Lightning-Based Search for Terrestrial Gamma-Ray Flashes in Fermi Gamma-Ray Burst Monitor Data

Abstract:

We analyze data from the Fermi Gamma-Ray Burst Monitor (GBM) around the times of VLF radio sferics, excluding already-known Terrestrial Gamma-Ray Flashes (TGFs) and searching for any remaining TGFs in the GBM data. We use sferic signals from both the World Wide Lightning Location Network (WWLLN) and the Earth Networks Total Lightning Network (ENTLN). Similar analyses have been done with the RHESSI and ADELE gamma-ray data and WWLLN radio data (Smith et al., 2014; Albrechtsen et al., 2015). GBM gamma-ray data that is temporally near sferics within GBM’s field-of-view is corrected for time of propagation of the signal to Fermi (light travel time), time-aligned to the radio signals, and stacked for plotting. This analysis is possible on data from late 2012 onward, as this is when the GBM Continuous Time-Tagged Event datatype became fully available. At least 250,000 WWLLN strokes and 600,000 ENTLN strokes are within the field-of-view of GBM each year, even with conservative distance cutoffs. Validation of the method is achieved by a test mode in which only sferics near already-known TGFs are selected, which reveals a very strong signal. Various selections are tried for the remaining sferics, e.g., altering the distance cutoff, geographic region, etc.