Fermi Gamma-ray Space Telescope

Highlights of Fermi GBM Terrestrial Gamma-ray Flash (TGF) Observations

Michael Briggs
V. Connaughton, T. Chronis, S. Cummer, M. Stanbro, N. Bhat, S. McBreen, G. Fitzpatrick, O. Roberts, J. Fishman, E. Grove

Abstract:

With the new Continuous Time-Tagged Event (CTTE) datatype and a ground-based search of these data for TGFs, GBM is detecting $sim$850 TGFs per year. The resulting large sample of TGFs has made several discoveries possible. Correlating with ground-based radio observations, we have shown that radio signals close in time ($pm 200 mu$s) to TGFs mostly originate from the TGFs themselves, while radio signals with millisecond separations are from associated lightning. High-resolution radio measurements of several TGFs show a source altitude of 12 km, lower than previously thought. A low source altitude implies increased atmospheric absorption of the gamma rays reaching space and thus more particle multiplication in the source. The temporal distribution of TGF-lightning separations supports the interpretation that TGFs occur while intra-cloud lightning is propagating between the main negative charge layer to the upper positive charge layer. The large sample also provides a small number ($sim$24) of TGFs within range of the NOAA NEXRAD weather radar system. Even with this small sample we find that TGFs originate from a diversity of storm types and convective strengths. One TGF was observed from Tropic Storm Julio in the Pacific.