On August 27, 1998, 10:22 UT, the most powerful cosmic burst ever to hit the solar system was recorded by the X-ray and gamma-ray detectors on the interplanetary network of spacecraft. The gamma-ray burst had a significant effect on the Earth's middle and upper atmosphere. Ionization in the upper atmosphere increased to near daytime levels. The effects were observed in the Very Low Frequency (VLF) radio region by Stanford University's Holographic Array for Ionospheric Lightning research (HAIL). Changes in the propagation characteristics of the VLF signals indicated an abrupt change in plasma densities in the lower ionosphere. This one-year SGER effort will examine the global effects of the gamma-ray burst on ionization, conductivities, and minor species chemistry in the middle and upper atmosphere. The primary tools to be used in this study are a coupled photon-electron transport model, and a comprehensive time-dependent model of ionospheric chemistry.
