  More than 3000 exoplanets have been detected so far, and more and more
spectroscopic observations of exoplanets are performed. Future instruments are
eagerly awaited as they will be able to provide spectroscopic data with a
greater accuracy and sensitivity than what is currently available. An important
aspect to consider is temporal stellar atmospheric disturbances that can
influence the planetary composition, and hence spectra, and potentially can
lead to incorrect assumptions about the steady-state atmospheric composition of
the planet. We focus on perturbations that come from the host star in the form
of flare events that significantly increase the photon flux impingement on the
exoplanet atmosphere. In some cases, and particularly for M stars, this sudden
increase may last for several hours. We aim at answering the question to what
extent a stellar flare is able to modify the chemical composition of the
planetary atmosphere and, therefore influence the resulting spectra. We use a
1D thermo-photochemical model to study the neutral atmospheric composition of
two hypothetic planets located around the star AD Leo. This active star has
already been observed during a flare. We use the spectroscopic data from this
flare event to simulate the evolution of the chemical composition of the
atmospheres of the two hypothetic planets. We compute synthetic spectra to
evaluate the implications for observations. The increase of the incoming photon
flux affects the chemical abundances of some important species down to
altitudes associated with an atmospheric pressure of 1 bar, that can lead to
variations in planetary spectra if performed during transit.
