Effects of a stellar flare on planet HD 209458b, irradiated by the star HD 209458 (using the Sun as a proxy). The top panel shows the variation of the intensity of the flare (in blue) and the stellar zenith angle (in red) with time, t = 0 representing the beginning of the flare. The bottom panel shows the ratio of electron density during the flare to that pre-flare, as a function of time and pressure within the planetary atmosphere. Electron densities are determined at 30° latitude and 12 LT; the planet is considered phase-locked with its star.