• solar flare;
  • EUV irradiance;
  • terrestrial photoelectrons

[1] The near-simultaneous observations of the solar extreme ultraviolet (EUV) irradiance and terrestrial photoelectron distribution during and after the X-class flare on 21 April 2002 provide for a distinctive study of the effects that a solar flare can have on Earth's upper atmosphere. The solar EUV irradiance from 0.1 to 195 nm was measured by the Solar EUV Experiment (SEE) aboard the NASA Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) satellite. The terrestrial photoelectron distribution from 10 to 1000 eV was measured by the Fast Auroral Snapshot (FAST) energetic electron sensor. The variations of the solar EUV irradiance from the X-class flare at ∼0200 UT on 21 April 2002 range from more than a factor of 10 for the X-ray emissions to less than 10% at longer EUV wavelengths. The measured ratio of the flare spectrum to the preflare spectrum has a spectral shape that is similar to that predicted for the Bastille Day 2000 flare. Most of the solar irradiance variation is in the X-ray range and is due to coronal emissions. The photoelectron distribution changed by a factor of about 10 for the high-energy Auger electrons and by very little for the low-energy electrons. Modeling the photoelectron distribution using the measured solar EUV irradiance provides results that are qualitatively consistent with the observed photoelectron distribution. This study is focused primarily on the relative changes during the flare compared to the preflare conditions.