| A novel photometric array with a high-speed triggered data acquisition system, bore-sighted image-intensified CCD video camera, and VLF radio receiver was built to detect a predicted signature of elves, the lower ionospheric (80 to 95 km altitude) flash due to heating by an impinging electromagnetic pulse launched by intense lightning currents. Data acquired by the array (named the “Fly's Eye”) settles several questions regarding the relationship between elves and lightning and, by measuring the spatial extent of ionospheric heating and the frequency of occurrence of elves, demonstrates their significance in causing sustained and cumulative modification of the nighttime lower ionospheric electron density profile over large thunderstorm systems.; The Fly's Eye, along with a telescopic imaging system developed in 1998, was also used to investigate sprites. Sprites are highly structured discharges lasting 5 to 100 ms and extending from 40 to 85 km altitude which result from intense electric fields following a major redistribution of electric charge in the troposphere usually a positive cloud-to-ground return stroke. Photometric, video, and radio (30 Hz to 20 kHz) measurements were used to detect the first sprites directly associated with negative cloud-to-ground lightning, implying a breakdown process that can propagate in upward and downward electric fields; this is consistent with only a subset of the theoretical descriptions for sprites. In addition, telescopic imagery shows clear evidence of both positive and negative corona streamer propagation in a sprite.; Detailed electromagnetic (finite difference time domain) modeling of both elves and sprites is used to interpret observations. Three events recorded by a high-speed (3000 frames per second) imaging system in 1997, combined with modeling results, led to the recognition of a widespread confusion in interpreting video signatures of elves and sprites and identified for the first time the diffuse upper portion of sprites, a hard-to-measure but likely ubiquitous form of heating and ionization in the upper mesosphere which is now called the sprite halo. |
