The Aditya-L1 is the first space-based solar observatory of the Indian Space Research Organization (ISRO). The spacecraft will carry seven payloads providing uninterrupted observations of the Sun from the first Lagrangian point. Aditya-L1 comprises four remote sensing instruments, viz. a coronagraph observing in visible and infrared, a full disk imager in Near Ultra-Violet (NUV), and two full-sun integrated spectrometers in soft X-ray and hard X-ray. In addition, there are three instruments for in-situ measurements, including a magnetometer, to study the magnetic field variations during energetic events. Aditya-L1 is truly a mission for multi-messenger solar astronomy from space that will provide comprehensive observations of the Sun across the electromagnetic spectrum and in-situ measurements in a broad range of energy, including magnetic field measurements at L1.

We present a statistical study of the dependencies of the shapes and sizes of the photospheric convective cells on the magnetic field properties. This analysis is based on a 2.5 hour long SST observations of active region NOAA 11768. We have blue continuum images taken with a cadence of 5.6 sec that are used for segmentation of individual granules and 270 maps of spectropolarimetric CRISP data allowing us to determine the properties of the magnetic field along with the line-of-sight velocities. The sizes and shapes of the granular cells are dependent on the the magnetic field strength, where the granules tend to be smaller in regions with stronger magnetic field. In the presence of highly inclined magnetic fields, the eccentricity of granules is high and we do not observe symmetric granules in these regions. The mean up-flow velocities in granules as well as the granules intensities decrease with increasing magnetic field strength.

We present a new determination of the solar nitrogen abundance making use of 3D hydrodynamical modelling of the solar photosphere, which is more physically motivated than traditional static 1D models. We selected suitable atomic spectral lines, relying on equivalent width measurements already existing in the literature. For atmospheric modelling we used the co5bold 3D radiation hydrodynamics code. We investigated the influence of both deviations from local thermodynamic equilibrium (non-LTE effects) and photospheric inhomogeneities (granulation effects) on the resulting abundance. We also compared several atlases of solar flux and centre-disc intensity presently available. As a result of our analysis, the photospheric solar nitrogen abundance is A(N) = 7.86 ± 0.12.

An abstract mathematical concept of fractal organization of certain complex objects received significant attention in astrophysics during last decades. The concept evolved into a broad field including multi-fractality and intermittency, percolation theory, self-organized criticality, theory of catastrophes, etc. Such a strong mathematical and physical approach provide new possibilities for exploring various aspects of astrophysics. In particular, in the solar and stellar magnetism, multi-fractal properties of magnetized plasma turned to be useful for understanding burst-like dynamics of energy release events, conditions for turbulent dynamo action, nature of turbulent magnetic diffusivity, and even the dual nature of solar dynamo. In this talk, I will briefly outline how the ideas of multi-fractality are used to explore the above mentioned aspects of solar magnetism.

Quiet-Sun magnetic fields are enigmatic in terms of their properties, and their origin is not well understood. One likely possibility is that they are a consequence of interactions with turbulent convective motions of various temporal and spatial scales. Here we investigate the relationship between small-scale magnetic fields and various convection flows. We demonstrate that in addition to granulation and supergranulation, mesogranulation also plays an important role in structuring quiet-Sun magnetic fields. We also study the vector magnetic fields in the quiet Sun, and propose that emerging granular-scale bipolar loops are major sources of the quiet-Sun magnetic fields.

Filamentary structure is a fundamental property of the magnetized solar plasma. Recent high-resolution observations and numerical simulations have revealed close links between the filamentary structures and plasma dynamics in large-scale solar phenomena, such as sunspots and magnetic network. A new emerging paradigm is that the mechanisms of the filamentary structuring and large-scale organization are natural consequences of turbulent magnetoconvection on the Sun. We present results of 3D radiative MHD large-eddy simulations (LES) of magnetic structures in the turbulent convective boundary layer of the Sun. The results show how the initial relatively weak and uniformly distributed magnetic field forms the filamentary structures, which under certain conditions gets organized on larger scales, creating stable long-living magnetic structures. We discuss the physics of magnetic self-organization in the turbulent solar plasma, and compare the simulation results with observations.

