Observations of the Effects of Solar Flares on Earth and Mars

Disturbances on the Sun affect the other bodies in the solar system and their environments. Solar flares cause sudden ionospheric disturbances at Earth and coronal mass ejections cause geomagnetic storms and auroral displays at Earth and other planets. We present the first observations of the effects of a solar flare on the upper atmosphere of another planet. A large solar flare on 15 April 2001 caused electron densities in the bottomside martian ionosphere to increase by 100 to 200 percent. Electron densities in the terrestrial ionosphere also increased in response to the flare. Comparative studies of events such as these will improve our knowledge of the solar X-ray spectrum and secondary ionization yields from the interaction of X-rays and molecules.