  Because of their activity, late-type stars are known to host powerful flares
producing intense high-energy radiation on short time-scales that may
significantly affect the atmosphere of nearby planets. We employ a
one-dimensional aeronomic model to study the reaction of the upper atmosphere
of the hot Jupiter HD 209458b to the additional high-energy irradiation caused
by a stellar flare. Atmospheric absorption of the additional energy produced
during a flare leads to local atmospheric heating, accompanied by the formation
of two propagating shock waves. We present estimates of the additional
atmospheric loss occurring in response to the flare. We find the mass loss rate
at the exobase level to significantly increase (3.8*10^10, 8*10^10, and
3.5*10^11 g s-1 for 10, 100, and 1000 times the high-energy flux of the quiet
star, respectively) in comparison to that found considering the inactive star
(2*10^10 g s-1).