We report on modelling in stellar astrophysics with the ANTARES code. First, we describe properties of turbulence in solar granulation as seen in high-resolution calculations. Then, we turn to the first 2D model of pulsation-convection interaction in a cepheid. We discuss properties of the outer and the HeII ionization zone. Thirdly, we report on our work regarding models of semiconvection in the context of stellar physics.

Long-term trends in the solar spectral irradiance are important to determine the impact on Earth's climate. These long-term changes are thought to be caused mainly by changes in the surface area covered by small-scale magnetic elements. The direct measurement of the contrast to determine the impact of these small-scale magnetic elements is, however, limited to a few wavelengths, and is, even for space instruments, affected by scattered light and instrument defocus. In this work we calculate emergent intensities from 3-D simulations of solar magneto-convection and validate the outcome by comparing with observations from Hinode/SOT. In this manner we aim to construct the contrast at wavelengths ranging from the NUV to the FIR.

Numerical simulations of convection near the solar surface are now advanced enough to reproduce both a large set of observational data and provide tests for convection models. We discuss the role of coherent structures in models of solar p-mode excitation, for which the analysis of numerical simulations has provided key inputs in the modelling. The robustness of these simulations is shown by a comparison illustrating the influence of boundary conditions on ensemble averaged quantities. In a concluding example advanced high resolution simulations are shown to resolve the onset of shear driven turbulence generated by up- and downflow structures.

The solar chemical composition has recently undergone a drastic revision, in particular in terms of the C, N, O and Ne abundances that have been lowered by almost a factor of two. In this invited review I will describe the different compounding reasons for this change (3D model atmospheres, non-LTE line formation, improved atomic/molecular data) and discuss some astrophysical implications thereof, which fall under both good (solar neighborhood) and bad (helioseismology) news. The most recent literature regarding the solar O abundance is surveyed and a critical evaluation whether or not these support the low solar abundance scale is presented. Finally I venture to make some predictions to what the real solar O abundance may be.

The results of applying two algorithmic techniques to time-averaged images of the solar granulation are presented. Some quasi-regular structures, which are hidden in the intense noise due to the presence of light blotches in the images, can be detected in this way.

In the quest for a better theoretical understanding of the dynamical processes operating in stellar envelopes, increasingly more realistic hydrodynamical models of stellar convection have been developed over the last 25 years. Based on 3D state-of-the-art simulations performed recently with the radiation hydrodynamics code CO5BOLD, we review the present status of hydrodynamical modeling of stellar surface convection and demonstrate the basic differences between 3D hydrodynamical models and 1D ‘classical’ hydrostatic atmospheres where convection is treated by the mixing-length theory. We briefly discuss some of the many possible applications across the Hertzsprung-Russell diagram, including spectroscopic abundance determinations using 3D stellar atmospheres, particularly with regard to the much debated question of the solar oxygen abundance.

In the Sun, the convection zone reaches up to the solar photosphere and can thus directly influence the emergent spectrum. Traditionally, the effects of convection has been modelled with the local mixing length theory in theoretical 1D hydrostatic model atmospheres. In a different approach, we have performed realistic time-dependent, 3D, radiative-hydrodynamical simulations of the outer layers of the solar convection zone, including the photosphere. Both the different mean stratification and the presence of atmospheric inhomogeneities in 3D impact the spectral line formation. In a series of papers, we have applied our 3D solar model atmosphere to the problem of the solar chemical composition. Furthermore, we have adopted the best possible atomic and molecular line data and taken into account departures from LTE in the line formation when necessary. The inferred C, N, O and Ne abundances are all significantly lower than estimated from previous 1D modelling by 0.2-0.3 dex. These results have significant implications for a range of topics in contemporary astrophysics, including causing a severe headache for helioseismology.

We present the essential features of the ANTARES code. ANTARES has been developed to perform the simulation of astrophysical flows in one, two, or three dimensions. Using, in particular, the option of grid refinement, we present results for solar granulation achieved at very high spatial resolution.

