Intergranular Lanes Research Articles

Comprehensive Synthesis of Magnetic Tornado: Cospatial Incidence of Chromospheric Swirls and Extreme-ultraviolet Brightening

Magnetic tornadoes, characterized as impulsive Alfvén waves initiated by photospheric vortices in intergranular lanes, are considered efficient energy channels to the corona. Despite their acknowledged importance for solar coronal heating, their observational counterparts from the corona have not been well understood. To address this issue, we use a radiative magnetohydrodynamic simulation of a coronal loop with footpoints rooted in the upper convection zone and synthesize the chromospheric and coronal emissions corresponding to a magnetic tornado. Considering Solar Dynamics Observatory/Atmospheric Imaging Assembly 171 Å and Solar Orbiter/Extreme Ultraviolet Imager 174 Å channels, our synthesis reveals that the coronal response to magnetic tornadoes can be observed as an EUV brightening of which the width is ∼2 Mm. This brightening is located above the synthesized chromospheric swirl observed in the Ca ii 8542 Å, Ca ii K, and Mg ii k lines, which can be detected by instruments such as the Swedish 1 m Solar Telescope/CRisp Imaging SpectroPolarimeter, Goode Solar Telescope/Fast Imaging Solar Spectrograph, and Interface Region Imaging Spectrograph. Considering the height correspondence of the synthesized brightening, magnetic tornadoes can be a mechanism for the small-scale EUV brightenings such as the solar “campfires.” Our findings indicate that coordinated observations encompassing the chromosphere to the corona are indispensable for comprehending the origin of coronal EUV brightenings.

MUSE observations of small-scale heating events

ABSTRACT Constraining the processes that drive coronal heating from observations is a difficult task due to the complexity of the solar atmosphere. As upcoming missions such as the Multi-slit Solar Explorer (MUSE) will provide coronal observations with unprecedented spatial and temporal resolution, numerical simulations are becoming increasingly realistic. Despite the availability of synthetic observations from numerical models, line-of-sight effects and the complexity of the magnetic topology in a realistic set-up still complicate the prediction of signatures for specific heating processes. 3D magnetohydrodynamic (MHD) simulations have shown that a significant part of the Poynting flux injected into the solar atmosphere is carried by small-scale motions, such as vortices driven by rotational flows inside intergranular lanes. MHD waves excited by these vortices have been suggested to play an important role in the energy transfer between different atmospheric layers. Using synthetic spectroscopic data generated from a coronal loop model incorporating realistic driving by magnetoconvection, we study whether signatures of energy transport by vortices and eventual dissipation can be identified with future missions such as MUSE.

Beyond Cryptic Equations: Reimagining Concepts in Physics Through Metaheuristics and Fantasy Stories using Neutrosophic Venn Diagram

Physics, the grand narrative of the universe, has long been viewed as a realm of cold, hard equations. But what if we looked beyond the formulas and considered a more imaginative origin for some of its concepts? This article explores the intriguing possibility that physics, and even cosmology, might share a surprising kinship with metaheuristics and fantastical fiction. Metaheuristics, a branch of computer science, deals with finding approximate solutions to complex problems. Perhaps the universe, in its vastness, employs a set of "rules" that lead to the most likely outcomes, much like an algorithm searching for the best solution within a vast space of possibilities. The connection strengthens when we consider the fantastical. Argentine writer Jorge Luis Borges, known for his thought-provoking short stories, often explored themes of infinity, labyrinths, and forking realities. In this article, we discuss, among other things, how to look at physics laws from an alternative fundamental viewpoint that is fluid dynamics perspective. As an example, we provide an outline for deriving the Newton gravitational law from the Kutta-Joukowski theorem, and then deriving the Kutta-Joukowski theorem from Bernoulli principles. In the meantime, it is known that vortex flows, related to solar convective turbulent dynamics at granular scales and their interplay with magnetic fields within intergranular lanes, occur abundantly on the solar surface and in the atmosphere above.

