  Stellar flares of active M dwarfs can affect the atmospheric composition of
close-orbiting gas giants, and can result in time-dependent transmission
spectra. We aim to examine the impact of a variety of flares, differing in
energy, duration, and occurrence frequency, on the composition and spectra of
close-orbiting, tidally locked gaseous planets with climates dominated by
equatorial superrotation. We used a series of pseudo-2D photo- and
thermochemical kinetics models, which take advection by the equatorial jet
stream into account, to simulate the neutral molecular composition of a gaseous
planet (effective temperature 800 K) that orbits a flaring M dwarf. We then
computed transmission spectra for the evening and morning limb. We find that
the upper regions of the dayside and evening limb are heavily depleted in CH4
and NH3 up to several days after a flare with a total radiative energy of $ 2
\times 10^{33} $ erg. Molar fractions of C2H2 and HCN are enhanced up to a
factor three on the nightside and morning limb after day-to-nightside advection
of photodissociated species. CH4 depletion reduces transit depths by 100-300
parts per million (ppm) on the evening limb and C2H2 production increases the
14 micron feature up to 350 ppm on the morning limb. We find that repeated
flaring drives the atmosphere to a composition that differs from its pre-flare
distribution and that this translates to a permanent modification of the
transmission spectrum. We show that single high-energy flares can affect the
atmospheres of close-orbiting gas giants up to several days after the flare
event, during which their transmission spectra are altered by several hundred
ppm. Repeated flaring has important implications for future retrieval analyses
of exoplanets around active stars, as the atmospheric composition and resulting
spectral signatures substantially differ from models that do not include
flaring.
