any high-frequency Alfvén waves. With the single-fluid model, we investigate the possible impact of NEQ ionization within the region where the FIP may occur, as well as the plasma properties in those regions. These models suggest that regions with strongly enhanced network and type II spicules are possible sites of large ponderomotive forces. We use the plasma properties of the single-fluid MHD model and the IRIS observations to initialize our multifluid models to investigate the multifluid effects on the ponderomotive force associated with Alfvén waves. Our multifluid analysis reveals that collisions and NEQ ionization effects dramatically impact the behavior of the ponderomotive force in the chromosphere, and existing theories may need to be revisited.

• Eklund, Henrik; Szydlarski, Mikolaj & Wedemeyer, Sven (2023). Using the slope of the brightness temperature continuum as a diagnostic tool for solar ALMA observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 669. doi: 10.1051/0004-6361/202244400. Fulltekst i vitenarkiv Vis sammendrag

  Context. The intensity of radiation from the solar atmosphere at millimetre wavelengths is closely related to the plasma temperature, and the formation height of the radiation is wavelength dependent. It follows from this that the slope of the intensity continuum, or the brightness temperature continuum, samples the local gradient of the gas temperature of the sampled layers in the solar atmosphere. Aims. We aim to show the added information and diagnostics potential of the solar atmosphere that comes with measuring the slope of the brightness temperature continuum. Methods. We used solar observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA) and estimated and predicted the slope using a numerical three-dimensional radiation-magnetohydrodynamic simulation. The slope was estimated by the differences between observables at wavelengths corresponding to different sub-bands at opposite sides of the ALMA receiver band 3 (2.8–3.2 mm) and band 6 (1.20–1.31 mm). Results. The sign of the brightness temperature slope indicates temperature changes with increasing height at the sampled layers. A positive sign implies an increase in temperature, while a negative sign implies a temperature decrease. The differences in brightness temperature between the sub-bands across the field of view of the simulation typically span from −0.4 kK to 0.75 kK for band 3 and −0.2 kK to 0.3 kK at band 6. The network patches are dominated by large positive slopes, while the quiet-Sun region shows a mixture of positive and negative slopes. As the slope of the continuum is coupled to the small-scale dynamics, a negative slope is seen typically under quiet-Sun conditions as a result of propagating shock waves and the corresponding post-shock regions. The temporal evolution of the slopes can therefore be used to identify shocks. The observability of the slope of the brightness temperatures is estimated at bands 3 and 6 for different angular resolutions corresponding to ALMA observations. The simulations also show that the intensity of the radiation at bands 3 and 6 can simultaneously originate from several major components at different heights, which is strongly dependent on the small-scale dynamics and is seen in both quiet-Sun and network patches. Our in-depth analysis of selected shock waves that propagating upward in the atmosphere shows that the delay of shock signatures between two wavelengths (e.g., bands 6 and 3) does not necessarily reflect the propagation speed of the shock front, but might be cause by the rate of change in opacity of higher layers at these wavelengths. Conclusions. The slope of the brightness temperature continuum sampled at different ALMA receiver sub-bands serves as an indicator of the slope of the local plasma temperature at the sampled heights in the atmosphere. This offers new diagnostic possibilities for measuring the underlying physical properties of small-scale dynamic features and thus contributes to the understanding of these features and the related transport of energy and heat in the chromosphere.

• Pandit, Sneha; Wedemeyer, Sven; Carlsson, Mats & Szydlarski, Mikolaj (2023). Comparison of chromospheric diagnostics in a 3D model atmosphere: Hα linewidth and millimetre continua. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 673. doi: 10.1051/0004-6361/202245412. Fulltekst i vitenarkiv Vis sammendrag

  Context. The H α line, one of the most studied chromospheric diagnostics, is a tracer of magnetic field structures, while the intensity of its line core provides an estimate of the mass density. The interpretation of H α observations is complicated by deviations from local thermodynamic equilibrium (LTE) or instantaneous statistical equilibrium conditions. Meanwhile, millimetre (mm) continuum radiation is formed in LTE, and therefore the brightness temperatures from Atacama Large Millimetre-submillimetre Array (ALMA) observations provide a complementary view of the activity and the thermal structure of stellar atmospheres. These two diagnostics together can provide insights into the physical properties of stellar atmospheres, such as their temperature stratification, magnetic structures, and mass density distribution. Aims. In this paper, we present a comparative study between synthetic continuum brightness temperature maps at mm wavelengths (0.3 mm to 8.5 mm) and the width of the H α 6565 Å line. Methods. We used the 3D radiative-transfer codes Multi3D and Advanced Radiative Transfer (ART) to calculate synthetic spectra for the H α line and the mm continua, respectively, from an enhanced network atmosphere model with non-equilibrium hydrogen ionisation generated with the state-of-the-art 3D radiation magnetohydrodynamics (rMHD) code Bifrost. We use a Gaussian point spread function (PSF) to simulate the effect of ALMA’s limited spatial resolution and calculate the H α versus mm continuum correlations and slopes of scatter plots for the original and degraded resolution of the whole box, quiet sun, and enhanced network patches separately. Results. The H α linewidth and mm brightness temperatures are highly correlated and the correlation is highest at a wavelength of 0.8 mm, that is, in ALMA Band 7. The correlation systematically increases with decreasing resolution. On the other hand, the slopes decrease with increasing wavelength. The degradation of resolution does not have a significant impact on the calculated slopes. Conclusions. With decreasing spatial resolution, the standard deviations of the observables, H α linewidth, and brightness temperatures decrease and the correlations between them increase, but the slopes do not change significantly. These relations may therefore prove useful in calibrating the mm continuum maps observed with ALMA.

