  Long-term high-cadence measurements of stellar spectral variability are
fundamental to better understand stellar atmospheric properties and stellar
magnetism. These, in turn, are fundamental for the detectability of exoplanets
as well as the characterization of their atmospheres and habitability. The Sun,
viewed as a star via disk-integrated observations, offers a means of exploring
such measurements while also offering the spatially resolved observations that
are necessary to discern the causes of observed spectral variations.
High-spectral resolution observations of the solar spectrum are fundamental for
a variety of Earth-system studies, including climate influences, renewable
energies, and biology. The Integrated Sunlight Spectrometer at SOLIS, has been
acquiring daily high-spectral resolution Sun-as-a-star measurements since
2006.More recently, a few ground-based telescopes with the capability of
monitoring the solar visible spectrum at high spectral resolution have been
deployed (e.g. PEPSI, HARPS, NEID). However, the main scientific goal of these
instruments is to detect exo-planets, and solar observations are acquired
mainly as a reference. Consequently, their technical requirements are not ideal
to monitor solar variations with high photometric stability, especially over
solar-cycle temporal scales.The goal of this white paper is to emphasize the
scientific return and explore the technical requirements of a network of
ground-based spectrographs devoted to long-term monitoring of disk-integrated
solar-spectral variability with high spectral resolution and high photometric
stability, in conjunction with disk-resolved observations in selected spectral
lines,to complement planet-hunter measurements and stellar-variability studies.
The proposed network of instruments offers the opportunity for a larger variety
of multidisciplinary studies.
