  Context. Stellar spectral variability on timescales of a day and longer,
arising from magnetic surface features such as dark spots and bright faculae,
is an important noise source when characterising extra-solar planets. Current
1D models of faculae do not capture the geometric properties and fail to
reproduce observed solar facular contrasts. Magnetoconvection simulations
provide facular contrasts accounting for geometry. Aims. We calculate facular
contrast spectra from magnetoconvection models of the solar photosphere with a
view to improve (a) future parameter determinations for planets with early G
type host stars and (b) reconstructions of solar spectral variability. Methods.
Regions of a solar twin (G2, log g=4.44) atmosphere with a range of initial
average vertical magnetic fields (100 to 500~G) were simulated using a 3D
radiation-magnetohydrodynamics code, MURaM, and synthetic intensity spectra
were calculated from the ultraviolet (149.5~nm) to the far infrared (160000~nm)
with the ATLAS9 radiative transfer code. Nine viewing angles were investigated
to account for facular positions across most of the stellar disc. Results.
Contrasts of the radiation from simulation boxes with different levels of
magnetic flux relative to an atmosphere with no magnetic field are a
complicated function of position, wavelength and magnetic field strength that
is not reproduced by 1D facular models. Generally, contrasts increase towards
the limb, but at UV wavelengths a saturation and decrease are observed close to
the limb. Contrasts also increase strongly from the visible to the UV; there is
a rich spectral dependence, with marked peaks in molecular bands and strong
spectral lines. At disc centre, a complex relationship with magnetic field was
found and areas of strong magnetic field can appear either dark or bright,
depending on wavelength...
