Observationally Weak TGFs in the RHESSI Data

Abstract

Terrestrial gamma ray flashes (TGFs) are sub‐millisecond bursts of high energetic gamma radiation associated with intracloud flashes in thunderstorms. In this paper we use the simultaneity of lightning detections by World Wide Lightning Location Network to find TGFs in the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data that are too faint to be identified by standard search algorithms. A similar approach has been used in an earlier paper, but here we expand the data set to include all years of RHESSI + World Wide Lightning Location Network data and show that there is a population of observationally weak TGFs all the way down to 0.22 of the RHESSI detection threshold (three counts in the detector). One should note that the majority of these are “normal” TGFs that are produced further away from the subsatellite point (and experience a 1/ r 2 effect) or produced at higher latitudes with a lower tropoause and thus experience increased atmospheric attenuation. This supports the idea that the TGF production rate is higher than currently reported. We also show that compared to lightning flashes, TGFs are more partial to ocean and coastal regions than over land.

Detecting an Upward Terrestrial Gamma Ray Flash from its Reverse Positron Beam

“…In addition to making the location data publicly available (Fermi Science Support Center; http://fermi.gsfc.nasa.gov/ssc/data/access/gbm/tgf), they examined the “coastline‐distance distribution” and found that locations 0–80 km inland in tropical regions in general have the highest density of TGFs. A similar result was found by Albrechtsen et al (). In general it seems that low‐latitude, mountainous ( 1 km) regions within 80 km of a coast would make good candidate detector sites.…”

“…On the order of 1,000 upward‐directed TGFs occur per day that could be visible by satellite (Briggs et al, ), and possibly more if an undetected population of slightly fainter ones exists (Albrechtsen et al, ). An estimate for the frequency of downward‐directed TGFs does not exist.…”

Special Classes of Terrestrial Gamma Ray Flashes From RHESSI

“…Searches for faint TGFs associated with lightning flashes identified by their radio emission have revealed a small number of events (Østgaard et al, 2015), but the summed gamma ray emission from lightning is far lower than would be expected if the power law distribution continues much below the sensitivity limit of the current satellites (Smith et al, 2016). Further analysis of this population of weak events indeed indicates that the power law flattens out at low luminosity (Albrechtsen et al, 2019).…”

“…The current database extends from the start of the mission to 30 November 2013 for the first catalog algorithm and the new algorithm, and to 10 September 2012 for the second catalog algorithm. RHESSI was still detecting TGFs after these dates, but the detector efficiency continued to decline due to radiation damage-see Albrechtsen et al (2019).…”

On the High‐Energy Spectral Component and Fine Time Structure of Terrestrial Gamma Ray Flashes

“…The discovery of TGFs simultaneous (within few hundreds of microseconds) to lightning sferics detected by ground‐based lightning detection networks (Connaughton et al, , ) allowed to use only the association to lightning itself for TGF identification, provided a minimum number of counts are present, without the need for additional selection criteria. In other words, if any cluster of counts is observed in close time association to a lightning, the probability of chance association is remote and we can be reasonably sure it is a TGF, regardless of all its other properties (Albrechtsen et al, ; Østgaard et al, ). A set of events firmly associated to lightning sferics would provide a reliable sample of TGFs unbiased by selection criteria based on gamma ray data only, and would provide a test bench to confirm or disprove the existence of photons with energy higher than 40 MeV in TGF spectra.…”