For the last 30 years there has been a very small controversy rumbling in the hallowed halls of physics. Way back in 1985, scientists from the then-USSR noted that whenever a thunder storm passed over their neutron detector, they observed an increased flux of neutrons. Unfortunately, they didn't have much in the way of monitoring equipment to really nail down much beyond the initial observation.

Since then, scientists have put forward a couple of potential explanations for the observed flux. One was that the high fields generated during lightning strikes was modifying the trajectories of muons from cosmic ray showers. In short: these are cosmic rays, and this is not interesting. The second was that the gamma rays emitted during the lightning strike generated neutrons, a photonuclear event. But new measurements show that neither of these explanations can explain the data.

The (now) Russian scientists have designed an entirely new experiment that significantly improves their previous results. They installed three neutron detectors that were sensitive to low energy neutrons: one above ground, one partially shielded in a building, and a third underground with heavier shielding. Sitting next to the underground detector was a more traditional neutron detector that is sensitive to high energy neutrons. Finally, the electrical activity of incoming storms was monitored using a variety of instruments, allowing for better correlation between the neutron measurements and the electrical activity of any passing storms.

Why the variety of neutron detectors? Essentially, the researchers need to get rid of the background noise from cosmic rays. The cosmic rays generate muons that collide with something in or very near the detector, resulting in neutrons that have the high energy of the muon being registered. Neutrons from lightning, on the other hand, can only have the energy given up by a fission event, which is then lost in collisions with molecules in the air as they travel to the detector.

The data obtained by the researchers show clear spikes in the low-energy neutron detectors at the same time as the electrical discharges from a storm. Unfortunately, the time resolution of the neutron detectors is only 1 minute, so it is impossible to extract any detailed information about the neutron flux. The use of the three shielded detectors, however, shows the expected decay behavior, indicating that the neutrons are not generated within the detectors themselves.

The high-energy neutron detector also showed some activity during the storm, but this is because the detector still has some sensitivity for low-energy neutrons. Once this was taken into account, the four detectors all agreed. In short, cosmic rays are not the source for the neutron flux observed during lightning strikes.

The new detectors also allowed the researchers to calculate the neutron flux from the storm activity. In the previous experiments, it had been assumed that each detection event corresponded to a single neutron. In a surprising turn up, the new data show that up to 5000 neutrons per cubic meter are produced every second by lightning strikes.

This is very high, and not very compatible with the alternate explanation, neutron production by high energy photons (gamma rays). To generate the number of neutrons the researchers observe would take about 10 million gamma ray photons m-3s-1. Unfortunately, lightning strikes only generate a tiny fraction of that.

At the moment, this research is not of Earth-shattering importance. But it does point to things going on thunderstorms that we just don't know about yet. And that is quite exciting. It is also important to realize that this isn't going to revolutionize our understanding of nuclear physics, so these observations aren't going to lead to new reactor designs or free energy. Still, we will learn more about thunderstorms, which is pretty cool.

Physical Review Letters, 2012, DOI: 10.1103/PhysRevLett.108.125001 (About DOIs)