Mountaintop gamma ray observations of two TGFs at the Santis Tower, Switzerland with coincident radio waveforms characteristic of a RREA current pulse

The electrodynamics of the Terrestrial Gamma Ray Flash (TGF) are not fully understood but the last decade has seen compelling observations in lightning leader (+IC) radio emission that have linked a subset of TGF satellite observations with unique waveform characteristics during lightning leader propagation. ‘Slow Pulse’ events (Cummer et al.,2011; Lu et al.,2011; Pu et al.,2019), energetic In-cloud pulses (+EIPs) (Cummer et al.,2017; Lyu et al.,2016), and most recently the possibility of mountain-top energetic pulses (MEPs) (Lyu et al.,2021) all give a unique perspective into the electrodynamic behavior of the TGF mechanism not possible with gamma-ray observations alone. Though the observation of -EIPs, negative polarity ‘slow pulses’, and MEPs have been inferred to correspond with downward TGFs, thus far there have only been two published observations that directly make this connection, Pu et al.,2020 and Wada et al. 2020 which reported on a negative ‘slow pulse’ and -EIP respectively. See also the abstract by Ortberg et al. at this meeting. We report on the mountain top observation of two TGF events that occurred on the 9th of June 2021, the only downward TGFs ever observed in a mountain top environment. A gamma-ray sensitive detector located at the base of the Santis Tower in Switzerland observed a 120us duration flash of gamma photons coincident with a -IC (~10kA) lightning leader at 15:25:21.165148 UTC located close to the Santis Tower. A second double peak gamma ray flash was observed lasting 400us in total. This second event was coincident with an unusually strong (~100kA) and complex IC event at 17:48:17.847036 UTC, also located close to the Santis Tower. We will examine RF waveforms for both events to determine what conclusions can be made of these gamma ray observations and their association with the three types of TGF linked radio pulse events mentioned above.