This work presents observations of the optical emission of a lightning flash in conjunction with
a downward-directed Terrestrial Gamma-ray Flash (TGF) at the Telescope Array detector. Previously
in ICRC-2021, we reported joint observations by the Telescope Array Surface Detector
(TASD), the Lightning Mapping Array, a sferic sensor, and a broadband interferometer of particle
showers in coincidence with lightning. These observations consisted of energetic showers of
approximately less than 10-microsecond duration with footprints on the ground of 3-6 kilometers
in diameter, originating in the first one to two milliseconds of downward lightning leaders and in
coincidence with the initial breakdown pulses stage of negative cloud-to-ground lighting leaders.
Scintillator waveform and simulation studies confirmed that these showers must consist primarily
of gamma radiations.
In this work, we use the TASD detector, together with a high-speed video camera, in conjunction
with multiple lightning instruments at the TASD site, to report on the optical emission associated
with a downward-directed terrestrial gamma-ray flash. Results from this study allow us to further
the understanding of the initiation and propagation mechanism of terrestrial gamma-ray flashes.
It will also further our ability to compare the most recent satellite optical emissions counterpart
of upward-directed TGFs to that of downward-directed TGFs.