• Gomez, Juan Camilo Guevara; Jafarzadeh, Shahin; Wedemeyer, Sven; Grant, Samuel D. T.; Eklund, Henrik & Szydlarski, Mikolaj (2023). The Sun at millimeter wavelengths: IV. Magnetohydrodynamic waves in small-scale bright features. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 671. doi: 10.1051/0004-6361/202244228. Fulltekst i vitenarkiv Vis sammendrag

  Aims. We used solar observations of a plage-enhanced network with the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Band 3 and Band 6, together with synthetic continuum maps from numerical simulations with Bifrost in the same bands, to carry out a detailed study of bright small-scale magnetic features. Methods. We made use of an algorithm to automatically identify and trace bright features within the field of view (FoV) of the ALMA observations and the simulation. In particular, the algorithm recovers information of the time evolution of the shape, motion of the centre of gravity, temperature, and size for each feature. These quantities are used to determine the oscillatory properties of each feature utilising wavelets analysis. Results. We found 193 and 293 features in the Bands 3 and 6 observations, respectively. In the degraded simulation, the total number of features were 24 for Band 3 and 204 for Band 6. In the original simulation, the total number of features were 36 for Band 3 and 392 for Band 6. Based on the simulation, we confirm the magnetic nature of the features. We have obtained average oscillation periods of 30–99 s for the temperature, 37–92 s for size, and 37–78 s for horizontal velocity. There are indications for the possible presence of transverse (kink) waves with average amplitude velocities of 2.1–5.0 km s −1 . We find a predominant anti-phase behaviour between temperature and size oscillations suggesting that the variations of the bright features are caused by compressible fast-sausage magnetohydrodynamics (MHD) modes. For the first time to our knowledge, we estimated the flux of energy of the fast-sausage waves at the chromospheric heights sampled by ALMA as 453–1838 W m −2 for Band 3 and 3640–5485 W m −2 for Band 6. Conclusions. We have identified MHD waves, both transverse (kink) and compressible sausage modes, in small-scale (magnetic) structures, independently, in both ALMA Band 3 and Band 6 observations, along with their corresponding synthetic images from simulations. The decrease of wave energy-flux with height (from Band 6 to Band 3) could possibly suggest energy dissipation at chromospheric heights, namely, wave heating, with the assumptions that the identified small-scale waves are typical at each band and they propagate upward through the chromosphere.

• Gomez, Juan Camilo Guevara; Jafarzadeh, Shahin; Wedemeyer, Sven & Szydlarski, Mikolaj (2022). Propagation of transverse waves in the solar chromosphere probed at different heights with ALMA sub-bands. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 665. doi: 10.1051/0004-6361/202244387. Fulltekst i vitenarkiv Vis sammendrag

  The Atacama Large Millimeter/sub-millimeter Array (ALMA) has provided us with an excellent diagnostic tool for studies of the dynamics of the Solar chromosphere, albeit through a single receiver band at one time presently. Each ALMA band consists of four sub-bands that are comprised of several spectral channels. To date, however, the spectral domain has been neglected in favour of ensuring optimal imaging, so that time-series observations have been mostly limited to full-band data products, thereby limiting studies to a single chromospheric layer. Here, we report the first observations of a dynamical event (i.e., wave propagation) for which the ALMA Band 3 data (centred at 3 mm; 100 GHz) is split into a lower and an upper sideband. In principle, this approach is aimed at mapping slightly different layers in the Solar atmosphere. The side-band data were reduced together with the Solar ALMA Pipeline (SoAP), resulting in time series of brightness-temperature maps for each side-band. Through a phase analysis of a magnetically quiet region, where purely acoustic waves are expected to dominate, the average height difference between the two side-bands is estimated as 73 ± 16 km. Furthermore, we examined the propagation of transverse waves in small-scale bright structures by means of wavelet phase analysis between oscillations at the two atmospheric heights. We find 6% of the waves to be standing, while 54% and 46% of the remaining waves are propagating upwards and downwards, respectively, with absolute propagating speeds on the order of ≈96 km s −1 , resulting in a mean energy flux of 3800 W m 2 .

• Henriques, Vasco Manuel de Jorge; Jafarzadeh, Shahin; Gomez, Juan Camilo Guevara; Eklund, Henrik; Wedemeyer, Sven & Szydlarski, Mikolaj [Vis alle 8 forfattere av denne artikkelen] (2022). The Solar ALMA Science Archive (SALSA): First release, SALAT, and FITS header standard. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 659. doi: 10.1051/0004-6361/202142291. Fulltekst i vitenarkiv Vis sammendrag

  In December 2016, the Atacama Large Millimeter/submillimeter Array (ALMA) carried out the first regular observations of the Sun. These early observations and the reduction of the respective data posed a challenge due to the novelty and complexity of observing the Sun with ALMA. The difficulties with producing science-ready, time-resolved imaging products in a format familiar to and usable by solar physicists based on the measurement sets delivered by ALMA had limited the availability of such data to this point. With the development of the Solar ALMA Pipeline, it has now become possible to routinely reduce such data sets. As a result, a growing number of science-ready solar ALMA data sets are now offered in the form of the Solar ALMA Science Archive (SALSA). So far, SALSA contains primarily time series of single-pointing interferometric images at cadences of one or two seconds, accompanied by the respective single-dish full-disc solar images. The data arrays are provided in FITS format. We also present the first version of a standardised header format that accommodates future expansions and fits within the scope of other standards including the ALMA Science Archive itself and SOLARNET. The headers include information designed to aid the reproduction of the imaging products from the raw data. Links to co-observations, if available, with a focus on those of the Interface Region Imaging Spectrograph, are also provided. SALSA is accompanied by the Solar ALMA Library of Auxiliary Tools (SALAT), which contains Interactive Data Language and Python routines for convenient loading and a quick-look analysis of SALSA data.

• Wedemeyer, Sven; Fleishman, Gregory; Rodríguez, Jaime de la Cruz; Gunár, Stanislav; Santos, João M. da Silva & Antolin, Patrick [Vis alle 9 forfattere av denne artikkelen] (2022). Prospects and challenges of numerical modeling of the Sun at millimeter wavelengths. Frontiers in Astronomy and Space Sciences. 9. doi: 10.3389/fspas.2022.967878. Fulltekst i vitenarkiv Vis sammendrag

  The Atacama Large Millimeter/submillimeter Array (ALMA) offers new diagnostic possibilities that complement other commonly used diagnostics for the study of the Sun. In particular, ALMA’s ability to serve as an essentially linear thermometer of the chromospheric gas at unprecedented spatial resolution at millimeter wavelengths and future polarization measurements has great diagnostic potential. Solar ALMA observations are therefore expected to contribute significantly to answering long-standing questions about the structure, dynamics, and energy balance of the outer layers of the solar atmosphere. In this regard, current and future ALMA data are also important for constraining and further developing numerical models of the solar atmosphere, which in turn are often vital for the interpretation of observations. The latter is particularly important given the Sun’s highly intermittent and dynamic nature that involves a plethora of processes occurring over extended ranges in spatial and temporal scales. Realistic forward modeling of the Sun therefore requires time-dependent three-dimensional radiation magnetohydrodynamics that account for non-equilibrium effects and, typically as a separate step, detailed radiative transfer calculations, resulting in synthetic observables that can be compared to observations. Such artificial observations sometimes also account for instrumental and seeing effects, which, in addition to aiding the interpretation of observations, provide instructive tools for designing and optimizing ALMA’s solar observing modes. In the other direction, ALMA data in combination with other simultaneous observations enable the reconstruction of the solar atmospheric structure via data inversion techniques. This article highlights central aspects of the impact of ALMA for numerical modeling of the Sun and their potential and challenges, together with selected examples.