Small-scale dynamo in cool stars

Context. Some of the quiet solar magnetic flux could be attributed to a small-scale dynamo (SSD) operating in the convection zone. An SSD operating in cool main-sequence stars is expected to affect the atmospheric structure, in particular, the convection, and should have observational signatures. Aims. We investigate the distribution of SSD magnetic fields and their effect on bolometric intensity characteristics, vertical velocity, and spatial distribution of the kinetic energy (KE) and magnetic energy (ME) in the lower photosphere of different spectral types. Methods. We analyzed the SSD and purely hydrodynamic simulations of the near surface layers of F3V, G2V, K0V, and M0V stars. We compared the time-averaged distributions and power spectra in SSD setups relative to the hydrodynamic setup. The properties of the individual magnetic fields are also considered. Results. The probability density functions with a field strength at the τ = 1 surface are quite similar for all cases. The M0V star displays the strongest fields, but relative to the gas pressure, the fields on the F3V star reach the highest values. In all stars, the horizontal field is stronger than the vertical field in the middle photosphere, and this excess becomes increasingly prominent toward later spectral types. These fields result in a decrease in the upflow velocities and a slight decrease in granule size, and also lead to formation of bright points in intergranular lanes. The spatial distribution of the KE and ME is also similar for all cases, implying that important scales are proportional to the pressure scale height. Conclusions. The SSD fields have rather similar effects on the photospheres of cool main-sequence stars: a significant reduction in convective velocities, as well as a slight reduction in granule size and a concentration of the field to kilogauss levels in intergranular lanes that is associated with the formation of bright points. The distribution of the field strengths and energies is also rather similar.

Coronal Heating and Solar Wind Generation by Flux Cancellation Reconnection

In this paper, we propose that flux cancellation on small granular scales (≲1000 km) ubiquitously drives reconnection at a multitude of sites in the low solar atmosphere, contributing to chromospheric/coronal heating and the generation of the solar wind. We analyze the energy conversion in these small-scale flux cancellation events using both analytical models and three-dimensional, resistive magnetohydrodynamic (MHD) simulations. The analytical models—in combination with the latest estimates of flux cancellation rates—allow us to estimate the energy release rates due to cancellation events, which are found to be on the order 106–107 erg cm−2 s−1, sufficient to heat the chromosphere and corona of the quiet Sun and active regions, and to power the solar wind. The MHD simulations confirm the conversion of energy in reconnecting current sheets, in a geometry representing a small-scale bipole being advected toward an intergranular lane. A ribbon-like jet of heated plasma that is accelerated upward could also escape the Sun as the solar wind in an open-field configuration. We conclude that through two phases of atmospheric energy release—precancellation and cancellation—the cancellation of photospheric magnetic flux fragments and the associated magnetic reconnection may provide a substantial energy and mass flux contribution to coronal heating and solar wind generation.

DKIST Unveils the Serpentine Topology of Quiet Sun Magnetism in the Photosphere

We present the first quiet Sun spectropolarimetric observations obtained with the Visible SpectroPolarimeter at the 4 m Daniel K. Inouye Solar Telescope. We recorded observations in a wavelength range that includes the magnetically sensitive Fe i 6301.5/6302.5 Å doublet. With an estimated spatial resolution of 0.″08, this represents the highest spatial resolution full-vector spectropolarimetric observations ever obtained of the quiet Sun. We identified 53 small-scale magnetic elements, including 47 magnetic loops and four unipolar magnetic patches, with linear and circular polarization detected in all of them. Of particular interest is a magnetic element in which the polarity of the magnetic vector appears to change three times in only 400 km and which has linear polarization signals throughout. We find complex Stokes V profiles at the polarity inversion lines of magnetic loops and discover degenerate solutions, as we are unable to conclusively determine whether these arise due to gradients in the atmospheric parameters or smearing of opposite-polarity signals. We analyze a granule that notably has linear and circular polarization signals throughout, providing an opportunity to explore its magnetic properties. On this small scale, we see the magnetic field strength range from 25 G at the granular boundary to 2 kG in the intergranular lane (IGL) and sanity-check the values with the weak and strong field approximations. A value of 2 kG in the IGL is among the highest measurements ever recorded for the internetwork.

