Milan Gošić

The magnetic fields in the solar polar region are important to our understanding of the internal dynamo process, the global coronal structure, and the origin of the solar wind. The inference of polar fields based on spectropolarimetric observation is highly model-dependent and can suffer from various systematic effects. Here, we analyze a raster map of the southern polar region taken by the Hinode Spectro-Polarimeter, utilizing the Stokes Inversion based on Response functions code. The inversions provide height-dependent vector magnetic field maps between optical depths log 10 τ = − 2 and 0. We examine the impacts on the total magnetic flux estimates from adopting: (1) one-versus-two-component atmospheric models via a “filling factor” parameter; and (2) different analysis schemes. At log 10 τ = − 1.5 , the polar magnetic flux is estimated to be (1.84 ± 0.03) × 10 ²¹ Mx and (1.38 ± 0.02) × 10 ²¹ Mx under the one- and two-component atmosphere assumptions, respectively. The magnetic flux is approximately constant or increases slightly with height, respectively. We find that the two-component (one-component) configuration is preferred for 58.3% (32.3%) of the pixels. Different initial guesses—including the input atmosphere model and the filling factor—as well as different inversion settings can significantly affect the results, especially for locations with weaker polarization signals. Our work highlights the importance of considering unresolved magnetic structures or stray light. Model degeneracy and the convergence to local minima limit the precision of the polar magnetic flux inference (no better than several tens of percent, in this case). Higher-resolution observations and advanced inversion and disambiguation algorithms may alleviate these limitations.

We introduce an improved and fast inversion tool that is able to provide the thermodynamics of the solar atmosphere from the photosphere to the top of the chromosphere, as well as the integrated radiative losses (IRLs) in the chromosphere for data observed by the Interface Region Imaging Spectrograph (IRIS). This NASA mission has been observing the Sun and providing, among other kinds of data, multiline spectral observations sensitive to changes in the lower solar atmosphere since 2013. In this paper, we explain the new inversion tool IRIS ²⁺ based on the IRIS ²⁺ database, which is based on 135,472 synthetic representative profiles (RPs), each of them consisting of six chromospheric lines and six photospheric lines, their corresponding representative model atmospheres (RMA), and the IRL associated with these atmospheres. A nearest neighbor (k-nn) model algorithm is trained with the synthetic representative profiles to predict the closest RP in the database to the one observed, at which point IRIS ²⁺ assigns the RMA and the IRL to the location of that observed profile. We have compared the results obtained by IRIS ²⁺ with results obtained from the state-of-the-art inversion code STiC, which is also used to build the IRIS ²⁺ database. We find that the thermodynamics and the IRL obtained with both methods are comparable in most cases. Therefore, IRIS ²⁺ is a fast and reliable inversion tool that provides approximate values of the thermodynamic state and the radiative losses in the lower solar atmosphere for a large variety of solar scenes observed with IRIS.

The spatiotemporal inhomogeneous-homogeneous transition in the dynamics and structures of solar photospheric turbulence is studied by applying the complexity-entropy analysis to Hinode images of a vortical region of supergranular junctions in the quiet Sun. During a period of supergranular vortex expansion lasting 37.5 min, the spatiotemporal dynamics of the line-of-sight magnetic field and the horizontal electromagnetic energy flux displayed characteristics of an inverse turbulent cascade, as evidenced by the formation of a large magnetic coherent structure via the merger of two small magnetic elements trapped by a long-duration vortex. Consistent with Hinode observations, the magnetic and Poynting fluxes both exhibited an admixture of chaos and stochasticity in the complexity-entropy plane involving a temporal transition from low to high complexity and a temporal transition from high to low entropy during the period of vortex expansion.

Studying the emergence of magnetic fields is essential for understanding the physical mechanisms behind various phenomena in the solar atmosphere. Most importantly, the emerging fields offer valuable insights into how energy and mass are transferred to the upper solar atmosphere. As a result, they have garnered significant attention from both observational and theoretical perspectives. In this article, we present two models of quiet-Sun-like magnetic fields generated by the Bifrost code. We compare these models with observations from the Swedish 1 m Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). By tracking the magnetic features in both the SST and Bifrost data, we determine the similarities and differences between the fields identified in the models and those observed. We conduct a quantitative comparison of various properties, such as flux content, flux densities, horizontal and line-of-sight velocities, lifetimes, sizes, and surface interactions. Additionally, we identify and analyze the properties of the largest emerging bipoles in the SST and Bifrost data. Our findings indicate that the magnetic bipoles in the Bifrost simulations are generally stronger than those observed with the SST. However, a qualitative comparison of the chromospheric and transition region responses to the emerging fields in the Bifrost models, SST, and IRIS observations shows similar heating processes occurring above and around the emerging fields. Finally, we outline our plans for future work aimed at studying the emergence of magnetic fields in the quiet Sun, with a particular focus on the chromosphere and upper atmospheric layers.

