This research consists of a summer campaign of observing high-energy radiation associated with thunderstorm lightning using ADELE, the Airborne Detector for Energetic Lightning Emissions. ADELE was built with funding from NSF's Major Research Instrumentation program. This study is more in an exploratory nature, and requires expeditious deployment of the research team and instruments to the field in this coming summer. ADELE will be carried to the vicinity of lightning discharges in a van. The campaign will take place mostly in central Florida, with a preliminary few days of observing at high-altitude sites in New Mexico. Collected data will be analyzed after the field campaign.
Intellectual Merit. Relativistic runaway, a process in which high-energy electrons multiply themselves and produce x-rays and gamma-rays when they collide with the atoms of air, occurs at various timescales and conditions in thunderstorms and may be involved in stepped-leader propagation, lightning initiation, and in limiting the maximum electric field attainable in air. Even where the high-energy radiation does not play a controlling role, it can serve as a diagnostic of the field conditions in regions, such as the head of a leader, which are not easily accessible.
By taking the instrument to the lightning in real time for the first time, ADELE's campaign should result in a much higher yield of gamma-ray data per day than can be obtained by stationary detectors. The gamma-ray data returned by ADELE will be combined with meteorological data and observations of radio emission by lightning provided by long-term collaborators at Duke University and Los Alamos National Laboratories.
Broader Impacts. This research may enhance the understanding of atmospheric electricity, and broaden the society's awareness of thunderstorm lightning activity. We expect this campaign to prove the value of storm-chasing measurements in this field of research and to result in larger programs down the road. The science teams at University of California, Santa Cruz and the Florida Institute of Technology are dedicated to the inclusion of graduate and undergraduate students in NSF-funded research projects, and have long histories of successful student involvement.
This was a small, experimental project designed to test a new way of studying lightning. Of the many mysteries still remaining about lightning, one of the foremost is how and why the electrical charge make a jagged, stepwise descent from the cloud to the ground in the first phase of a lightning flash, called a "stepped leader". It was recently discovered that there is a burst of x-rays each time the charge takes a "step" of a few meters. We don't know exactly how this relates to the cause of the stepping behavior yet, and we need to learn how to get more observations of this phenomenon. This has, in the past, been accomplished by leaving x-ray detectors in one place and waiting for lighting to strike nearby, but only a few events per summer can be recorded this way. We took an x-ray detector that had been developed for another project and decided to try putting it into a van and chasing thunderstorms, so as to increase the amount of time spent near lightning compared to a detector at a fixed position. We chased storms in New Mexico and Florida in the summer of 2010, for a total of 8 days of driving. Using real-time weather feeds and a lightning detector to guide us to storms, we managed to be close enough to lightning to see four stepped-leader x-ray events. This is about the most that has ever been seen in an entire summer using a fixed station, and it suggests that we could get 20-30 events in a summer if we were storm-chasing for the whole Florida thunderstorm season. Since this was intended to be a test of the technique, and we didn't have enough funding to conduct a full-scale, all-summer campaign, we consider this to be a clear success. One of the events we saw had one burst of x-rays from a step near the ground and another from a step a few hundred feet in the air. What was interesting about this was that the step from further up had x-rays of higher energy (about 20 times as high as the energy of x-rays used in medical imaging). This suggests that x-ray bursts even further up, toward the thundercloud, might be even more energetic (the reason would probably be that the electric field is much higher up there). This is important because there is a very poorly understood natural phenomenon called a "terrestrial gamma-ray flash" (TGF). TGFs are incredibly bright flashes of gamma-rays that originate at the tops of thunderclouds, usually beamed upwards, and are observed from satellites in orbit (our detector made the first detection of a TGF from an airplane in 2009). One of the most important hypotheses for us to test in the near future is whether a TGF is the same sort of event as the bursts of x-rays from stepped leaders, only magnified enormously by the electric fields high in a thunderstorm. By making more measurements from a van, we hope to find "missing link" events that are halfway between stepped-leader x-ray bursts and TGFs. A promising way to do this might be to set up our instrument in a storm-chasing van in Japan, since Japanese winter thunderstorms have their clouds very near the ground -- thus we will be looking close-up at the inner parts of the storm, even though we will be on the road and not in a plane. We were able to involve five students at three universities in this project, teaching them about the physics of lightning, techniques of x-ray detection, and practical issues relating to working in the field. We presented the results at the Fall 2010 meeting of the American Geophysical Union, and are currently working on a paper summarizing them that will be submitted to the Journal of Geophysical Research.