• Druett, Malcolm K.; Leenarts, Jorrit; Carlsson, Mats & Szydlarski, Mikolaj (2022). Formation and heating of chromospheric fibrils in a radiation-MHD simulation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 665. doi: 10.1051/0004-6361/202142399. Fulltekst i vitenarkiv Vis sammendrag

  Aims. We examine the movements of mass elements within dense fibrils using passive tracer particles (corks) in order to understand the creation and destruction processes of fibrils. Methods. Simulated fibrils were selected at times when they were visible in a H α image proxy. The corks were selected within fibril H α formation regions. From this set, we selected a cork and constructed the field line passing through it. Other fibrilar corks close to this field line were also selected and pathlines were constructed, revealing the locations of the mass elements forwards and backwards in time. Finally, we analysed the forces acting on these mass elements. Results. The main process of fibrilar loading in the simulation is different to the mass loading scenario in which waves steepen into shocks and push material upwards along the field lines from locations near their footpoints. The twisted, low-lying field lines were destabilised and then they untwisted, lifting the material trapped above their apexes via the Lorentz force. Subsequently, the majority of the mass drained down the field lines towards one or both footpoints under the influence of gravity. Material with large horizontal velocities could also be elevated in rising field lines, creating somewhat parabolic motions, but the material was not generally moving upward along a stationary magnetic field line during loading. Conclusions. The processes observed in the simulation are additional scenarios that are plausible. The criteria for observing such events are described in this work. We note that it is desirable for our simulations to also be able to form more densely packed fibrils from material fed from the base of field footpoints. The experimental parameters required to achieve this are also discussed in this paper.

• Narang, Nancy; Chandrashekhar, Kalugodu; Jafarzadeh, Shahin; Fleck, Bernhard; Szydlarski, Mikolaj & Wedemeyer, Sven (2022). Power distribution of oscillations in the atmosphere of a plage region: Joint observations with ALMA, IRIS, and SDO. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 661. doi: 10.1051/0004-6361/202142080. Fulltekst i vitenarkiv Vis sammendrag

  Context. Joint observations of the Atacama Large Millimeter/Submillimeter Array (ALMA) with other solar observatories can provide a wealth of opportunities for understanding the coupling between different layers of the solar atmosphere. Aims. We present a statistical analysis of the power distribution of oscillations in a plage region in active region NOAA AR12651, which was observed jointly with ALMA, the Interface Region Imaging Spectrograph (IRIS), and the Solar Dynamics Observatory (SDO). Methods. We employ coordinated ALMA Band 6 (1.25 mm) brightness temperature maps, IRIS slit-jaw images in the 2796 Å passband, and observations in six passbands (1600 Å, 304 Å, 131 Å, 171 Å, 193 Å, and 211 Å) from the Atmospheric Imaging Assembly (AIA) on board SDO. We perform Lomb-Scargle transforms to study the distribution of oscillation power by means of dominant period maps and power maps. We study the spatial association of oscillations through the atmosphere, with a focus on the correlation of the power distribution of ALMA oscillations with others. Results. We do not observe any significant association of ALMA oscillations with IRIS and AIA oscillations. While the global behavior of the dominant ALMA oscillations shows a similarity with that of the transition region and coronal passbands of AIA, the ALMA dominant period maps and power maps do not show any correlation with those from the other passbands. The spatial distribution of dominant periods and power in different period intervals of ALMA oscillations is uncorrelated with those of any other passbands. Conclusions. We speculate that the non-association of ALMA oscillations with those of IRIS and AIA is due to significant variations in the height of formation of the millimeter continuum observed by ALMA. Additionally, the fact that ALMA directly maps the brightness temperature, in contrast to the intensity observations by IRIS and AIA, can result in the very different intrinsic nature of the ALMA oscillations compared to the IRIS and AIA oscillations.

• Eklund, Henrik; Wedemeyer, Sven; Szydlarski, Mikolaj & Jafarzadeh, Shahin (2021). The Sun at millimeter wavelengths: III. Impact of the spatial resolution on solar ALMA observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 656. doi: 10.1051/0004-6361/202140972. Fulltekst i vitenarkiv Vis sammendrag

  Context. Interferometric observations of the Sun with the Atacama Large Millimeter/sub-millimeter Array (ALMA) provide valuable diagnostic tools for studying the small-scale dynamics of the solar atmosphere. Aims. The aims are to perform estimations of the observability of the small-scale dynamics as a function of spatial resolution for regions with different characteristic magnetic field topology facilitate a more robust analysis of ALMA observations of the Sun. Methods. A three-dimensional model of the solar atmosphere from the radiation-magnetohydrodynamic code Bifrost was used to produce high-cadence observables at millimeter and submillimeter wavelengths. The synthetic observables for receiver bands 3–10 were degraded to the angular resolution corresponding to ALMA observations with different configurations of the interferometric array from the most compact, C1, to the more extended, C7. The observability of the small-scale dynamics was analyzed in each case. The analysis was thus also performed for receiver bands and resolutions that are not commissioned so far for solar observations as a means for predicting the potential of future capabilities. Results. The minimum resolution required to study the typical small spatial scales in the solar chromosphere depends on the characteristic properties of the target region. Here, a range from quiet Sun to enhanced network loops is considered. Limited spatial resolution affects the observable signatures of dynamic small-scale brightening events in the form of reduced brightness temperature amplitudes, potentially leaving them undetectable, and even shifts in the times at which the peaks occur of up to tens of seconds. Conversion factors between the observable brightness amplitude and the original amplitude in the fully resolved simulation are provided that can be applied to observational data in principle, but are subject to wavelength-dependent uncertainties. Predictions of the typical appearance at the different combinations of receiver band, array configuration, and properties of the target region are conducted. Conclusions. The simulation results demonstrate the high scientific potential that ALMA already has with the currently offered capabilities for solar observations. For the study of small-scale dynamic events, however, the spatial resolution is still crucial, and wide array configurations are preferable. In any case, it is essential to take the effects due to limited spatial resolution into account in the analysis of observational data. Finally, the further development of observing capabilities including wider array configurations and advanced imaging procedures yields a high potential for future ALMA observations of the Sun.