Effects of solar evolution on finite acquisition time of Fabry–Perot interferometers in high resolution solar physics

Context. The Visible Tunable Filter (VTF) imaging spectropolarimeter will be operated at the Daniel K. Inouye Solar Telescope (DKIST) in Hawaii. Due to its capability in resolving dynamic fine structure of smaller than 0.05 arcsec, the finite acquisition time of typically 11 s affects the measurement process and potentially causes errors in deduced physical parameters. Aims. We estimate these errors and investigate ways of minimising them. Methods. We mimicked the solar surface using a magnetohydrodynamic simulation with a spatially averaged vertical field strength of 200 G. We simulated the measurement process scanning through successive wavelength points with a temporal cadence of 1 s. We synthesised Fe 1617.3 nm for corresponding snapshots. In addition to the classical composition of the line profile, we introduce a novel method where the intensity in each wavelength point is normalised using the simultaneous continuum intensity, and then multiplied by the temporal mean of the continuum intensity. Milne-Eddington inversions were used to infer the line-of-sight velocity, vlos, and the vertical (longitudinal) component of the magnetic field, Blos. Results. We quantify systematic errors, defining the temporal average of the simulation during the measurement as the truth. We find that with the classical composition of the line profiles, errors exceed the sensitivity for vlos, and in filigree regions also for Blos. The novel method that includes normalisation reduces the measurement errors in all cases. Spatial binning without reducing the acquisition time decreases the measurement error slightly. Conclusions. The evolutionary timescale in inter-granular lanes, in particular in areas with magnetic features (filigree), is shorter than the timescale within granules. Hence, depending on the science objective, fewer accumulations could be used for strong magnetic field in inter-granular lanes and more accumulations could be used for the weak granular magnetic fields. As a key result of this investigation, we suggest including the novel method of normalisation in corresponding data pipelines.

Length Scale of Photospheric Granules in Solar Active Regions

Investigating the length scales of granules could help understand the dynamics of granules in the photosphere. In this work, we detected and identified granules in an active region near disk center observed at wavelength of TiO (7057 Å) by the 1.6 m Goode Solar Telescope (GST). By a detailed analysis of the size distribution and flatness of granules, we found a critical size that divides the granules in motions into two regimes: convection and turbulence. The length scales of granules with sizes larger than 600 km follow Gauss function and demonstrate “flat” in flatness, which reveal that these granules are dominated by convection. Those with sizes smaller than 600 km follow power-law function and behave power-law tendency in flatness, which indicate that the small granules are dominated by turbulence. Hence, for the granules in active regions, they are originally convective in large length scale, and directly become turbulent once their sizes turn to small, likely below the critical size of 600 km. Comparing with the granules in quiet regions, they evolve with the absence of the mixing motions of convection and turbulence. Such a difference is probably caused by the interaction between fluid motions and strong magnetic fields in active regions. The strong magnetic fields make high magnetic pressure which creates pressure walls and slows down the evolution of convective granules. Such walls cause convective granules extending to smaller sizes on one hand, and cause wide intergranular lanes on the other hand. The small granules isolated in such wide intergranular lanes are continually sheared, rotated by strong downflows in surroundings and hereby become turbulent.

Towards the Identification and Classification of Solar Granulation Structures Using Semantic Segmentation

Solar granulation is the visible signature of convective cells at the solar surface. The granulation cellular pattern observed in the continuum intensity images is characterised by diverse structures e.g., bright individual granules of hot rising gas or dark intergranular lanes. Recently, the access to new instrumentation capabilities has given us the possibility to obtain high-resolution images, which have revealed the overwhelming complexity of granulation (e.g., exploding granules and granular lanes). In that sense, any research focused on understanding solar small-scale phenomena on the solar surface is sustained on the effective identification and localization of the different resolved structures. In this work, we present the initial results of a proposed classification model of solar granulation structures based on neural semantic segmentation. We inspect the ability of the U-net architecture, a convolutional neural network initially proposed for biomedical image segmentation, to be applied to the dense segmentation of solar granulation. We use continuum intensity maps of the IMaX instrument onboard the Sunrise I balloon-borne solar observatory and their corresponding segmented maps as a training set. The training data have been labeled using the multiple-level technique (MLT) and also by hand. We performed several tests of the performance and precision of this approach in order to evaluate the versatility of the U-net architecture. We found an appealing potential of the U-net architecture to identify cellular patterns in solar granulation images reaching an average accuracy above 80% in the initial training experiments.

