  Exoplanet host star activity, in the form of unocculted star spots or
faculae, alters the observed transmission and emission spectra of the
exoplanet. This effect can be exacerbated when combining data from different
epochs if the stellar photosphere varies between observations due to activity.
redHere we present a method to characterize and correct for relative changes
due to stellar activity by exploiting multi-epoch ($\ge$2 visits/transits)
observations to place them in a consistent reference frame. Using measurements
from portions of the planet's orbit where negligible planet transmission or
emission can be assumed, we determine changes to the stellar spectral
amplitude. With the analytical methods described here, we predict the impact of
stellar variability on transit observations. Supplementing these forecasts with
Kepler-measured stellar variabilities for F-, G-, K-, and M-dwarfs, and
predicted transit precisions by JWST's NIRISS, NIRCam, and MIRI, we conclude
that stellar activity does not impact infrared transiting exoplanet
observations of most presently-known or predicted TESS targets by current or
near-future platforms, such as JWST.
