This investigation is focused on analysis of both archived and ongoing observations of Terrestrial Gamma-ray Flashes (TGFs), which are brief but intense bursts of radiation associated with lightning and visible from Earth orbit by suitably instrumented satellite platforms. These gamma rays were first detected in the 1990's and are thought to result from electrons accelerated to very high energies through a process termed "relativistic runaway." They may conceivably be either a cause of lightning (by serving as a trigger that first overcomes the insulating property of air, allowing larger currents to flow) or an effect of lightning (owing to a secondary breakdown process within zones of strong electric fields set up by the main lightning discharge). Establishing which of these situations applies is one objective of this work. Data on TGFs from three spacecraft will be analyzed: (1) The Compton Gamma Ray Observatory (no longer in orbit), whose BATSE instrument first discovered TGFs; (2) RHESSI (currently in orbit), which represents the largest source of available TGF observations; and (3) the recently launched GLAST spacecraft, which carries the Gamma-ray Burst Monitor (GBM) instrument. Plans call for a more complete statistical description and survey of background meteorological conditions for all RHESSI-observed events, and will extend to improved spectral modeling of the atmosphere's role in TGF propagation. Intercomparisons are also planned with observations from a recently developed lower altitude instrument (ADELE, the Airborne Detector for Energetic Lightning Emissions), which will be carried aboard the NSF-NCAR GV aircraft in the vicinity of Florida thunderstorms during August-September 2009.
The intellectual merit of this effort will center on improved physical understanding of mechanisms by which highly energetic radiation (of the sort generally associated with cosmic sources) may be generated within Earth's atmosphere in conjunction with electrified clouds. Broader Impacts will come primarily from student involvement, but could conceivably extend to issues of aviation/human safety in those instances where aircraft venture close to TGF-emitting thunderstorms.
This project involved the analysis of archival data from a satellite (the Reuven Ramaty High Energy Solar Spectroscopic Imager, or RHESSI), a network of ground-based lightning sensors (the World-Wide Lightning Location Network, or WWLLN), and an aircraft (the Airborne Detector for Energetic Lightning Emissions, or ADELE). RHESSI detects x-rays and gamma-rays; while designed to study those from the Sun, it can also detect them coming from any direction, including the Earth. It's been known since 1994 that bright flashes of gamma-rays lasting on the order of a millisecond or less are associated with thunderstorms. These Terrestrial Gamma-ray Flashes (TGFs) have since been shown to extend to high gamma-ray energies (to almost 1000 times the energy of the x-rays used by dentists) and to have such a high intensity at the source that there could be a health risk to aircraft passengers and crew if they were to fly directly through the production region of the TGF. In this program we revisted the RHESSI data to look for TGFs both fainter than the ones that are known and brighter. To search for bright TGFs, we scanned thousands of events to look for cases where the instrument was temporarily paralyzed by high gamma-ray intensities during the event. We found over 80 of these events. The proof that they represent really bright TGFs comes in a "tail" of lower-energy gammas that are delayed by tens of microseconds after the main TGF pulse. These are the gamma-rays that scattered around one or more times in Earth's atmosphere before arriving at the satellite; they are late because of the extra distance they have to travel and low in energy because each scattering removes energy from the gamma-ray. By studying these scattered tails in detail and comparing them to detailed simulations, we were able to show that a few of these TGFs are probably about 100 times as intense at the point of their production in the atmosphere as what was normally thought to be typical. An even more unexpected result came from our search for weak TGFs. Since each of these might produce as few as one -- or even zero -- gamma-rays in the instrument, they cannot be individually identified. But we added up ("stacked") the gamma-ray data corresponding to the times of millions of lightning flashes that occurred below the RHESSI spacecraft over a nine-year period from 2004 through 2012. We aligned the arrival times of the gamma-rays at RHESSI relative to the time of each lightning flash detected by WWLLN; we expected a signal, then, near t=0, representing no time offset from the detected lightning event. We did find a strong signal, but surprisingly, it was spread between about -75 and +75 milliseconds from the WWLLN lightning signal (which is a very-low-frequency radio signal), suggesting that different parts of the lightning process produce the lightning detection on the ground and the gamma-rays in space. But we believe that what we are seeing is an entirely new phenomenon, and not just a collection of weak TGFs because of the way the signal varies with distance. We define distance as the distance between the spot on Earth directly below the satellite and the position of the lightning flash. A source of gamma-rays coming from a single point at the lightning position will fall off in brightness rapidly as a function of distance. But we found that the stacked signal remains strong out to a distance of 1500 km, much farther than the TGF signal should theoretically be able to go. We therefore conclude that we are seeing a new phenomenon in which lightning causes gamma-rays to be emitted from a spread-out source of large size, rather than from the position of the lightning itself as in the case of TGFs. The likely explanation for this is that the electric field of the lightning's electromagnetic pulse (EMP) is causing the acceleration of electrons over a larger and larger ring as it expands at the speed of light, so that some of the emission occurs directly below the satellite even at these large distances. In conclusion, both the search for bright TGFs and the search for faint ones have greatly expanded our vision of the kinds and amounts of high-energy radiation that thunderstorms can produce.
