  Stellar flares are a frequent occurrence on young low-mass stars around which
many detected exoplanets orbit. Flares are energetic, impulsive events, and
their impact on exoplanetary atmospheres needs to be taken into account when
interpreting transit observations. We have developed a model to describe the
upper atmosphere of Extrasolar Giant Planets (EGPs) orbiting flaring stars. The
model simulates thermal escape from the upper atmospheres of close-in EGPs.
Ionisation by solar radiation and electron impact is included and photochemical
and diffusive transport processes are simulated. This model is used to study
the effect of stellar flares from the solar-like G star HD209458 and the young
K star HD189733 on their respective planets. A hypothetical HD209458b-like
planet orbiting the active M star AU Mic is also simulated. We find that the
neutral upper atmosphere of EGPs is not significantly affected by typical
flares. Therefore, stellar flares alone would not cause large enough changes in
planetary mass loss to explain the variations in HD189733b transit depth seen
in previous studies, although we show that it may be possible that an extreme
stellar proton event could result in the required mass loss. Our simulations do
however reveal an enhancement in electron number density in the ionosphere of
these planets, the peak of which is located in the layer where stellar X-rays
are absorbed. Electron densities are found to reach 2.2 to 3.5 times pre-flare
levels and enhanced electron densities last from about 3 to 10 hours after the
onset of the flare. The strength of the flare and the width of its spectral
energy distribution affect the range of altitudes that see enhancements in
ionisation. A large broadband continuum component in the XUV portion of the
flaring spectrum in very young flare stars, such as AU Mic, results in a broad
range of altitudes affected in planets orbiting this star.
