The Connection Between Terrestrial Gamma-Ray Flashes and Lightning Activity in Their Originating Thunderstorms

Abstract:

Terrestrial Gamma-Ray Flashes (TGFs) are extremely bright, short bursts of high-energy gamma-rays that are produced in association with some lightning flashes. Since their discovery in 1994, they have been the subject of intense investigation, but the question “why do some lightning flashes produce TGFs while others do not?” remains unanswered. Here we present an analysis of thunderstorm lightning activity in storms that produce these TGFs in an attempt to help answer this question. We provide analyses of how the production of TGFs is related to interflash intervals, lightning flash rates, and lightning flash amplitude in their respective thunderstorms. We used a clustering algorithm and lightning location data to identify a sample size of 219 TGF-producing storms, for which we performed an interflash interval analysis. We found that intervals immediately prior to the TGFs are typically 24% longer than average in a 20-minute window surrounding the time of the TGF, implying that stronger than normal electric fields might be necessary for the production of TGFs. We then manually tracked 418 TGF-producing storms to examine how TGF production depends on the flash rate and flash amplitude. We found that TGFs tend to occur in the declining phases of storms as 65% of the TGFs occur after the time of the peak flash rate. We also showed that TGFs tend to occur when the lightning flash rates are lower than average in their respective storms. Finally, we found that flashes near the time of the TGF tend to have larger peak currents that most other flashes in the same storms. These results together imply that there is something special about the production of a TGF: it’s not just that any lightning flash can or will produce a TGF, but rather that the environment plays a large part in whether or not a TGF will actually be produced.