Downward Terrestrial Gamma-Ray Flash Observed in a Winter Thunderstorm

Area of Science:

• Atmospheric Physics
• High-Energy Astrophysics
• Nuclear Physics

Background:

• Terrestrial gamma-ray flashes (TGFs) are intense bursts of gamma rays produced by lightning discharges.
• Most TGFs are observed from space, appearing as "upward" events originating from thunderstorms.
• The study investigates a rare "downward" TGF detected at ground level.

Purpose of the Study:

• To analyze the characteristics of a downward terrestrial gamma-ray flash (TGF) detected at sea level.
• To determine the altitude and electron population involved in the downward TGF event.
• To compare the properties of downward TGFs with previously observed upward TGFs.

Main Methods:

• Detection of gamma-ray bursts using scintillation detectors during a winter thunderstorm.
• Measurement of total radiation dose using ionization chambers across multiple monitoring posts.
• Analysis of burst characteristics, including duration and temporal structure, and Monte Carlo simulations.

Main Results:

• A strong gamma-ray burst (up to ~10 MeV) was detected coincident with a lightning discharge, exhibiting subsecond duration.
• The burst comprised four distinct gamma-ray bunches, each lasting less than 1 ms and associated with radio pulses.
• The downward TGF is estimated to have occurred at an altitude of 2500±500 m, involving approximately 8×10^18 avalanche electrons.

Conclusions:

• The detected event represents a rare downward terrestrial gamma-ray flash (TGF), analogous to upward TGFs observed from space.
• The energy and electron population involved in this downward TGF are comparable to those found in upward TGFs.
• This study provides valuable ground-level data on the mechanisms and characteristics of TGFs.