Chromospheric Recurrent Jets in a Sunspot Group and Their Intergranular Origin

We report on high-resolution observations of recurrent fan-like jets by the Goode Solar Telescope in multiple wavelengths inside a sunspot group. The dynamics behavior of the jets is derived from the Hα line profiles. Quantitative values for one well-identified event have been obtained, showing a maximum projected velocity of 42 km s−1 and a Doppler shift of the order of 20 km s−1. The footpoints/roots of the jets have a lifted center on the Hα line profile compared to the quiet Sun, suggesting a long-lasting heating at these locations. The magnetic field between the small sunspots in the group shows a very high resolution pattern with parasitic polarities along the intergranular lanes accompanied by high-velocity converging flows (4 km s−1) in the photosphere. Magnetic cancellations between the opposite polarities are observed in the vicinity of the footpoints of the jets. Along the intergranular lanes horizontal magnetic field around 1000 G is generated impulsively. Overall, all the kinetic features at the different layers through the photosphere and chromosphere favor a convection-driven reconnection scenario for the recurrent fan-like jets and evidence a site of reconnection between the photosphere and chromosphere corresponding to the intergranular lanes.

Quasiperiodic Microjets Driven by Granular Advection as Observed With High-resolution Imaging at He i 10830 Å

Abstract With high-resolution narrowband He i 10830 Å filtergrams from Goode Solar Telescope, we give an extensive analysis for four granule-sized microeruptions which appear as the gentle ejection of material in He i 10830 Å band. The analysis was aided with the EUV data from Atmospheric Imaging Assembly and line-of-sight magnetograms from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. The microeruptions are situated on magnetic polarity inversion lines (PILs), and their roots are accurately traced down to intergranular lanes. Their durations are different: two microeruptions are repetitive microjets, lasting ∼50 and 27 minutes respectively, while the other two events are singular, lasting ∼5 minutes. For the two microjets, they are continuous and recurrent in the He i 10830 Å band, and the recurrence is quasiperiodic with a period of ∼5 minutes. We found that only transient cospatial EUV brightenings are observed for the longer duration microjets and EUV brightenings are absent for the two singular microeruptions. What is essential to the longer duration microjets is that granules with the concentration of a positive magnetic field persistently transport the magnetic field to the PILs, canceling the opposite magnetic flux and making the base of the two microjets and the underlying granules migrate with the speed of ∼0.25 and 1.0 km s−1. The observations support the scenario of magnetic reconnection for the quasiperiodic microjets and further show that the reconnection continuously generates multitemperature components, especially the cool component with chromospheric temperature. In addition, the ongoing reconnection is modulated by p-mode oscillations inside the Sun.

The Importance of Horizontal Poynting Flux in the Solar Photosphere

Abstract The electromagnetic energy flux in the lower atmosphere of the Sun is a key tool to describe the energy balance of the solar atmosphere. Current investigations on energy flux in the solar atmosphere focus primarily on the vertical electromagnetic flux through the photosphere, ignoring the Poynting flux in other directions and its possible contributions to local heating. Based on a realistic Bifrost simulation of a quiet-Sun (coronal hole) atmosphere, we find that the total electromagnetic energy flux in the photosphere occurs mainly parallel to the photosphere, concentrating in small regions along intergranular lanes. Thereby, it was possible to define a proxy for this energy flux based on only variables that can be promptly retrieved from observations, namely, horizontal velocities of the small-scale magnetic elements and their longitudinal magnetic flux. Our proxy accurately describes the actual Poynting flux distribution in the simulations, with the electromagnetic energy flux reaching 1010 erg cm−2 s−1. To validate our findings, we extended the analysis to Sunrise/IMaX data. First, we show that Bifrost realistically describes photospheric quiet-Sun regions, as the simulation presents similar distributions for line-of-sight magnetic flux and horizontal velocity field. Second, we found very similar horizontal Poynting flux proxy distributions for the simulated photosphere and observational data. Our results also indicate that the horizontal Poynting flux in the observations is considerably larger than the vertical electromagnetic flux from previous observational estimates. Therefore, our analysis confirms that the electromagnetic energy flux in the photosphere is mainly horizontal and is most intense in localized regions along intergranular lanes.

