  Transmission spectra probe the atmospheres of transiting exoplanets, but
these observations are also subject to signals introduced by magnetic active
regions on host stars. Here we outline scientific opportunities in the next
decade for providing useful constraints on stellar photospheres and inform
interpretations of transmission spectra of the smallest ($R<4\,R_{\odot}$)
exoplanets. We identify and discuss four primary opportunities: (1) refining
stellar magnetic active region properties through exoplanet crossing events;
(2) spectral decomposition of active exoplanet host stars; (3) joint retrievals
of stellar photospheric and planetary atmospheric properties with studies of
transmission spectra; and (4) continued visual transmission spectroscopy
studies to complement longer-wavelength studies from $\textit{JWST}$. We make
five recommendations to the Astro2020 Decadal Survey Committee: (1) identify
the transit light source (TLS) effect as a challenge to precise exoplanet
transmission spectroscopy and an opportunity ripe for scientific advancement in
the coming decade; (2) include characterization of host star photospheric
heterogeneity as part of a comprehensive research strategy for studying
transiting exoplanets; (3) support the construction of ground-based extremely
large telescopes (ELTs); (4) support multi-disciplinary research teams that
bring together the heliophysics, stellar physics, and exoplanet communities to
further exploit transiting exoplanets as spatial probes of stellar
photospheres; and (5) support visual transmission spectroscopy efforts as
complements to longer-wavelength observational campaigns with $\textit{JWST}$.