• Chintzoglou, Georgios; Pontieu, Bart Walter De; Martinez-Sykora, Juan; Hansteen, Viggo; Rodriguez, Jaime de la Cruz & Szydlarski, Mikolaj Marcin [Vis alle 10 forfattere av denne artikkelen] (2021). ALMA and IRIS Observations of the Solar Chromosphere. II. Structure and Dynamics of Chromospheric Plages. The Astrophysical Journal (ApJ). ISSN 0004-637X. 906(2). doi: 10.3847/1538-4357/abc9b0. Fulltekst i vitenarkiv
• Eklund, Henrik; Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Jafarzadeh, Shahin & Gomez, Juan Camilo Guevara (2020). The Sun at millimeter wavelengths: II. Small-scale dynamic events in ALMA Band 3. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 644. doi: 10.1051/0004-6361/202038250. Fulltekst i vitenarkiv Vis sammendrag

  Context. Solar observations with the Atacama Large Millimeter/sub-millimeter Array (ALMA) facilitate studies of the atmosphere of the Sun at chromospheric heights at high spatial and temporal resolution at millimeter wavelengths. Aims. ALMA intensity data at millimeter(mm)-wavelengths are used for a first detailed systematic assessment of the occurrence and properties of small-scale dynamical features in the quiet Sun. Methods. We analyzed ALMA Band 3 data (∼3 mm/100 GHz) with a spatial resolution of ∼1.4–2.1 arcsec and a duration of ∼40 min together with SDO/HMI magnetograms. The temporal evolution of the mm maps is studied to detect pronounced dynamical features, which then are connected to dynamical events via a k -means clustering algorithm. We studied the physical properties of the resulting events and explored whether or not they show properties consistent with propagating shock waves. For this purpose, we calculated observable shock wave signatures at mm wavelengths from one- and three-dimensional model atmospheres. Results. We detect 552 dynamical events with an excess in brightness temperature (Δ T b ) of at least ≥400 K. The events show a large variety in size up to ∼9, amplitude Δ T b up to ∼1200 K with typical values in the range ∼450–750 K, and lifetime at full width at half maximum of Δ T b of between ∼43 and 360 s, with typical values between ∼55 and 125 s. Furthermore, many of the events show signature properties suggesting that they are likely produced by propagating shock waves. Conclusions. There are a lot of small-scale dynamic structures detected in the Band 3 data, even though the spatial resolution sets limitations on the size of events that can be detected. The number of dynamic signatures in the ALMA mm data is very low in areas with photospheric footpoints with stronger magnetic fields, which is consistent with the expectation for propagating shock waves.

• Martinez-Sykora, Juan; Leenarts, Jorrit; Pontieu, Bart Walter De; Siverio, Daniel Elias Nóbrega; Hansteen, Viggo & Carlsson, Mats [Vis alle 7 forfattere av denne artikkelen] (2020). Ion–neutral Interactions and Nonequilibrium Ionization in the Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 889(2). doi: 10.3847/1538-4357/ab643f. Fulltekst i vitenarkiv
• Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Jafarzadeh, Shahin; Eklund, Henrik; Gomez, Juan Camilo Guevara & Bastian, Tim [Vis alle 10 forfattere av denne artikkelen] (2020). The Sun at millimeter wavelengths: I. Introduction to ALMA Band 3 observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 635. doi: 10.1051/0004-6361/201937122. Fulltekst i vitenarkiv Vis sammendrag

  Context. The Atacama Large Millimeter/submillimeter Array (ALMA) started regular observations of the Sun in 2016, first offering receiver Band 3 at wavelengths near 3 mm (100 GHz) and Band 6 at wavelengths around 1.25 mm (239 GHz). Aims. Here we present an initial study of one of the first ALMA Band 3 observations of the Sun. Our aim is to characterise the diagnostic potential of brightness temperatures measured with ALMA on the Sun. Methods. The observation covers a duration of 48 min at a cadence of 2 s targeting a quiet Sun region at disc-centre. Corresponding time series of brightness temperature maps are constructed with the first version of the Solar ALMA Pipeline and compared to simultaneous observations with the Solar Dynamics Observatory (SDO). Results. The angular resolution of the observations is set by the synthesised beam, an elliptical Gaussian that is approximately 1.4 × 2.1 in size. The ALMA maps exhibit network patches, internetwork regions, and elongated thin features that are connected to large-scale magnetic loops, as confirmed by a comparison with SDO maps. The ALMA Band 3 maps correlate best with the SDO/AIA 171 Å, 131 Å, and 304 Å channels in that they exhibit network features and, although very weak in the ALMA maps, imprints of large-scale loops. A group of compact magnetic loops is very clearly visible in ALMA Band 3. The brightness temperatures in the loop tops reach values of about 8000−9000 K and in extreme moments up to 10 000 K. Conclusions. ALMA Band 3 interferometric observations from early observing cycles already reveal temperature differences in the solar chromosphere. The weak imprint of magnetic loops and the correlation with the 171, 131, and 304 SDO channels suggests, however, that the radiation mapped in ALMA Band 3 might have contributions from a wider range of atmospheric heights than previously assumed, but the exact formation height of Band 3 needs to be investigated in more detail. The absolute brightness temperature scale as set by total power measurements remains less certain and must be improved in the future. Despite these complications and the limited angular resolution, ALMA Band 3 observations have a large potential for quantitative studies of the small-scale structure and dynamics of the solar chromosphere.

