Terrestrial Gamma-Ray Flashes Observed up to 20 MeV.

Terrestrial gamma-ray flashes (TCFs) from Earth's upper atmospherehave been detected with the Reuven Ramaty High Energy SolarSpectroscopic Imager (RHESSI) satellite. The gamma-ray spectra typicallyextend up to 10 to 20 megaelectron volts (MeV); a simple bremsstrahlungmodel suggests that most of the electrons that produce the gamma rayshave energies on the order of 20 to 40 MeV. RHESSI detects 10 to 20 TCFsper month, corresponding to ∼50 per day globally, perhaps many moreif they are beamed. Both the frequency of occurrence and maximum photonenergy are more than an order of magnitude higher than previously knownfor these events. [ABSTRACT FROM AUTHOR]