Terrestrial gamma-ray flashes observed up to 20 MeV

Area of Science:

• Atmospheric Physics
• High-Energy Astrophysics
• Space Science

Background:

• Terrestrial gamma-ray flashes (TGFs) are brief, intense bursts of gamma radiation originating from Earth's upper atmosphere.
• Previous observations provided limited data on TGF occurrence rates and maximum photon energies.

Purpose of the Study:

• To report on the detection of TGFs using the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite.
• To characterize the energy spectra and frequency of these atmospheric gamma-ray events.

Main Methods:

• Utilized data from the RHESSI satellite to detect and analyze TGF events.
• Examined gamma-ray spectra to determine photon energies and applied a bremsstrahlung model to estimate electron energies.

Main Results:

• Detected 10 to 20 TGFs per month with RHESSI, suggesting a global rate of approximately 50 per day.
• Observed gamma-ray spectra extending up to 10-20 MeV, with estimated electron energies of 20-40 MeV.
• TGF occurrence frequency and maximum photon energy are significantly higher (over an order of magnitude) than previously reported.

Conclusions:

• RHESSI observations reveal a higher-than-expected frequency and energy for terrestrial gamma-ray flashes.
• These findings indicate potentially new or underestimated physical processes governing energetic particle interactions in Earth's atmosphere.