• Hudson, Hugh S.; MacKinnon, Alec; Szydlarski, Mikolaj Marcin & Carlsson, Mats (2020). Cosmic ray interactions in the solar atmosphere. Monthly notices of the Royal Astronomical Society. ISSN 0035-8711. 491(4), s. 4852–4856. doi: 10.1093/mnras/stz3373. Fulltekst i vitenarkiv Vis sammendrag

  High-energy particles enter the solar atmosphere from Galactic or solar coronal sources, and produce ‘albedo’ emission from the quiet Sun that is now observable across a wide range of photon energies. The interaction of high-energy particles in a stellar atmosphere depends essentially upon the joint variation of the magnetic field and plasma density, which heretofore has been characterized parametrically as P ∝ Bα with P the gas pressure and B the magnitude of the magnetic field. We re-examine that parametrization by using a self-consistent 3D MHD model (Bifrost) and show that this relationship tends to P ∝ B3.5 ± 0.1 based on the visible portions of the sample of open-field flux tubes in such a model, but with large variations from point to point. This scatter corresponds to the strong meandering of the open-field flux tubes in the lower atmosphere, which will have a strong effect on the prediction of the emission anisotropy (limb brightening). The simulations show that much of the open flux in coronal holes originates in weak-field regions within the granular pattern of the convective motions seen in the simulations.

• Martinez-Sykora, Juan; Szydlarski, Mikolaj Marcin; Hansteen, Viggo & Pontieu, Bart Walter De (2020). On the Velocity Drift between Ions in the Solar Atmosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 900(2). doi: 10.3847/1538-4357/ababa3. Fulltekst i vitenarkiv
• Kohutova, Petra; Antolin, Patrick; Popovas, Andrius; Szydlarski, Mikolaj Marcin & Hansteen, Viggo (2020). Self-consistent 3D radiative magnetohydrodynamic simulations of coronal rain formation and evolution. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 639. doi: 10.1051/0004-6361/202037899. Fulltekst i vitenarkiv Vis sammendrag

  Context. Coronal rain consists of cool and dense plasma condensations formed in coronal loops as a result of thermal instability. Aims. Previous numerical simulations of thermal instability and coronal rain formation have relied on the practice of artificially adding a coronal heating term to the energy equation. To reproduce large-scale characteristics of the corona, the use of more realistic coronal heating prescription is necessary. Methods. We analysed coronal rain formation and evolution in a three-dimensional radiative magnetohydrodynamic simulation spanning from convection zone to corona which is self-consistently heated by magnetic field braiding as a result of convective motions. Results. We investigate the spatial and temporal evolution of energy dissipation along coronal loops which become thermally unstable. Ohmic dissipation in the model leads to the heating events capable of inducing sufficient chromospheric evaporation into the loop to trigger thermal instability and condensation formation. The cooling of the thermally unstable plasma occurs on timescales that are comparable to the duration of the individual impulsive heating events. The impulsive heating has sufficient duration to trigger thermal instability in the loop but does not last long enough to lead to coronal rain limit cycles. We show that condensations can either survive and fall into the chromosphere or be destroyed by strong bursts of Joule heating associated with a magnetic reconnection events. In addition, we find that condensations can also form along open magnetic field lines. Conclusions. We modelled, for the first time, coronal rain formation in a self-consistent 3D radiative magnetohydrodynamic simulation, in which the heating occurs mainly through the braiding and subsequent Ohmic dissipation of the magnetic field. The heating is stratified enough and lasts for long enough along specific field lines to produce the necessary chromospheric evaporation that triggers thermal instability in the corona.

• Gomez, Juan Camilo Guevara; Jafarzadeh, Shahin; Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Stangalini, M. & Fleck, Bernhard [Vis alle 7 forfattere av denne artikkelen] (2020). High-frequency oscillations in small chromospheric bright features observed with Atacama Large Millimetre/Submillimetre Array. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. ISSN 1364-503X. 379(2190). doi: 10.1098/rsta.2020.0184. Fulltekst i vitenarkiv Vis sammendrag

  We report detection of oscillations in brightness temperature, size and horizontal velocity of three small bright features in the chromosphere of a plage/enhanced-network region. The observations, which were taken with high temporal resolution (i.e. 2 s cadence) with the Atacama large millimetre/ submillimetre array (ALMA) in Band 3 (centred at 3 mm; 100 GHz), exhibit three small-scale features with oscillatory behaviour with different, but overlapping, distributions of period on the order of, on average, 90 ± 22 s, 110 ± 12 s and 66 ± 23 s, respectively. We find anti-correlations between perturbations in brightness, temperature and size of the three features, which suggest the presence of fast sausage-mode waves in these small structures. In addition, the detection of transverse oscillations (although with a larger uncertainty) may also suggest the presence of Alfvénic oscillations which are likely representative of kink waves. This work demonstrates the diagnostic potential of high-cadence observations with ALMA for detecting high-frequency magnetohydrodynamic waves in the solar chromosphere. Such waves can potentially channel a vast amount of energy into the outer atmosphere of the Sun. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’.

• Eklund, Henrik; Wedemeyer, Sven; Snow, Ben; Jess, David B.; Jafarzadeh, Shahin & Grant, Samuel D. T. [Vis alle 8 forfattere av denne artikkelen] (2020). Characterization of shock wave signatures at millimetre wavelengths from Bifrost simulations. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. ISSN 1364-503X. 379(2190). doi: 10.1098/rsta.2020.0185. Fulltekst i vitenarkiv Vis sammendrag

  Observations at millimetre wavelengths provide a valuable tool to study the small-scale dynamics in the solar chromosphere. We evaluate the physical conditions of the atmosphere in the presence of a propagating shock wave and link that to the observable signatures in mm-wavelength radiation, providing valuable insights into the underlying physics of mm-wavelength observations. A realistic numerical simulation from the three-dimensional radiative magnetohydrodynamic code Bifrost is used to interpret changes in the atmosphere caused by shock wave propagation. High-cadence (1 s) time series of brightness temperature (Tb) maps are calculated with the Advanced Radiative Transfer code at the wavelengths 1.309 mm and 1.204 mm, which represents opposite sides of spectral band 6 of the Atacama Large Millimeter/submillimeter Array (ALMA). An example of shock wave propagation is presented. The brightness temperatures show a strong shock wave signature with large variation in formation height between approximately 0.7 and 1.4 Mm. The results demonstrate that millimetre brightness temperatures efficiently track upwardly propagating shock waves in the middle chromosphere. In addition, we show that the gradient of the brightness temperature between wavelengths within ALMA band 6 can potentially be used as a diagnostics tool in understanding the small-scale dynamics at the sampled layers. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’.

• Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Rodriguez, Jaime de la Cruz & Jafarzadeh, Shahin (2020). Observing the Sun with the Atacama Large Millimeter/submillimeter Array-from continuum to magnetic fields. Proceedings of the International Astronomical Union. ISSN 1743-9213. 15(S354), s. 24–37. doi: 10.1017/S1743921319009906. Fulltekst i vitenarkiv
• Jafarzadeh, Shahin; Wedemeyer, Sven; Fleck, Bernhard; Stangalini, M.; Jess, D. B. & Morton, R. J. [Vis alle 15 forfattere av denne artikkelen] (2020). An overall view of temperature oscillations in the solar chromosphere with ALMA. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. ISSN 1364-503X. 379(2190). doi: 10.1098/rsta.2020.0174. Fulltekst i vitenarkiv Vis sammendrag

  By direct measurements of the gas temperature, the Atacama Large Millimeter/submillimeter Array (ALMA) has yielded a new diagnostic tool to study the solar chromosphere. Here, we present an overview of the brightness-temperature fluctuations from several high-quality and high-temporal-resolution (i.e. 1 and 2 s cadence) time series of images obtained during the first 2 years of solar observations with ALMA, in Band 3 and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz), respectively. The various datasets represent solar regions with different levels of magnetic flux. We perform fast Fourier and Lomb–Scargle transforms to measure both the spatial structuring of dominant frequencies and the average global frequency distributions of the oscillations (i.e. averaged over the entire field of view). We find that the observed frequencies significantly vary from one dataset to another, which is discussed in terms of the solar regions captured by the observations (i.e. linked to their underlying magnetic topology). While the presence of enhanced power within the frequency range 3–5 mHz is found for the most magnetically quiescent datasets, lower frequencies dominate when there is significant influence from strong underlying magnetic field concentrations (present inside and/or in the immediate vicinity of the observed field of view). We discuss here a number of reasons which could possibly contribute to the power suppression at around 5.5 mHz in the ALMA observations. However, it remains unclear how other chromospheric diagnostics (with an exception of Hα line-core intensity) are unaffected by similar effects, i.e. they show very pronounced 3-min oscillations dominating the dynamics of the chromosphere, whereas only a very small fraction of all the pixels in the 10 ALMA datasets analysed here show peak power near 5.5 mHz. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’.

• Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Carlsson, Mats; Rodriguez, Jaime de la Cruz; Bose, Souvik & Chintzoglou, Georgios [Vis alle 33 forfattere av denne artikkelen] (2020). High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/202038732. Fulltekst i vitenarkiv Vis sammendrag

  NASA’s Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe  I lines and spectrally resolved imaging in the chromospheric Ca  II 8542 Å, H α , and Ca  II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes.

• Jafarzadeh, Shahin; Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Pontieu, Bart Walter De; Rezaei, Rezza & Carlsson, Mats (2019). The solar chromosphere at millimetre and ultraviolet wavelengths: I. Radiation temperatures and a detailed comparison. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 622. doi: 10.1051/0004-6361/201834205. Fulltekst i vitenarkiv Vis sammendrag

  Solar observations with the Atacama Large Millimeter/submillimeter Array (ALMA) provide us with direct measurements of the brightness temperature in the solar chromosphere. We study the temperature distributions obtained with ALMA Band 6 (in four sub-bands at 1.21, 1.22, 1.29, and 1.3 mm) for various areas at, and in the vicinity of, a sunspot, comprising quasi-quiet and active regions with different amounts of underlying magnetic fields. We compare these temperatures with those obtained at near- and far-ultraviolet (UV) wavelengths (and with the line-core intensities of the optically-thin far-UV spectra), co-observed with the Interface Region Imaging Spectrograph (IRIS) explorer. These include the emission peaks and cores of the Mg II k 279.6 nm and Mg II h 280.4 nm lines as well as the line cores of C II 133.4 nm, O I 135.6 nm, and Si IV 139.4 nm, sampling the mid-to-high chromosphere and the low transition region. Splitting the ALMA sub-bands resulted in an slight increase of spatial resolution in individual temperature maps, thus, resolving smaller-scale structures compared to those produced with the standard averaging routines. We find that the radiation temperatures have different, though somewhat overlapping, distributions in different wavelengths and in the various magnetic regions. Comparison of the ALMA temperatures with those of the UV diagnostics should, however, be interpreted with great caution, the former is formed under the local thermodynamic equilibrium (LTE) conditions, the latter under non-LTE. The mean radiation temperature of the ALMA Band 6 is similar to that extracted from the IRIS C II line in all areas with exception of the sunspot and pores where the C II poses higher radiation temperatures. In all magnetic regions, the Mg II lines associate with the lowest mean radiation temperatures in our sample. These will provide constraints for future numerical models.

• Rodger, Andrew S.; Labrosse, Nicolas; Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Simões, Paulo J. A. & Fletcher, Lyndsay (2019). First Spectral Analysis of a Solar Plasma Eruption Using ALMA. The Astrophysical Journal (ApJ). ISSN 0004-637X. 875(2). doi: 10.3847/1538-4357/aafdfb. Fulltekst i vitenarkiv Vis sammendrag

  The aim of this study is to demonstrate how the logarithmic millimeter continuum gradient observed using the Atacama Large Millimeter/submillimeter Array (ALMA) may be used to estimate optical thickness in the solar atmosphere. We discuss how using multiwavelength millimeter measurements can refine plasma analysis through knowledge of the absorption mechanisms. Here we use subband observations from the publicly available science verification (SV) data, while our methodology will also be applicable to regular ALMA data. The spectral resolving capacity of ALMA SV data is tested using the enhancement coincident with an X-ray bright point and from a plasmoid ejection event near active region NOAA12470 observed in Band 3 (84–116 GHz) on 2015 December 17. We compute the interferometric brightness temperature light curve for both features at each of the four constituent subbands to find the logarithmic millimeter spectrum. We compared the observed logarithmic spectral gradient with the derived relationship with optical thickness for an isothermal plasma to estimate the structures' optical thicknesses. We conclude, within 90% confidence, that the stationary enhancement has an optical thickness between 0.02 ≤ τ ≤ 2.78, and that the moving enhancement has 0.11 ≤ τ ≤ 2.78, thus both lie near to the transition between optically thin and thick plasma at 100 GHz. From these estimates, isothermal plasmas with typical Band 3 background brightness temperatures would be expected to have electron temperatures of ~7370–15300 K for the stationary enhancement and between ~7440 and 9560 K for the moving enhancement, thus demonstrating the benefit of subband ALMA spectral analysis.

