Spectral Observations of Optical Emissions Associated With Terrestrial Gamma-Ray Flashes.

Area of Science:

• Atmospheric physics
• Space physics
• Electromagnetism

Background:

• Terrestrial Gamma-Ray Flashes (TGFs) are intense bursts of gamma rays produced in Earth's atmosphere.
• TGFs are known to be associated with thunderstorms and lightning activity.
• Simultaneous measurements of TGFs and optical lightning emissions are crucial for understanding their relationship.

Purpose of the Study:

• To investigate the temporal and spatial relationship between TGFs and associated optical emissions from lightning.
• To determine the origin and characteristics of optical pulses linked to TGFs.
• To provide evidence supporting or refuting existing models of TGF generation.

Main Methods:

• Utilized data from the Atmosphere-Space Interactions Monitor (ASIM) to analyze 69 TGF events.
• Correlated TGF observations with simultaneous optical emissions measured at specific wavelengths (180-230, 337, and 777.4 nm).
• Applied a photon delay model incorporating cloud scattering to estimate the source location and timing of optical pulses relative to TGFs.

Main Results:

• All analyzed TGFs were associated with optical emissions.
• 90% of TGFs occurred at the onset of large optical pulses, indicating a connection to current surge initiation.
• Optical pulse sources were located 1-5 km below cloud top, with a median delay of -10 ± 80 µs relative to TGFs.
• Optical pulses exhibited lightning-like rise times but longer durations, with higher amplitudes at 337 nm compared to 777.4 nm.

Conclusions:

• The findings strongly support the leader-streamer mechanism as the primary process for TGF generation.
• The close association between TGFs and the initiation of optical pulses suggests a causal link.
• The study provides valuable insights into the complex interplay between lightning and high-energy phenomena in the atmosphere.