Atmospheric physicist Joseph Dwyer had a harrowing close encounter with a thunderstorm several years back while conducting research aboard a specially modified Gulfstream V airplane. After taking a wrong turn into the heart of the storm, the plane’s particle detector lit up to indicate the presence of a cloud of antimatter, but the origin of these antiparticles are still a mystery. Dwyer and his colleagues have tried for several years to model the scenario, but have yet to find a satisfactory explanation.

Antimatter is the name we give to particles with the same mass, but opposite charge, as the particles of which we are composed. When an antiparticle comes in contact with its corresponding “normal” particle, they annihilate each other and release gamma rays. In this case, the team detected a large number of positrons (the antiparticle of an electron) in that storm.

The particle detector on Dwyer’s plane recorded three gamma ray spikes at the right level to indicate positron-electron collisions. It also picked up additional gamma rays at slightly lower energy levels, indicating they traveled a short distance to the detector from the site of collision. The researchers estimated the cloud of antimatter around the plane was between one and two kilometers across, which is huge. Finding a mechanism for such an event has proven difficult, though.

The power electrical fields in a thunderstorm usually produce a small number of positrons, but not at this scale. The electrons released by clouds in powerful thunderstorms are accelerated to near the speed of light and produce energetic gamma rays. These gamma rays can produce electron-positron pairs when they impact an atomic nucleus. The readings from Dwyer’s 2009 flight don’t appear to be caused by this known process, as no gamma rays of sufficiently high energy were seen.

Another possibility is that positrons from the upper atmosphere were somehow funneled toward the aircraft. The impact of cosmic rays is always producing a few particles of antimatter here and there, but it wouldn’t naturally cluster together in the way Dwyer observed.

You might be wondering, did they actually see positrons in the data? Other researchers who have examined the readings say the signature of positrons is unmistakable. However, the estimation of the cloud size might be off. Perhaps the skin of the plane became charged and increased the production of positrons in its immediate vicinity (not for a kilometer in all directions). Even if that’s the case, it’s an interesting phenomenon.

Meanwhile, Dwyer is busy sending weather balloons into the middle of violent thunderstorms to look for more clues. The National Science Foundation also plans to equip a heavily armored A-10 Warthog assault aircraft with a particle detector to recreate Dwyer’s accidental storm encounter.