• Jafarzadeh, Shahin; Rutten, Robert; Solanki, S. K.; Wiegelmann, T.; Riethmüller, T. L. & Noort, M. van [Vis alle 17 forfattere av denne artikkelen] (2017). Slender Ca II H Fibrils Mapping Magnetic Fields in the Low Solar Chromosphere. Astrophysical Journal Supplement Series. ISSN 0067-0049. 229(1). doi: 10.3847/1538-4365/229/1/11. Fulltekst i vitenarkiv Vis sammendrag

  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. © 2017. The American Astronomical Society. All rights reserved.

• Szydlarski, Mikolaj Marcin; Grigori, L. & Stompor, R. (2014). Accelerating the cosmic microwave background map-making procedure through preconditioning. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 572. doi: 10.1051/0004-6361/201323210. Fulltekst i vitenarkiv
• Szydlarski, Mikolaj Marcin (2013). Algebraic Domain Decomposition methods for highly heterogeneous problem. SIAM Journal on Scientific Computing. ISSN 1064-8275. doi: 10.1137/110842648. Fulltekst i vitenarkiv
• Szydlarski, Mikolaj Marcin (2013). Parallel spherical harmonic transforms on heterogeneous architectures (graphics processing units/multi‐core CPUs). Concurrency: Practice and Experience. doi: 10.1002/cpe.3038. Fulltekst i vitenarkiv
• Szydlarski, Mikolaj Marcin (2012). A parallel two-level preconditioner for Cosmic Microwave Background map-making. I Hollingsworth, Jeffrey K. (Red.), SC '12: Proceedings of the International Conference on High Performance Computing, Networking, Storage and Analysis. IEEE (Institute of Electrical and Electronics Engineers). ISSN 9781467308069. doi: 10.1109/SC.2012.10. Fulltekst i vitenarkiv

Se alle arbeider i NVA

• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj & Zhou, Yixiao (2026). Towards Resolving the Solar Convective Conundrum: A staged approach in progress. doi: 10.5281/ZENODO.20642947. Fulltekst i vitenarkiv Vis sammendrag

  The solar convective conundrum — the order-of-magnitude discrepancy between helioseismically inferred deep convective velocities and those predicted by global simulations — has resisted resolution for over a decade. Three suspects have been proposed: missing physics, insufficient resolution, and the way simulations are configured. ReCon² addresses all three together in a single controlled programme, with the third element implemented through a novel staged column-relaxation strategy: solar convection is first developed in a narrow horizontal column, where large-scale unstable modes are geometrically excluded, then progressively expanded to production widths while carrying the equilibrated small-scale physics forward. This poster, presented at Cool Stars 23 (Tokyo, 15–19 June 2026), reports first-stage results from the cz_f25_d100 run: a 25 Mm wide column at Δx = 100 km finest resolution, with fully realistic physics (OPAL/FreeEOS equation of state with ionization, OP opacities and radiative diffusion in the interior, multi-bin short-characteristics radiative transfer at the surface, fully compressible MHD, entropy-based HLLS solver), initialized from rest. Horizontally-averaged entropy perturbation statistics over 300 hours of simulated time show a coherent thermal mixing layer descending ~140 Mm into the convection zone. The active layer is strongly negative-skewed (≤ −2) — the statistical signature of entropy rain (Käpylä 2025), indicating distributions dominated by narrow cold extremes. A compensating positive-skew layer at the leading edge marks warm material displaced by penetrating cold plumes, and discrete late-time arcs at z ≈ 480–540 Mm correspond to individual extreme plumes reaching ~200 Mm depth, exhibiting Käpylä's predicted "tree" morphology. A complementary photospheric kinetic-energy spectrum from an exploratory wider-box run shows that the surface convective physics matches established surface-tracking diagnostics within the box-resolved range.

• Popovas, Andrius; Nordlund, Åke & Szydlarski, Mikolaj (2026). Recent updates on global Sun modeling with DISPATCH. Fulltekst i vitenarkiv Vis sammendrag

  In this talk i present an update on our global simulations of the solar convection. I demonstrate that we can simulate the entire convection zone from below the tachocline all the way to lower chromosphere with smallest cell size near the surface of 98km. Then I discuss "entropy rain" and it's effects on the convection morphology.

• Kohutova, Petra; Antolin, Patrick; Szydlarski, Mikolaj & Poirier, Nicolas Aurelien Corentin (2024). Can we rely on EUV emission to identify coronal waveguides? Fulltekst i vitenarkiv
• Cherry, George; Gudiksen, Boris Vilhelm & Szydlarski, Mikolaj (2024). Implementation of thermal conduction energy transfer codes in the Bifrost Solar atmosphere MHD solver. Fulltekst i vitenarkiv Vis sammendrag

  Thermal conductivity provides important contributions to the energy evolution of the upper solar atmosphere, behaving as a non-linear concentration-dependent diffusion equation. Computational discretisation limits the operation of solving such terms due to numerical instabilities and other error build-up. Recently, different methods have been offered as best-fit solutions to these problems in specific situations, but their limitations and total range in other scenarios is rarely discussed. Therefore, we rigorously test the different implementations of solving the conductivity flux, in the massively parallel MHD solver code, Bifrost. We compare the differences and limitations of explicit vs. implicit methods, and analyse the convergence of a hyperbolic approximation. Among the tests, we use a newly derived 1st-order self-similar approximation to compare the efficacy of each method analytically in a 1D pure-thermal scenario. The results give guidelines for when to use each method, and the variables that might affect a certain method’s efficiency or accuracy. We discuss the optimisation of parameters in each method, and weaknesses that are not covered suitably by the current implementations.

