Analysis of Terrestrial Gamma-Ray Flashes

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

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.