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We compare two observations of gamma rays before, during, and after lightning flashes initiated by upward leaders from a tower during low‐altitude winter thunderstorms on the western coast of Honshu, Japan. While the two leaders appear similar, one produced a terrestrial gamma ray flash (TGF) so bright that it paralyzed the gamma ray detectors while it was occurring and could be observed only via the weaker flux of neutrons created in its wake, while the other produced no detectable TGF gamma rays at all. The ratio between the indirectly derived gamma ray fluence for the TGF and the 95% confidence gamma ray upper limit for the gamma ray quiet flash is a factor of 1.0 × 10 7 . With the only two observations of this type providing such dramatically different results—a TGF probably as bright as those seen from space and a powerful upper limit—we recognize that weak, subluminous TGFs in this situation are probably not common, and we quantify this conclusion. While the gamma ray quiet flash appeared to have a faster leader and more powerful initial continuous current pulse than the flash that produced a TGF, the TGF‐producing flash occurred during a weak gamma ray  glow , while the gamma ray quiet flash did not, implying a higher electric field aloft when the TGF was produced. We suggest that the field in the high‐field region approached by a leader may be more important for whether a TGF is produced than the characteristics of the leader itself.

Characterizing Upward Lightning With and Without a Terrestrial Gamma Ray Flash