Impact of magnetic field on emergent solar spectra
Solar brightness varies at all measured timescales and wavelengths, and can
affect terrestrial atmosphere and climate. Variations on timescales longer than a day
are driven by the solar surface magnetic activity. Solar magnetic field modifies the
structure of the solar atmosphere and its radiative properties, appearing at the surface
as dark spots and bright faculae. These features continuously evolve with time and
modulate solar brightness. Although significant progress has been made in modeling
solar brightness variations, their amplitude in the ultraviolet (UV) range remains
controversial. IMagE aims at resolving this controversy.

A crucial ingredient of the irradiance models are brightness spectra of the various
magnetic components. Spectra that have been used until now relied on a number of
simplifications that are not valid in the UV. To properly account for the physical
mechanisms which influence the solar variability in the UV, including the
line blanketing and departures from local thermodynamic equilibrium (LTE),
non-LTE computations of spectra from realistic 3D magnetohydrodynamic (MHD)
atmospheres are needed. This is
computationally extremely challenging. IMagE will exploit state-of-the-art MHD
and radiative transfer simulations to device a method for efficient, yet accurate,
synthesis of the non-LTE
brightness spectra of the different magnetic components. This method will be validated
against high spatial resolution observations of the Sun. Incorporation of the spectra
computed with this method in the physics-based irradiance models
will 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 solar
disc positions produced within IMagE can also be used to analyze the data from
future missions, for instance SUNRISE III and the maiden Indian solar mission Aditya-L1.