  In its first 2 years of operation, the ground-based Terrestrial gamma ray
flash and Energetic Thunderstorm Rooftop Array(TETRA)-II array of gamma ray
detectors has recorded 22 bursts of gamma rays of millisecond-scale duration
associated with lightning. In this study, we present the TETRA-II observations
detected at the three TETRA-II ground-level sites in Louisiana, Puerto Rico,
and Panama together with the simultaneous radio frequency signals from the
VAISALA Global Lightning Data set, VAISALA National Lightning Detection
Network, Earth Networks Total Lightning Network, and World Wide Lightning
Location Network. The relative timing between the gamma ray events and the
lightning activity is a key parameter for understanding the production
mechanism(s) of the bursts. The gamma ray time profiles and their correlation
with radio sferics suggest that the gamma ray events are initiated by lightning
leader activity and are produced near the last stage of lightning leader
channel development prior to the lightning return stroke.
