An NLDN-Based Search for Observationally Dim Terrestrial Gamma-Ray Flashes Using Low Frequency Radio Sensors and Fermi’s GBM

Abstract:

Terrestrial gamma-ray flashes (TGFs) are bursts of energetic radiation that have been known to occur in association with in-cloud lightning (IC). These bursts last only fractions of milliseconds, but have such strong photon emissions that they can be observed by satellites in low-Earth orbit. One such satellite, the Fermi Gamma-ray Space Telescope and its Gamma-ray Burst Monitor (GBM), is currently being used to detect the gamma-rays of these intense events. Our current research aims to determine the rarity of TGFs by searching for ones that are considered observationally dim, that is, they exist below the threshold of the current detection algorithms of instruments such as the GBM. It will do this by combining data from ground-based, low frequency (LF) radio sensors, the National Lightning Detection Network (NLDN), and the Fermi satellite. LF sensor data is crucial to this search because it allows us to exploit the known link LF data has to TGFs in a way that can’t be done using Fermi and NLDN alone. First we use NLDN data to identify in-cloud events that have a peak current >15 kA and occur within 600 km of Fermi’s nadir point. These are chosen because they have a known association with TGFs. This is followed by examining LF data at the time of the NLDN events to identify probable early stage IC events, a sequence of fast pulses followed by ionospheric reflections, which have been linked to TGFs. Once these TGF-like events are found in LF data, their signal peaks are used to determine the time of their NLDN sources to tens of μs. From this, the time the TGF-like events’ gamma-rays would have reached Fermi, had they been a TGF, is calculated using the Fermi position at the times of the TGF-like events. Fermi’s GBM data is then used in conjunction with the LF radio signal to create an observed photon distribution of all the events that were found to be TGF-like. The photon distribution will be statistically compared to a random distribution of Fermi photons to determine the overall likelihood that observationally dim TGFs were found. So far, many thousands of events have been considered TGF-like in terms of their LF signals, a number of which are known TGFs, thereby verifying the performance of this method. The results found through this research will provide us with vital scientific insight into the rarity of TGFs as a whole.