Ground-level terrestrial gamma-ray flashes produced by upward positive leaders

Terrestrial gamma-ray flashes (TGFs) are bright sub-millisecond bursts of gamma rays with energies extending to many tens of MeV. The vast majority of observed TGFs, measured by spacecraft in low-Earth orbit, are directed upwards and originate from between the main negative and positive charge centers of thunderstorms. In recent years, several downward directed TGF-like bursts of gamma rays have also been measured on the ground, presumably originating from below the main negative charge centers. These events have similar properties as the TGFs seen from space, including similar source luminosities, durations and energy spectra, suggesting the same mechanism(s) may be responsible for the production of the high-energy radiation. It is generally accepted that relativistic runaway electron avalanches (RREAs), accelerated in strong electric fields produced by thunderstorms and/or lightning leaders, are a key component of TGFs. However, RREAs require a large number of energetic seed electrons to be injected into the source region, and it is still not clear how and where these seed particles are generated. Two sources currently under investigation are the relativistic feedback and lightning leader mechanisms. For the former, energetic seed particles are produced by a positive feedback effect caused by backward propagating positrons and backscattered x-rays. For the latter, energetic seed particles are generated by the cold runaway electron mechanism in the very high fields associated with leaders and/or streamers. To investigate TGF produced by upward positive leaders initiated near the ground, a 3-D particle-in-cell (PIC) code is used to model the runaway electron and gamma ray production and propagation via the relativistic feedback and lightning leader mechanisms. Model results will be compared with ground-level TGF observations made at the International Center for Lightning Research and Testing.