Studying the emergence of magnetic fields is essential for understanding the physical mechanisms behind various phenomena in the solar atmosphere. Most importantly, the emerging fields offer valuable insights into how energy and mass are transferred to the upper solar atmosphere. As a result, they have garnered significant attention from both observational and theoretical perspectives. In this article, we present two models of quiet-Sun-like magnetic fields generated by the Bifrost code. We compare these models with observations from the Swedish 1-meter Solar Telescope (SST) and the Interface Region Imaging Spectrograph (IRIS). By tracking the magnetic features in both the SST and Bifrost data, we determine the similarities and differences between the fields identified in the models and those observed. We conduct a quantitative comparison of various properties, such as flux content, flux densities, horizontal and line-of-sight velocities, lifetimes, sizes, and surface interactions. Additionally, we identify and analyze the properties of the largest emerging bipoles in the SST and Bifrost data. Our findings indicate that the magnetic bipoles in the Bifrost simulations are generally stronger than those observed with the SST. However, a qualitative comparison of the chromospheric and transition region responses to the emerging fields in the Bifrost models, SST, and IRIS observations shows similar heating processes occurring above and around the emerging fields. Finally, we outline our plans for future work aimed at studying the emergence of magnetic fields in the quiet Sun, with a particular focus on the chromosphere and upper atmospheric layers.

Small-scale, newly emerging internetwork (IN) magnetic fields are considered a viable source of energy and mass for the solar chromosphere and possibly the corona. Multiple studies show that single events of flux emergence can indeed locally heat the low solar atmosphere through interactions of the upward propagating magnetic loops and the preexisting ambient field lines. However, the global impact of the newly emerging IN fields on the solar atmosphere is still unknown. In this paper, we study the spatiotemporal evolution of IN bipolar flux features and analyze their impact on the energetics and dynamics of the quiet-Sun atmosphere. We use high-resolution, multiwavelength, coordinated observations obtained with the Interface Region Imaging Spectrograph, Hinode, and the Solar Dynamics Observatory to identify emerging IN magnetic fields and follow their evolution. Our observational results suggest that only the largest IN bipoles are capable of heating locally the low solar atmosphere, while the global contribution of these bipoles appears to be marginal. However, the total number of bipoles detected and their impact estimated in this work is limited by the sensitivity level, spatial resolution, and duration of our observations. To detect smaller and weaker IN fields that would maintain the basal flux, and examine their contribution to the chromospheric heating, we will need higher resolution, higher sensitivity, and longer time series obtained with current and next-generation ground- and space-based telescopes.

We present the most comprehensive collection of stratified-in-optical-depth thermodynamic models of the lower solar atmosphere. This database, named IRIS²⁺, consists of 40,320 synthetic representative profiles (RPs) and their corresponding representative model atmospheres (RMAs). The latter result from inverting the observed RPs, which are the result of clustering multiline spectral profiles observed by the Interface Region Imaging Spectrograph in 126 active regions (ARs). Each AR was clustered in 320 RPs considering five solar features: umbra, pore or pore-like, penumbra, plage, and quiet Sun. The multiline spectral profiles contain 12 lines with encoded physical information from the top of the chromosphere to the midphotosphere. These 12 spectral lines, six chromospheric and six photospheric, were simultaneously inverted with the Stockholm Inversion Code. Therefore, the stratified-in-optical-depth low solar atmosphere models obtained in this study are better constrained than those obtained from the nonsimultaneous inversion of individual lines. Each tuple in the database contains an inverted RP, its corresponding RMA, the solar feature label where the observed RP was clustered, its heliocentric angle, and its date and time of observation. The database may be used as an inversion code for any of the lines present in the RPs, and the RMAs may be used for synthesizing spectral lines in the chromosphere and photosphere or as mean constraints for numerical models of the solar atmosphere.

Whitepaper #333 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: space weather applications; basic research. Additional topics: space weather research/operations/research loop; space weather research/applications/operations pipeline; […]

The Mg ii h and k lines are among the best diagnostic tools of the upper solar chromosphere. This region of the atmosphere is of particular interest, as it is the lowest region of the Sun’s atmosphere where the magnetic field is dominant in the energetics and dynamics, defining its structure. While highly successful in the photosphere and lower to mid-chromosphere, numerical models have produced synthetic Mg ii lines that do not match the observations well. We present a number of large-scale models with magnetic field topologies representative of the quiet Sun, ephemeral flux regions and plage, and also models where the numerical resolution is high and where we go beyond the MHD paradigm. The results of this study show models with a much improved correspondence with IRIS observations in terms of both intensities and widths, especially underscoring the importance of chromospheric mass loading and of capturing the magnetic field topology and evolution in simulations. This comes in addition to the importance of capturing the generation of small-scale velocity fields and including nonequilibrium ionization and ion−neutral interaction effects. However, it should be noted that difficulties in achieving a good correspondence remain, especially when considering the width of Mg ii h and k lines in plage. Understanding and modeling all these effects and their relative importance is necessary in order to reproduce observed spectral features and in isolating the missing pieces necessary to fully comprehend Mg ii formation.