Classical theories of turbulence do not describe accurately inertial range scaling laws in turbulent convection and notably fail to model the shape of the turbulent spectrum of solar photospheric convection. To understand these discrepancies, a detailed study of scale-by-scale budgets in turbulent Rayleigh-Bénard convection is presented, with particular emphasis placed on anisotropy and inhomogeneity. A generalized Kolmogorov equation applying to convection is derived and its various terms are computed using numerical simulations of turbulent Boussinesq convection. The analysis of the isotropic part of the equation shows that the third-order velocity structure function is significantly affected by buoyancy forcing and large-scale inhomogeneities. Anisotropic contributions to this equation are also shown to be comparable to their isotropic counterpart at moderate to large scales. Implications of these results for convection in the solar photosphere, mesogranulation and supergranulation are discussed.

Reversed granulation is predicted by numerical simulations and has been observed in the wings of chromospheric lines. Here we present reversed granulation in high spatial, spectral and temporal resolution, extracted from the photospheric Fe I 7090.4 Å line core intensities, once taking into account the velocity effects. Correlations with the continuum are studied in terms of smoothing, time delay and temporal evolution. The latter appears to be superimposed with an oscillatory signal that suggests the presence of gravity waves in the mid-photosphere.

High spatial resolution observations of the solar granulation up to 3 μm are possible on existing vacuum solar telescopes. They permit the analysis of the deepest photospheric layers near 1.7 μm. A high photometric accuracy is achieved using scanning techniques with a pinhole photometer; imaging methods are used for 2D-analysis. Statistically significant results on granulation, including power spectrum and histogram analysis, center-limb variations and lifetime analysis, are presented. Temperature fluctuations of periods near 5 minute are considered at 1.7 μm, as well as large-scale variations at the scale of the meso- and, especially, the super-granulation. We also discuss the “abnormal” granulation in magnetic regions and the umbral granulation in the cores of sunspots.

Three-dimensional simulations of solar convection are described. The simulations show that viewing convection as a hierarchy of eddies does not properly represent the large scale topology. A better picture is to view convection as a broad warm upflow with embedded cool, narrow, downdrafts. These downdrafts penetrate many scale heights through the convection zone and carry most of the net convective flux. Near the solar surface there are extremely large fluctuations in the temperature (5000-11000 K), entropy and pressure (factor of four). Radiation temperature does not provide an accurate measure of the gas temperature at a given geometric depth, because the opacity is very temperature sensitive. The emergent intensity in the infrared is smaller and has a smaller contrast than in the visible. However, in terms of radiation temperature the infrared is hotter and has a higher contrast than the visible.

Line shifts and asymmetries of spectral lines have been found in the ATMOS spectra. These are due to the granulation at the solar surface. A two-component model for the solar photosphere is used to calculate theoretical profiles which are compared to the observations. The ATMOS lines provide additional constraints on models for the solar photosphere.

We report on a first analysis of the horizontal motions in a 45 minute (32 × 22 arcsec2 field of view) granulation time series (movie presented at the present IAU Symposium) obtained at 1.6 μm with the Vacuum Tower Telescope (VTT) of the National Solar Observatory at Sacramento Peak (NSO/SP). High signal/noise flow maps are obtained by use of the local cross correlation technique (November 1986) which incorporates efficient attenuation of seeing and 5-min oscillations. The flow pattern, showing a (≈ 30 arcsec diameter) supergranule with (≈ 8-15 arcsec) mesogranules superposed, is long lived compared to the 45 min of observations. The computed flows (velocity, divergence, vorticity) resemble the ones obtained at visible wavelengths (e.g., by Brandt et al. 1991, November 1989, November and Simon 1988, Simon et al. 1988). The high quality of the the flow maps (due to a large number of selected images (1500), and (supposedly) smaller 5-min oscillations and better seeing conditions at 1.6 μm) allows us to study time evolution (resolution ≈ 15 min) of the details of the flow (spatial resolution ≈ 3 arcsec). An interesting new finding is the short lifetime (< 45 min) of vorticity as opposed to the long lived (≫ 45 min) divergence of the flow. Our study demonstrates the possibility of using the 1.6 μm window to the opacity minimum region to study the horizontal flows at these deep layers of the photosphere.