Sunday, 6 January 2013
Exhibit Hall 3 (Austin Convention Center)
Terrestrial gamma ray flashes (TGFs) are known to be associated with thunderstorms since their discovery and are specifically identified with individual lightning events. While this is know, there remains a multitude of uncertainty in the meteorology of a TGF producing storm. TGFs are observed by the Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope (Fermi) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Characteristics of TGF producing thunderstorms were determined using data from the polar-orbiting Moderate-Resolution Imaging Spectroradiometer (MODIS) meteorological satellite. Because of the association between TGFs and lightning, by examining lightning data we obtain the spatial proximity of the TGF. Out of a possible 230 TGFs observed by GBM and RHESSI that were geo-located using coincidences with World Wide Lightning Location Network (WWLLN), only three separate events spanning from 13 August 2008 to 13 May 2010 (one GBM and two RHESSI) occurred within seventeen minutes of MODIS passing over the same footprint. Each thunderstorm that produced a TGF during these three events was associated with tropical thunderstorm systems with cloud top heights ranging from 15.9 km to 17.5 km. This indicates that the TGF sources were consistent with models of relativistic runaway electron avalanches causing breakdown near the tops of cumulonimbus clouds. A comparison for each storm at the latitude and longitude by use of the MODIS Cloud Product that corresponds to the TGF-WWLLN correlation was constructed indicating that each storm appeared to be associated with considerably high tropopause altitudes as well as in the vicinity of coastal regions. In a second analysis, a histogram of flash rate as a function of time over a thirty-minute snapshot was prepared to determine what phase of the storm evolution that the TGF occurred. By this, it was found that the TGF does not appear to happen at a preferred spot in the lightning flash rate temporal development close to the TGF.
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