Hunting for Gamma Rays above Thunderstorms: The ALOFT Campaign

Abstract An internationally collaborative airborne campaign in July 2023—led by the University of Bergen (Norway) and NASA, with contributions from many other institutions—discovered that thunderstorms near Florida and Central America produce gamma rays far more frequently than previously thought. The campaign was called Airborne Lightning Observatory for Fly’s Eye Geostationary Lightning Mapper (GLM) Simulator (FEGS) and Terrestrial Gamma-ray Flashes (TGFs), which shortens to ALOFT. The campaign employed a unique sampling strategy with NASA’s high-altitude ER-2 aircraft, equipped with gamma-ray and lightning sensors, flying near ground-based lightning sensors. Real-time updates from instruments, downlinked to mission scientists on the ground, enabled immediate return to thunderstorm cells found to be producing gamma rays. This maximized the observations of radiation created by strong electric fields in clouds and showed how gamma-ray production may be physically linked to the thunderstorm life cycle. ALOFT also sampled storms entirely within the stereo-viewing region of the GLM instruments on GOES-16 / GOES-18 and performed multiple underflights of the International Space Station Lightning Imaging Sensor (ISS LIS), while using an upgraded FEGS instrument that demonstrated the operational value of observing multiple wavelengths (including ultraviolet) with future spaceborne lightning mappers. In addition, a robust complement of airborne active and passive microwave sensors—including X- and W-band Doppler radars, as well as radiometers spanning 10–684 GHz—sampled some of the most intense convection ever overflown by the ER-2. These observations will benefit planned convection-focused NASA spaceborne missions. ALOFT is an exemplar of a high-risk, high-reward field campaign that achieved results far beyond original expectations. Significance Statement Though it has been known for years that thunderstorms sometimes produce gamma rays, the Airborne Lightning Observatory for Fly’s Eye Geostationary Lightning Mapper (GLM) Simulator (FEGS) and Terrestrial Gamma-ray Flashes (TGFs) (ALOFT) campaign discovered that this high-energy radiation is ubiquitous and highly dynamic in tropical thunderstorms. As these thunderstorms intensify, strong electric fields generate gamma-ray glows, and within those glows, powerful TGFs often occur. When the storms weaken, the gamma-ray production weakens. This means gamma rays can be an indicator of thunderstorm evolution, like lightning flash rate. Thus, thunderstorm radiation is not just a boutique topic for lightning physicists, but also relevant to forecasters, storm scientists, and those impacted by aviation hazards. Moreover, ALOFT gathered important validation data for spaceborne lightning sensors and sampled some of the most intense convection ever overflown by NASA aircraft.