  Transmission spectroscopy during planetary transits, which is based on the
measurements of the variations of planet-to-star radius ratio as a function of
wavelength, is a powerful technique to study the atmospheric properties of
transiting planets. One of the main limitation of this technique is the effects
of stellar activity, which up until now, have been taken into account only by
assessing the effect of non-occulted stellar spots on the estimates of
planet-to-star radius ratio. In this paper, we study, for the first time, the
impact of the occultation of a stellar spot and plage on the transmission
spectra of transiting exoplanets. We simulated this effect by generating a
large number of transit light curves for different transiting planets, stellar
spectral types, and for different wavelengths. Results of our simulations
indicate that the anomalies inside the transit light curve can lead to a
significant underestimation or overestimation of the planet-to-star radius
ratio as a function of wavelength. At short wavelengths, the effect can reach
to a difference of up to 10% in the planet-to-star radius ratio, mimicking the
signature of light scattering in the planetary atmosphere. Atmospheric
scattering has been proposed to interpret the increasing slopes of transmission
spectra toward blue for exoplanets HD 189733b and GJ 3470b. Here we show that
these signatures can be alternatively interpreted by the occultation of stellar
plages. Results also suggest that the best strategy to identify and quantify
the effects of stellar activities on the transmission spectrum of a planet is
to perform several observations during the transit epoch at the same
wavelength. This will allow for identifying the possible variations in transit
depth as a function of time due to stellar activity variability.
