IMagE · Impact of Magnetic field on Emergent solar spectra

Solar brightness varies at all measured timescales and wavelengths, and canaffect terrestrial atmosphere and climate. Variations on timescales longer than a dayare driven by the solar surface magnetic activity. Solar magnetic field modifies thestructure of the solar atmosphere and its radiative properties, appearing at the surfaceas dark spots and bright faculae. These features continuously evolve with time andmodulate solar brightness. Although significant progress has been made in modelingsolar brightness variations, their amplitude in the ultraviolet (UV) range remainscontroversial. IMagE aims at resolving this controversy.A crucial ingredient of the irradiance models are brightness spectra of the variousmagnetic components. Spectra that have been used until now relied on a number ofsimplifications that are not valid in the UV. To properly account for the physicalmechanisms which influence the solar variability in the UV, including theline blanketing and departures from local thermodynamic equilibrium (LTE),non-LTE computations of spectra from realistic 3D magnetohydrodynamic (MHD)atmospheres are needed. This iscomputationally extremely challenging. IMagE will exploit state-of-the-art MHDand radiative transfer simulations to device a method for efficient, yet accurate,synthesis of the non-LTEbrightness spectra of the different magnetic components. This method will be validatedagainst high spatial resolution observations of the Sun. Incorporation of the spectracomputed with this method in the physics-based irradiance modelswill lead to a breakthrough in our understanding of the solar UV irradiance variability.The grid of non-LTE spectra for different magnetic field strengths and solardisc positions produced within IMagE can also be used to analyze the data fromfuture missions, for instance SUNRISE III and the maiden Indian solar mission Aditya-L1.