• Szydlarski, Mikolaj (2024). Linking ASH and Bifrost for Better Solar Simulations & A Quick Update on the Whole SUN Benchmark. Fulltekst i vitenarkiv
• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj; Kohutova, Petra; Hansteen, Viggo Haraldson & Noraz, Quentin Jean Francois Alain [Vis alle 7 forfattere av denne artikkelen] (2023). Update on global MHD simulations of the solar convective region. Fulltekst i vitenarkiv
• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj; Kohutova, Petra; Hansteen, Viggo Haraldson & Noraz, Quentin Jean Francois Alain [Vis alle 7 forfattere av denne artikkelen] (2023). Global MHD simulation of the solar convective region. Fulltekst i vitenarkiv
• Popovas, Andrius; Nordlund, Åke; Szydlarski, Mikolaj; Kohutova, Petra & Noraz, Quentin Jean Francois Alain (2023). Global MHD simulations of the solar convective zone using a volleyball mesh decomposition: Pilot. Fulltekst i vitenarkiv
• Kohutova, Petra; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2023). Damping of Coronal Oscillations in Self-Consistent 3D Radiative MHD Simulations of the Solar Atmosphere. Fulltekst i vitenarkiv
• Kohutova, Petra; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2023). Damping of coronal oscillations in self-consistent 3D radiative MHD simulations of the solar atmosphere. Fulltekst i vitenarkiv
• Wedemeyer, Sven & Szydlarski, Mikolaj (2023). Artificial observations of the Sun – The scientific potential of optimizing future solar observatories. Fulltekst i vitenarkiv
• Popovas, Andrius; Szydlarski, Mikolaj & Nordlund, Åke (2022). First results from global MHD simulations of Solar convective zone. Fulltekst i vitenarkiv
• Kohutova, Petra; Popovas, Andrius; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2022). Coronal oscillations in the self-consistent 3D MHD simulations of the solar atmosphere. Fulltekst i vitenarkiv
• Kohutova, Petra; Popovas, Andrius; Antolin, Patrick; Szydlarski, Mikolaj & Carlsson, Mats (2022). Coronal oscillations in the self-consistent 3D MHD simulations of the solar atmosphere. Fulltekst i vitenarkiv
• Popovas, Andrius; Szydlarski, Mikolaj & Nordlund, Åke (2022). Multi-scale simulations of the Sun. Fulltekst i vitenarkiv
• Szydlarski, Mikolaj (2022). Understanding the Sun. Numerical models of our local star. Fulltekst i vitenarkiv
• Szydlarski, Mikolaj (2022). Understanding the Sun. Numerical models of our local star. Fulltekst i vitenarkiv
• Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Henriques, Vasco Manuel de Jorge; Eklund, Henrik & Gomez, Juan Camilo Guevara [Vis alle 10 forfattere av denne artikkelen] (2021). Observing the Sun with ALMA - First results and future opportunities. Fulltekst i vitenarkiv
• Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Henriques, Vasco Manuel de Jorge; Eklund, Henrik & Gomez, Juan Camilo Guevara [Vis alle 10 forfattere av denne artikkelen] (2021). Five years observing the solar chromosphere with ALMA - First results and future opportunities. Fulltekst i vitenarkiv
• Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Henriques, Vasco; Gomez, Juan Camilo Guevara & Eklund, Henrik [Vis alle 10 forfattere av denne artikkelen] (2021). Lessons learned from 5 years of solar observing with ALMA. Fulltekst i vitenarkiv
• Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Henriques, Vasco; Eklund, Henrik & Gomez, Juan Camilo Guevara [Vis alle 11 forfattere av denne artikkelen] (2021). A brief introduction to solar observing with ALMA. Fulltekst i vitenarkiv
• Wedemeyer, Sven; Szydlarski, Mikolaj; Jafarzadeh, Shahin; Henriques, Vasco; Eklund, Henrik & Gomez, Juan Camilo Guevara [Vis alle 10 forfattere av denne artikkelen] (2021). Five years observing the solar chromosphere with ALMA - First results and future opportunities. Fulltekst i vitenarkiv
• Jafarzadeh, Shahin; Gafeira, Ricardo; Wedemeyer, Sven & Szydlarski, Mikolaj (2021). MHD waves in chromospheric fibrillar structures as observed with ALMA. Fulltekst i vitenarkiv
• Saberi, Maryam; Jafarzadeh, Shahin; Gafeira, Ricardo; Wedemeyer, Sven & Szydlarski, Mikolaj (2021). MHD waves in chromospheric fibrillar structures as observed with ALMA. Fulltekst i vitenarkiv
• Eklund, Henrik; Szydlarski, Mikolaj Marcin & Wedemeyer, Sven (2020). Image reconstruction of interferometric ALMA data. Fulltekst i vitenarkiv
• Jafarzadeh, Shahin; Wedemeyer, Sven; Szydlarski, Mikolaj Marcin; Fleck, Bernhard & Stangalini, Marco (2019). Wave heating of the solar chromosphere from observations with ALMA. Fulltekst i vitenarkiv
• Jafarzadeh, Shahin; Wedemeyer, Sven; Szydlarski, Mikolaj Marcin & Fleck, Bernhard (2019). Wave heating of the lower solar atmosphere. Fulltekst i vitenarkiv
• Jafarzadeh, Shahin; Wedemeyer, Sven & Szydlarski, Mikolaj Marcin (2019). Temperature Fluctuations in the Solar Chromosphere with ALMA. Fulltekst i vitenarkiv
• Gomez, Juan Camilo Guevara; Wedemeyer, Sven; Jafarzadeh, Shahin & Szydlarski, Mikolaj Marcin (2019). Dynamics of solar isolated features observed by the Atacama Large Millimeter/submillimeter Array (ALMA). Fulltekst i vitenarkiv
• Eklund, Henrik; Szydlarski, Mikolaj Marcin & Wedemeyer, Sven (2018). Interferometric ALMA observations and data processing. Fulltekst i vitenarkiv
• Szydlarski, Mikolaj Marcin & Eide, Vegard (2017). Stellar Atmosphere Simulation code Bifrost on Intel Xeon Phi Knights Landing. PRACE - Partnership for Advanced Computing in Europe. Fulltekst i vitenarkiv

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