Granular-scale Magnetic Flux Emergence and its Associated Features in an Emerging Active Region

Abstract Using the high-resolution photosphere and chromosphere observations made by the 1 m New Vacuum Solar Telescope, we studied the granular-scale magnetic flux emergence occurring in emerging active region NOAA 12579. Supplementary observations are also provided by the spacecraft Solar Dynamics Observatory. The studied granular-scale flux emergence took place at two different locations. One is completely embedded in the unipolar region of the following sunspots (Case 1), while another is located at the central part in the active region (Case 2). We find that both cases initially emerge from a dark patch like a wide intergranular lane, but showing the different subsequent features. In Case 1, the emerging granule grows in an elongated feature and reaches its maximum size of almost of 5″ × 3″, with an elongated speed of about 2–3 km s−1. An eruption (i.e., surge) with bright footpoints is observed after the emerging granule reaches its maximum scale. There is a time delay of more than 10 minutes between the appearance of the abnormal granule and the Hα surge. Furthermore, its footpoints are clearly rooted at the intergranular lane. We propose that the eruptive surge could be triggered by the reconnection between the emerging magnetic flux and the preexisting ambient field, leading to the localized heating and bidirectional flows. In Case 2, the granular cell emerging is simultaneously associated with bright points with opposite magnetic polarity, showing the separating motion between them and a bunch of newly formed arch filament systems. We infer that the bright points are due to the strong-field magnetic concentration in the dark intergranular lanes rather than the instantaneous Ellerman bombs.

Newly formed downflow lanes in exploding granules in the solar photosphere

Context. Exploding granules have drawn renewed interest because of their interaction with the magnetic field (either emerging or already present). Especially the newly forming downflow lanes developing in their centre seem to be eligible candidates for the intensification of magnetic fields. We analyse spectroscopic data from two different instruments in order to study the intricate velocity pattern within the newly forming downflow lanes in detail. Aims. We aim to examine general properties of a number of exploding granules, such as their lifetime and extend. To gain a better understanding of the formation process of the developing intergranular lane in exploding granules, we study the temporal evolution and height dependence of the line-of-sight velocities at their formation location. Additionally, we search for evidence that exploding granules act as acoustic sources. Methods. We investigated the evolution of several exploding granules using data taken with the Interferometric Bidimensional Spectrometer and the Imaging Magnetograph eXperiment. Velocities for different heights of the solar atmosphere were determined by computing bisectors of the Fe I 6173.0 Å and the Fe I 5250.2 Å lines. We performed a wavelet analysis to study the intensity and velocity oscillations within and around exploding granules. We also compared our observational findings with predictions of numerical simulations. Results. Exploding granules have significantly longer lifetimes (10 to 15 min) than regular granules. Exploding granules larger than 3.8″ form an independent intergranular lane during their decay phase, while smaller granules usually fade away or disappear into the intergranular area (we find only one exception of a smaller exploding granule that also forms an intergranular lane). For all exploding granules that form a new intergranular downflow lane, we find a temporal height-dependent shift with respect to the maximum of the downflow velocity. Our suggestion that this results from a complex atmospheric structure within the newly forming downflow lane is supported by the comparison with synthesised profiles inferred from the simulations. We found an enhanced wavelet power with periods between 120 s to 190 s seen in the intensity and velocity oscillations of high photospheric or chromospheric spectral lines in the region of the dark core of an exploding granule.

Solar atmosphere radiative transfer model comparison based on 3D MHD simulations

Context. The reconstruction of the solar spectral irradiance (SSI) on various time scales is essential for the understanding of the Earth’s climate response to the SSI variability. Aims. The driver of the SSI variability is understood to be the intensity contrast of magnetic features present on the Sun with respect to the largely non-magnetic quiet Sun. However, different spectral synthesis codes lead to diverging projections of SSI variability. In this study we compare three different radiative transfer codes and carry out a detailed analysis of their performance. Methods. We perform the spectral synthesis at the continuum wavelength of 665 nm with the Code for Solar Irradiance, and the Rybicki-Hummer, and Max Planck University of Chicago Radiative MHD codes for three 3D MHD simulations snapshots, a non-magnetic case, and MHD simulations with 100 G, and 200 G magnetic field strength. Results. We determine the intensity distributions, the intensity differences and ratios for the spectral synthesis codes. We identify that the largest discrepancies originate in the intergranular lanes where the most field concentration occurs. Conclusions. Overall, the applied radiative transfer codes give consistent intensity distributions. Also, the intensity variation as a function of magnetic field strength for the particular 100 G and 200 G snapshots agree within the 2–3% range.

