Accelerators in the clouds. Terrestrial gamma-ray flashes, other fantastic beasts, and where to find them.

Abstract:

Thunderclouds host the most energetic natural particle accelerators on Earth. Few kilometers above hour heads, yet less studied and understood than many astrophysical phenomena much farther away from us. Terrestrial Gamma-ray Flashes (TGF) first discovered in 1994 by the BATSE instrument onboard the Compton gamma-ray observatory, are the brightest manifestation of these particle accelerators. Initially considered a rare phenomenon, basically uninfluential for the atmospheric system, now we know that TGFs are much more common than previously thought, and that tropical thunderclouds can shine in gamma-rays for hours and over thousands of square kilometers. New ‘weird’ phenomena have been discovered, showing that particle acceleration is a crucial and pervasive phenomenon in thundercloud electrodynamics, together with cloud electrification and lightning activity. In this presentation I will show how our understanding of these phenomena has evolved over the past thirty years, what is the current state of the art, what are the yet unanswered questions and the steps forward. This will be also a personal journey bridging astrophysics and atmospheric sciences, taking me from the Italian AGILE satellite, to Norway, to the ASIM mission onboard the International Space Station, eventually to the ALOFT aircraft campaign over the Gulf of Mexico, and beyond.