  Brightness inhomogeneities in the stellar photosphere (dark spots or bright
regions) affect the measurements of the planetary transmission spectrum. To
investigate the star spots of the M dwarf GJ 1214, we conducted a multicolor
photometric monitoring from 2012 to 2016. The measured variability shows a
periodicity of 125 +- 5 days, which we interpret as the signature of the
stellar rotation period. This value overrules previous suggestions of a
significantly shorter stellar rotation period. A light curve inversion of the
monitoring data yields an estimation of the flux dimming of a permanent spot
filling factor not contributing to the photometric variability, a temperature
contrast of the spots of about 370 K and persistent active longitudes. The
derived surface maps over all five seasons were used to estimate the influence
of the star spots on the transmission spectrum of the planet from 400 nm to
2000 nm. The monitoring data presented here do not support a recent
interpretation of a measured transmission spectrum of GJ 1214b as to be caused
by bright regions in the stellar photosphere. Instead, we list arguments as to
why the effect of dark spots likely dominated over bright regions in the period
of our monitoring. Furthermore, our photometry proves an increase in
variability over at least four years, indicative for a cyclic activity
behavior. The age of GJ 1214 is likely between 6 and 10 Gyr. The long-term
photometry allows for a correction of unocculted spots. For an active star such
as GJ 1214, there remains a degeneracy between occulted spots and the transit
parameters used to build the transmission spectrum. This degeneracy can only be
broken by high-precision transit photometry resolving the spot crossing
signature in the transit light curve.