Correction of atmospheric stray light in restored slit spectra

Context. A long-standing issue in solar ground-based observations has been the contamination of data due to stray light, which is particularly relevant in inversions of spectropolarimetric data. Aims. We aim to build on a statistical method of correcting stray-light contamination due to residual high-order aberrations and apply it to ground-based slit spectra. Methods. The observations were obtained at the Swedish Solar Telescope, and restored using the multi-frame blind deconvolution restoration procedure. Using the statistical properties of seeing, we created artificially degraded synthetic images generated from magneto-hydrodynamic simulations. We then compared the synthetic data with the observations to derive estimates of the amount of the residual stray light in the observations. In the final step, the slit spectra were deconvolved with a stray-light point spread function to remove the residual stray light from the observations. Results. The RMS granulation contrasts of the deconvolved spectra were found to increase to approximately 12.5%, from 9%. Spectral lines, on average, were found to become deeper in the granules and shallower in the inter-granular lanes, indicating systematic changes to gradients in temperature. The deconvolution was also found to increase the redshifts and blueshifts of spectral lines, suggesting that the velocities of granulation in the solar photosphere are higher than had previously been observed.

Observation of solar coronal heating powered by magneto-acoustic oscillations in a moss region

In this paper, we report the observed temporal correlation between extreme-ultraviolet (EUV) emission and magneto-acoustic oscillations in an EUV moss region, which is the footpoint region only connected by magnetic loops with million-degree plasma. The result is obtained from a detailed multi-wavelength data analysis of the region with the purpose of resolving fine-scale mass and energy flows that come from the photosphere, pass through the chromosphere and finally heat the solar transition region or the corona. The data set covers three atmospheric levels on the Sun, consisting of high-resolution broad-band imaging at TiO 7057 Å and the line of sight magnetograms for the photosphere, high-resolution narrow-band images at helium i 10830 Å for the chromosphere and EUV images at 171 Å for the corona. The 10830 Å narrow-band images and the TiO 7057 Å broad-band images are from a much earlier observation on 2012 July 22 with the 1.6 meter aperture Goode Solar Telescope (GST) at Big Bear Solar Observatory (BBSO) and the EUV 171 Å images and the magnetograms are from observations made by Atmospheric Imaging Assembly (AIA) or Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We report the following new phenomena: (1) Repeated injections of chromospheric material appearing as 10830 Å absorption are squirted out from inter-granular lanes with a period of ∼ 5 minutes. (2) EUV emissions are found to be periodically modulated with similar periods of ∼ 5 minutes. (3) Around the injection area where 10830 Å absorption is enhanced, both EUV emissions and strength of the magnetic field are remarkably stronger. (4) The peaks on the time profile of the EUV emissions are found to be in sync with oscillatory peaks of the stronger magnetic field in the region. These findings may give a series of strong evidences supporting the scenario that coronal heating is powered by magneto-acoustic waves.

High-resolution He I 10830 Å Narrowband Imaging for a Small-scale Chromospheric Jet

Abstract Solar jets are ubiquitous phenomena in the solar atmosphere. They are important in mass and energy transport to the upper atmosphere and interplanetary space. Here, we report a detailed analysis of a small-scale chromospheric jet with high-resolution He i 10830 Å and TiO 7057 Å images observed by the 1.6 m aperture Goode Solar Telescope at the Big Bear Solar Observatory. The observation reveals the finest dark threads inside the jet are rooted in the intergranular lanes. Their width is equal to the telescope’s diffraction limit at 10830 Å (∼100 km). The jet is recurrent and its association with the emergence and convergence of magnetic flux is observed. Together with other important features like photospheric flow toward the magnetic polarity inversion line, a bald-patch magnetic configuration, and earlier excitation of helium atoms, we propose that the jet might be initiated by magnetic reconnection in a U-shaped loop configuration. The plasmoid configuration results from the possible buoyancy of the magnetic reconnection, which reoccurs in a second step with an overlying magnetic field line. Notably, the second-step magnetic reconnection produces not only bidirectional cool or hot flows but also a new U-shaped loop configuration. The feature may be used to explain the recurrent behavior of the jet, since the new U-shaped loop can be driven to reconnect again.

