Solar Radiance Chain in the April 2002 Series of Flares

The April 2002 series of solar storms included a number of GOES M- and X-class flares, in particular, a X1.5 flare on April 21 that was observed by many spacecraft - SoHO, TRACE, Wind, and the recently launched RHESSI (Ramaty High Energy Solar Spectroscopic Imager), which is designed to study flare hard X-ray/gamma-ray emissions. At the same time the effects of these emissions on the Earth were observed by an fleet of spacecraft, including TIMED, and by ground-based instrumentation. The effects of the increased energetic photon flux on the Earth's atmosphere during solar flares is usually small in comparison to geomagnetic disturbances, but large X-class flares (such as the one on April 21, 2002) that are several hours in duration can have significant consequences. When the 1 to 10 nm region of the of the solar spectrum is enhanced during a flare, the energy is deposited largely in the E-region of the ionosphere, ~100 to ~120 km altitude, where increases in ion density, photoelectron production, airglow emission, and odd-nitrogen production can result. Higher energy photons (such as observed by RHESSI) penetrate to lower altitudes, where they have less effect on the atmosphere but can still create additional ionization in the D-region of the ionosphere, ~80 to ~100 km altitude, that is disruptive of certain types of radio communication. Here we present the observations of solar photon emissions and their effects on the Earth's ionosphere and atmosphere.