“Lightning is the most beautiful and dramatic demonstration of electricity in nature, yet its origin has long eluded a complete explanation,” Victor Pasko said, both capturing the beauty and the long-standing enigma that has always encircled this atmospheric event. Scientists have stood in awe of the sheer force of a lightning bolt for generations, but when it comes to how storm clouds build up enough energy to release such a discharge, the answer has until now remained elusive.
Recent advances in computational modeling have illuminated a remarkable sequence of events that begins far beyond Earth’s atmosphere. The new research, published in the Journal of Geophysical Research: Atmospheres, presents a quantitative model demonstrating that cosmic rays high-energy particles originating from sources like supernovas and pulsars seed the electron avalanches that ultimately trigger lightning. These cosmic rays, mostly protons, bombard the upper atmosphere and initiate a cascade of secondary particles in a process known as an extensive air shower. When these energetic particles encounter the electric fields inside thunderclouds, they can act as “seed” electrons, setting off a runaway chain reaction.
The model, constructed by Pasko and colleagues, combines measurements from ground sensors, satellites, and high-flying aircraft to model the preconditions for the initiation of lightning. Interestingly, the model’s predictions agree with field measurements revealing that the measured electric field inside storm clouds is about ten times weaker than what was previously thought necessary to trigger lightning. The discrepancy had confounded atmospheric scientists for many decades, yet the new results propose that the elusive ingredient is the injection of cosmic-ray–induced electrons.
Lying at the root of the process is the relativistic runaway electron avalanche mechanism. When such an electron generated by a cosmic ray encounters a region of strong electric field, it is accelerated to relativistic velocities. Moving, it collides with air molecules and generates X-rays and gamma rays by bremsstrahlung emission. With each collision, more electrons are set free, quickly increasing the number of high-energy particles in what is now an electron avalanche. The electric field threshold to this avalanche is impressively close to the maximum threshold observed in thunderclouds, so the process is not only possible but almost certain to be widespread during storms.
One of the most intriguing aspects of this model is its explanation for the mysterious gamma-ray and X-ray flashes terrestrial gamma-ray flashes that precede many lightning strikes. These fleeting bursts, first detected by satellites in the 1990s, had challenged conventional wisdom about thunderstorm physics. Pasko explained, “In our modeling, the high-energy X-rays produced by relativistic electron avalanches generate new seed electrons driven by the photoelectric effect in air, rapidly amplifying these avalanches.” This runaway process can occur in highly localized regions, often producing detectable X-rays and gamma rays with minimal accompanying optical or radio emissions. As a result, gamma-ray flashes can emerge from source regions that appear optically dim and radio silent, a phenomenon confirmed by both satellite and ground-based observations.
The coupling among lightning and high-energy atmospheric events goes beyond the instant flash. Equipment on board the International Space Station, like that on the ASIM mission, has captured the chain of events that precede TGFs, with the development of a lightning leader preceding a TGF in a matter of milliseconds. The follow-on lightning discharge generates an electromagnetic pulse that has the capability to excite ultraviolet emissions in the ionosphere, called elves, further connecting the microphysics of electron avalanches to larger-scale atmospheric electrodynamics.
The modeling and detection of cosmic-ray interactions in the atmosphere rely on sophisticated computational frameworks, such as CORSIKA and Geant4, which simulate the propagation and interaction of primary and secondary particles across varying atmospheric and geomagnetic conditions. These tools have enabled researchers to analyze the total background flux of secondary particles at specific geographic locations, factoring in variables like altitude, local magnetic field, and atmospheric composition. This comprehensive approach has been essential for validating the new lightning initiation models against real-world data.
At last, the combination of observation data, advanced models, and greater insight into cosmic-ray physics has given a compelling quantitative account for the initiation of lightning. As Pasko recapitulated, “It connects the dots between X-rays, electric fields and the physics of electron avalanches.” Cosmic chain reaction, seeded by particles from far-away astrophysical processes, highlights the vast interconnectedness of Earth’s weather and the universe at large.
