Thunderstorm characteristics associated with RHESSI identified terrestrial gamma ray flashes

Abstract

[1] The characteristics of thunderstorms that produce terrestrial gamma-ray flashes (TGFs) observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are determined using climatological and meteorological data. RHESSI observed TGFs follow diurnal, seasonal, and geographic patterns that are very similar to those of thunderstorms confirming, in part, that these events are directly connected to thunderstorm activity. The TGF producing thunderstorms are shown to be closely associated with tall (ranging from 13.6 km to 17.3 km) tropical thunderstorm systems, a finding that is consistent with theoretical expectations from models of relativistic breakdown that relate the source region to the spectral signatures observed by RHESSI. Unlike sprites, there appears to be no predilection for TGFs to occur with large thunderstorm complexes. Rather, TGF producing thunderstorms are shown to range in areal extent by several orders of magnitude. Analysis of a single TGF event within the Mozambique Channel indicates an elevated mixed phase (both liquid water and ice present) level of approximately 6 km which is consistent with the climatological findings.

High-Energy Atmospheric Physics: Terrestrial Gamma-Ray Flashes and Related Phenomena

“…Splitt et al (2010) found enhanced production of TGFs relative to lightning between 0300 and 0900 hours local time. Otherwise, the diurnal and seasonal cycle of TGFs is similar to that of lightning (Hazelton 2009;Splitt et al 2010).…”

“…Splitt et al (2010) found enhanced production of TGFs relative to lightning between 0300 and 0900 hours local time. Otherwise, the diurnal and seasonal cycle of TGFs is similar to that of lightning (Hazelton 2009;Splitt et al 2010). used WWLLN data to place RHESSI TGFs in the context of the evolution of flash rate within the storm that produced them, finding that TGFs tend to come after the peak flash rate of the storm, with an average delay of 38 min but a standard deviation of 98 min.…”

“…At thundercloud altitudes, the flux of seed particles from the ambient atmospheric cosmic-rays flux is about 10 4 m 2 s −1 (Hillas 1972). Therefore, in order to produce the 10 17 runaway electrons in 200 µs known from TGF observations (Dwyer and Smith 2005), the RREA mechanism would require a lateral diameter of the TGF source region to be at least 800 km, much too large for realistic thunderstorms, especially those that are known to be associated with TGFs (Splitt et al 2010). It is possible that a rapid electric field increase above a thundercloud, caused by lightning charge transfer, could temporarily circumvent the relativistic feedback threshold, but, as discussed in Sect.…”

“…The prevalence of TGFs in the tropics may therefore have a more complicated explanation A significant part of the difference between lightning and TGF distributions may therefore lie in meteorological conditions that favor the production of TGFs, not just their escape into space. Splitt et al (2010) made an extensive study of storms associated with individual RHESSI TGFs and compared the TGF map to maps of water vapor and ice content, showing that ice alone had a poor correlation with the TGF map but that liquid water at 10-14 km, indicating deep convection, gave a much better match. Elevated mixed phase (liquid water plus ice), which is expected to correlate with electrification, might be the most physically relevant parameter; in a case study in Splitt et al (2010), one TGF-producing storm had several regions of elevated mixed phase.…”

“…Splitt et al (2010) made an extensive study of storms associated with individual RHESSI TGFs and compared the TGF map to maps of water vapor and ice content, showing that ice alone had a poor correlation with the TGF map but that liquid water at 10-14 km, indicating deep convection, gave a much better match. Elevated mixed phase (liquid water plus ice), which is expected to correlate with electrification, might be the most physically relevant parameter; in a case study in Splitt et al (2010), one TGF-producing storm had several regions of elevated mixed phase. The authors found that storm systems of all sizes could produce TGFs, from storms with a cloud-top size of <300 km 2 to large mesoscale convective complexes (MCCs) of 10 5 km 2 .…”