The influence of NLTE effects in Fe I lines on an inverted atmosphere

Context. This paper forms the second part of our study of how neglecting non-local thermodynamic equilibrium (NLTE) conditions in the formation of Fe I 6301.5 Å and the 6302.5 Å lines affects the atmosphere that is obtained by inverting the Stokes profiles of these lines in LTE. The main cause of NLTE effects in these lines is the line opacity deficit that is due to the excess ionisation of Fe I atoms by ultraviolet (UV) photons in the Sun. Aims. In the first paper, these photospheric lines were assumed to have formed in 1D NLTE and the effects of horizontal radiation transfer (RT) were neglected. In the present paper, the iron lines are computed by solving the RT in 3D. We investigate the effect of horizontal RT on the inverted atmosphere and how it can enhance or reduce the errors that are due to neglecting 1D NLTE effects. Methods. The Stokes profiles of the iron lines were computed in LTE, 1D NLTE, and 3D NLTE. They were all inverted using an LTE inversion code. The atmosphere from the inversion of LTE profiles was taken as the reference model. The atmospheres from the inversion of 1D NLTE profiles (testmodel-1D) and 3D NLTE profiles (testmodel-3D) were compared with it. Differences between reference and testmodels were analysed and correspondingly attributed to NLTE and 3D effects. Results. The effects of horizontal RT are evident in regions surrounded by strong horizontal temperature gradients. That is, along the granule boundaries, regions surrounding magnetic elements, and its boundaries with intergranular lanes. In some regions, the 3D effects enhance the 1D NLTE effects, and in some, they weaken these effects. In the small region analysed in this paper, the errors due to neglecting the 3D effects are lower than 5% in temperature. In most of the pixels, the errors are lower than 20% in both velocity and magnetic field strength. These errors also persist when the Stokes profiles are spatially and spectrally degraded to the resolution of the Swedish Solar Telescope (SST) or Daniel K. Inouye Solar Telescope (DKIST). Conclusions. Neglecting horizontal RT introduces errors not only in the derived temperature, but also in other atmospheric parameters. The error sizes depend on the strength of the local horizontal temperature gradients. Compared to the 1D NLTE effect, the 3D effects are more localised in specific regions in the atmosphere and are weaker overall.

Spatially resolved measurements of the solar photospheric oxygen abundance

Aims. We report the results of a novel determination of the solar oxygen abundance using spatially resolved observations and inversions. We seek to derive the photospheric solar oxygen abundance with a method that is robust against uncertainties in the model atmosphere. Methods. We use observations with spatial resolution obtained at the Vacuum Tower Telescope to derive the oxygen abundance at 40 different spatial positions in granules and intergranular lanes. We first obtain a model for each location by inverting the Fe I lines with the NICOLE inversion code. These models are then integrated into a hierarchical Bayesian model that is used to infer the most probable value for the oxygen abundance that is compatible with all the observations. The abundance is derived from the [O I] forbidden line at 6300 Å taking into consideration all possible nuisance parameters that can affect the abundance. Results. Our results show good agreement in the inferred oxygen abundance for all the pixels analyzed, demonstrating the robustness of the analysis against possible systematic errors in the model. We find a slightly higher oxygen abundance in granules than in intergranular lanes when treated separately (log(ϵO) = 8.83 ± 0.02 vs. log(ϵO) = 8.76 ± 0.02), which is a difference of approximately 2-σ. This tension suggests that some systematic errors in the model or the radiative transfer still exist but are small. When taking all pixels together, we obtain an oxygen abundance of log(ϵO) = 8.80 ± 0.03, which is compatible with both granules and lanes within 1-σ. The spread of results is due to both systematic and random errors.