Characterisation of Weak TGFs in RHESSI Data

Terrestrial gamma-ray flashes (TGFs) are short sub-millisecond bursts of high energy gamma radation, associated with intracloud positive lightning, bringing negative charge upwards. They were first detected by the Burst and Transient Source Experiment (BATSE) in 1991 and were thought to be a rare phenomena, as BATSE only detected 87 TGFs during its 9 years of operation ( 9 TGFs/yr). With the launch of the RHESSI satellite in 2002, the detection rate of TGFs improved dramatically to 160 TGFs/yr due to the ability to apply search algorithms on raw telemetered data. Later on, an improved search algorithm was applied and the detection rate increased to 350 TGFs/yr. Other notable satellites are the Fermi GBO with a detection rate of 850 TGFs/yr and AGILE with 1000 TGFs/yr. As new experiments and algorithms are able to find more and more TGFs, an unanswered question in the community is: 'how common are TGFs?'We have used the World Wide Lightning Location Network (WWLLN) in addition to RHESSI to extract 100ms interval of RHESSI data centred at the time of each lightning flash. By superposing the RHESSI intervals of data (excluding already identified TGFs) between August 2004 and December 2014, we find a 9.03σ increase in gamma-rays at the time of lightning, indicating that there exists a new population of TGFs that cannot be detected by current RHESSI search algorithms. We will present the characteristics of these weak TGFs, in particular their spatial distribution and energy spectrum in relation to already identified TGFs and see how they compare.