Characterizing upward lightning with and without a terrestrial gamma-ray flash

TL;DR

This study compares two upward lightning events from a tower, one producing a bright terrestrial gamma-ray flash (TGF) and the other not, revealing that high electric fields aloft may be key to TGF production.

Contribution

It provides the first direct comparison of upward lightning with and without TGFs, highlighting the importance of high-altitude electric fields over leader characteristics.

Findings

One event produced a TGF bright enough to paralyze detectors.

The other event had no detectable TGF, with a 10^7 times lower gamma-ray fluence.

Higher electric fields aloft are likely crucial for TGF generation.

Abstract

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 $1\times10^7$. 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, sub-luminous 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.