This is a program to observe x-rays and gamma-rays emitted by thunderstorms. Two known phenomena will be observed: the incredibly bright, fast terrestrial gamma-ray flashes (TGFs) and ?glows? of gamma-rays emanating from the tops of active thunderstorm cells. The Airborne Detector for Energetic Lightning Emissions (ADELE) is a set of gamma-ray detectors and supporting electronics. In previous flights, it was found that the first TGF seen from an aircraft, set strong upper limits showing that TGFs are associated with < 1% of lightning flashes, and established that glows are quite common and extend to energies >5 MeV.
ADELE will be adapted to fly on a Global Hawk drone at 18 km as part of NASA?s Hurricane and Severe Storm Sentinel (HS3) mission, which will provide 300 hours of flight time over tropical storms and hurricanes in the Atlantic. The activities to be performed include adapting the instrument for the new aircraft, integrating it onboard, participating in the campaigns by monitoring our instrument in real time from NASA?s Dryden flight facility, analyzing the data, and publishing and disseminating the results. Scientists from Duke University will provide simultaneous broad-band radio-frequency data to interpret the behavior of lightning corresponding to TGFs and the periods in which glows are observed. Scientists from Florida Tech will provide meteorological support and interpretation and will model the high-energy processes associated with TGFs and glows. NASA Collaborator Richard Blakeslee and colleagues will provide detailed storm electrical measurements, including lightning, electric fields, air conductivity, and storm currents, from the Lightning Instrument Package which will be simultaneously flown on the Global Hawk during HS3.
INTELLECTUAL MERIT The scientific goals embrace understanding the physical mechanism behind TGFs and glows and searching for high-energy emission associated with two other phenomena: gigantic jets and elves. The measurements of the intensity and spectrum of glows and the comparison of the gamma-ray data with lightning data from the same storm and in-situ electric field and conductivity data (provided by other instruments on HS3) will determine whether the glows represent a significant current caused by relativistic breakdown with feedback. A new tool for estimating the magnitude and extent of the electric field in the upper layers of storms via the spectrum, intensity, and shape of the gamma-ray emission will be developed. The ADELE flights will give dramatically improved views of recently-discovered phenomena that may play a role in thundercloud discharging and the physics of lightning initiation. The discovery of high-energy radiation from gigantic jets and elves, both of which should involve high electric fields, is a possibility, since this would represent the first time a sensitive high-energy detector was taken into the vicinity of these phenomena.
BROADER IMPACTS The program will train several graduate and undergraduate students, some already involved with ADELE and some new, in the skills of instrument development, programming, field work, data analysis, and publication. The TGF measurements (both detections and non-detections) will also help us address the public health question of how often (if ever) aircraft passengers are exposed to the radiation dose of a TGF. Directly within the region where TGFs are formed (probably less than a square kilometer), the dose could be up to 100 times the annual limit for non-radiation-workers.
