By Tess Lavezzi Light
When thunderstorm season rolls around and lightning streaks the sky, creating its dazzling display, we likely don’t ponder the mysteries it presents. Lightning seems to be one of those things we’ve got figured out. Didn’t we learn everything we need to know when Benjamin Franklin flew his kite on a stormy day in a Pennsylvania field in 1752?
Not quite.
We know that lightning is an energetic electric discharge, creating a current that flows briefly within a cloud or between a cloud and the ground and heating the air to temperatures about five times hotter than the sun’s surface. We know that more than 2,000 thunderstorms are active throughout the world at a given moment, producing up to 100 flashes per second. We also know that lightning is dangerous: estimates vary, but the lowest available suggest lightning causes 4,000 deaths worldwide each year and millions of dollars in property damage. Furthermore, about 22,600 wildfires per year in the United States alone are started by lightning. In New Mexico’s San Mateo Mountains last summer, a lightning-initiated wildfire burned more than 65 square miles of land—that’s over 40,000 acres.
Despite all this knowledge, lightning is still poorly understood.
Los Alamos National Laboratory is working to change that. Because lightning produces optical and radio frequency signals similar to those from a nuclear explosion, it’s important to be able to distinguish whether such signals are caused by lightning or a nuclear event. As part of the global security mission at Los Alamos, scientists use lightning to help develop better instruments for nuclear test-ban treaty monitoring and, in the process, have learned a lot about lightning itself.
For example, how is lightning initiated? Think about that: it happens up to 100 times per second, but we don’t quite know how. In a thunderstorm, electric charge separates into layers. A lightning discharge is a flow of electrons from one layer to another (or to the ground) that slightly relieves the imbalance. Air is normally an insulator, so getting such a current to flow requires the presence of a large electric field to break the air down and allow it to conduct electricity. But measurements of the electric fields within thunderstorms come up short, by about a factor of 10, compared to the needed breakdown fields. Perhaps the fields are being enhanced within very small regions so that we simply haven’t observed them, or maybe something is predisposing the air to break down more easily.
One popular theory depends on cosmic rays — energetic particles born in places like supernovae — that continually strike our atmosphere. This theory suggests that when cosmic rays impact near a thunderstorm, they strip off enough extra electrons to “seed” the air, tipping the balance and allowing an electrical current to flow. Experimental evidence has been sought for decades to support this theory. But recent research by Los Alamos and partner institutions may show the opposite to be true. In a paper published in Nature, the researchers present data that strongly suggest lightning arises in very small regions of extremely high, localized electric fields, and does not develop over the long paths that a cosmic-ray-induced event would require.
The team’s measurements identify a little-studied process called fast positive breakdown as the initiator of most, if not all, lightning flashes that we see in thunderstorms. To better resolve this issue, the lightning team at Los Alamos is preparing a side-by-side study of lightning and cosmic rays.
If even “routine” lightning isn’t well understood, it shouldn’t be surprising that many other equally baffling mysteries about those bright cracks in the sky exist. For example, it was determined years ago that a particularly strong subset of these fast positive breakdown events produce little, if any, light—despite being the biggest producer of radio frequency emission. Dark lightning? That’s odd. But even more fascinating—and beautiful—is volcanic lightning.
Volcanic lightning strikes during an eruption of Japan’s Sakurajima volcano in February 2013.
Los Alamos leads a multi-institution collaboration to determine whether the lightning that commonly arises in volcanic ash plumes can be used to improve volcano monitoring. The research also seeks to understand the physics of the radio frequency noise that emanates from ash plumes, which appear to be distinct from the volcanic lightning.
Lightning is also a useful tool. For example, some research shows that lightning might offer a long-distance measurement of total rain rate, allowing for precipitation monitoring in otherwise unobservable regions. Los Alamos has also used lightning signals as “probes” that can map small structures in the Earth’s ionosphere. Further, researchers at Los Alamos and elsewhere have seen that enhanced lightning rates can indicate severe weather. In particular, lightning rates in the eye-wall of hurricanes increase shortly before storm intensification, similar to the jumps in lightning activity about five to 20 minutes before the onset of tornado activity.
These radio frequency signals from lightning that we typically observe from space give us a bird’s eye view of the whole earth. So might we someday use space-based lightning detection to globally predict tornadoes and other severe weather events, with enough lead-time to prevent casualties? We’re not there yet, but could be someday.
Tess Lavezzi Light is the chief scientist for the Space-based Nuclear Detonation Detection program at Los Alamos and leads the Radiofrequency On-orbit Operations team. Her lightning research has focused on joint optical and radiofrequency lightning phenomenology. Other members of the team include Xuan-Min Shao, who leads the 3D electromagnetic pulse modeling effort at Los Alamos; Sonja Behnke, who leads the volcanic lightning project at Los Alamos, funded by the National Science Foundation; and Erin Lay, who is the recipient of an early career research grant to model ionospheric signal transport.