Lower TGF Detection Threshold for Fermi GBM: Sample Properties

From shortly after the launch of Fermi in 2008 the Gamma-ray Burst Monitor has detected TGFs by the flight software monitoring the detector rates. The sensitivity of this method is limited by the 16 ms resolution of the in-orbit data, which is far longer than the durations of TGFs. Since the summer of 2010 individual photon data has been downlinked for selected portions of the orbit, from TGF active regions and seasons. An offline search of this data at high temporal resolution has increased the TGF detection rate by a factor of ten. The reliability of the sample was verified using control regions and VLF observations of the World Wide Lightning Location Network (WWLLN). In this sample, we find TGFs that are shorter than those in the triggered population, and TGFs at greater distances from the spacecraft nadir. Maps of the source locations, as determined with WWLLN, show distinct geographic and meteorological features.