  In this paper, we present the first high-speed video observation of a
cloud-to-ground lightning flash and its associated downward-directed
Terrestrial Gamma-ray Flash (TGF). The optical emission of the event was
observed by a high-speed video camera running at 40,000 frames per second in
conjunction with the Telescope Array Surface Detector, Lightning Mapping Array,
interferometer, electric-field fast antenna, and the National Lightning
Detection Network. The cloud-to-ground flash associated with the observed TGF
was formed by a fast downward leader followed by a very intense return stroke
peak current of -154 kA. The TGF occurred while the downward leader was below
cloud base, and even when it was halfway in its propagation to ground. The
suite of gamma-ray and lightning instruments, timing resolution, and source
proximity offer us detailed information and therefore a unique look at the TGF
phenomena.
