Abstract
Results of a full electrodynamic model developed by Lehtinen et al [1999] suggested the possibility of optical emissions in regions geomagnetically conjugate to intense lightning discharges. These intense discharges create a transient electric field between the cloud and the ionosphere, which drives an avalanche ionization process and creates a beam of upward going relativistic electrons. Such beams are believed to be the sources for gamma-ray flashes observed on the Compton Gamma Ray Observatory (CGRO) during some of its passages over intense thunderstorms. The upward going relativistic electron beam traverses the earth's magnetic field and impinges onto the ionosphere in the geomagetically conjugate hemisphere and is estimated to produce detectable optical emissions. Up to now, two different campaigns have been conducted in an attempt to uncover experimental evidence for such conjugate effects. An initial campaign was conducted in Japan in January 2003, looking for effects of lightning in Australia. In July and August 2003, a second experiment was conducted in South Africa in an attempt to detect such optical emissions produced by lightning activity in central Europe. Very Low Frequency (VLF) electromagnetic data was recorded at new sites in France and Crete to identify ionospheric disturbances associated with such events and to document the waveforms of causative lightning discharges at high resolution. High time-resolution optical data was recorded in South Africa using a portable array of photometers. Additionally, images of sprites, an optical event directly above large lightning discharges, were taken in Europe. Optical measurements of sprites, VLF radio atmospherics from lightning discharges, ionospheric disturbances, and optical measurements in the conjugate region are now under analysis.