  Searches for small exoplanets around solar-type stars are limited by stellar
physical variability. While chromospheric variability is well studied,
observing, modeling. and understanding the much smaller fluctuations in
photospheric spectral line strengths, shapes, and shifts is challenging.
Extreme precision radial-velocity spectrometers now enable extreme precision
stellar spectroscopy and time series of the Sun seen as a star permit
monitoring of its photospheric variability. Fluctuations in their line
strengths may well correlate with radial-velocity excursions and identify
observable proxies for their monitoring. From three years of HARPS-N
observations of the Sun-as-a-star, one thousand low-noise spectra are selected,
and line absorption measured in Fe I, Fe II, Mg I, Mn I, H-alpha, H-beta,
H-gamma, Na I, and the G-band. Their variations and likely atmospheric origins
are examined, also with respect to simultaneously measured chromospheric
emission and apparent radial velocity. Systematic line-strength variability is
seen, largely shadowing the solar-cycle evolution of Ca II H & K emission, but
with smaller amounts, typically on a sub-percent level. Among iron lines,
greatest amplitudes are for Fe II in the blue, while the trends change sign
among differently strong lines in the green Mg I triplet and between Balmer
lines. Variations in the G-band core are greater than of the full G-band, in
line with theoretical predictions. No variation is detected in the
semi-forbidden Mg I 457.1 nm. Hyperfine split Mn I behaves largely similar to
Fe I. For lines at longer wavelengths, telluric absorption limits the
achievable precision. Microvariability in the solar photospheric spectrum thus
displays systematic signatures among various features. These measure something
different than the classical Ca II H & K index while still reflecting a strong
influence from magnetic regions.
