  The Sun provides the main energy input to the terrestrial atmosphere, and yet
the impact of solar variability on long-term changes remains a controversial
issue. Direct radiative forcing is the most studied mechanism. Other much
weaker mechanisms, however, can have a significant leverage, but the underlying
physics is often poorly known.
  We review the main mechanisms by which solar variability may impact the
terrestrial atmosphere, on time scales ranging from days to millennia. This
includes radiative forcing, but also the effect of interplanetary perturbations
and energetic particle fluxes, all of which are eventually driven by the solar
magnetic field.
