Mystery of what triggers lightning solved

 Lightning's cosmic origin solved finally by Scientists 

Scientists appear to have found the final puzzle pieces to unravel the mystery behind how lightning strikes occur. Around the world, every day, roughly three and a half million bolts of lightning course through the atmosphere. They spark within thunderstorms, bridge the gaps between storm clouds, exchange charge between the clouds and the ground, and even shoot off into space. Studies of lightning from the mid-18th century up until now have revealed a lot about this phenomenon. Up until very recently, though, one aspect has remained elusive: exactly how a bolt of lightning initiates. Though scientists have long understood how lightning strikes, the precise atmospheric events which trigger it within thunderclouds remained a perplexing mystery. The mystery may be solved, thanks to a team of researchers led by Victor Pasko, professor of electrical engineering in the Penn State School of Electrical Engineering and Computer Science, which has revealed the powerful chain reaction that triggers lightning. Thunderstorms get an extraterrestrial kick to produce lightning.

In the study in the Journal of Geophysical Research, the authors described how they determined strong electric fields in thunderclouds accelerate electrons which crash into molecules like nitrogen and oxygen, producing X-rays and initiating a deluge of additional electrons and high-energy photons, the perfect storm from which lightning bolts are born. As a thunderstorm develops, friction between ice crystals lofted by updrafts and snow pellets falling towards the ground cause a static charge build-up within the cloud. From this charge separation, a powerful electric field forms, which can grow to over 100 million volts in strength. This electric field is the “power source” behind every stroke of lightning the storm will produce during its lifetime. However, all on its own, the storm cloud is incapable of generating even one bolt, due to the insulating properties of the air itself. For a spark of electricity to jump through the air from one point to another, the electric field between those two points needs to exceed air’s breakdown voltage. That’s at least 3,000 volts for every millimetre of distance the lightning will cross. So, for a bolt of lightning to jump the distance of just one km, the electric field would need to exceed 3 billion volts, or a thousand times stronger than what the most powerful thunderstorm can produce. 

"Our findings provide the first precise, quantitative explanation for how lightning initiates in nature," Pasko said. "It connects the dots between X-rays, electric fields and the physics of electron avalanches." The team used mathematical modelling to confirm and explain field observations of photoelectric phenomena in Earth's atmosphere, when relativistic energy electrons, which are seeded by cosmic rays entering the atmosphere from outer space, multiply in thunderstorm electric fields and emit brief high-energy photon bursts. This phenomenon, known as a terrestrial gamma-ray flash, comprises the invisible, naturally occurring bursts of X-rays and accompanying radio emissions. "By simulating conditions with our model that replicated the conditions observed in the field, we offered a complete explanation for the X-rays and radio emissions that are present within thunderclouds," Pasko said. "We demonstrated how electrons, accelerated by strong electric fields in thunderclouds, produce X-rays as they collide with air molecules like nitrogen and oxygen, and create an avalanche of electrons that produce high-energy photons that initiate lightning."

 Apparently, the storms get help from the cosmos. A cosmic ray is usually a proton, or more specifically, the nucleus of a hydrogen atom. They originate from the Sun, other stars in our galaxy and beyond, as well as more extreme features of the universe, like supernovae and black holes. Travelling at immense speeds, trillions of these particles flow past Earth every day. When a cosmic ray plunges into the top of the planet’s atmosphere, it smacks into an atom of oxygen or nitrogen in the air, resulting in one or more of the atom’s electrons being ejected at high speed. The overall effect is an avalanche of high-energy electrons cascading down towards the ground. A shower of charged particles rains down through the atmosphere following a cosmic ray particle hitting the top of the atmosphere. As these electrons pass through a thunderstorm cloud, they would result in what is known as a "runaway breakdown", bypassing air’s breakdown voltage and allowing gigajoules of energy to lance out in a brilliant bolt that can span a distance of many kilometres. The study also reveals the origins of other phenomena we see emitted from thunderstorms, such as x-rays (TGFs) and radio waves.

Zaid Pervez, a doctoral student in electrical engineering, used the model to match field observations, collected by other research groups using ground-based sensors, satellites and high-altitude spy planes, to the conditions in the simulated thunderclouds.  "To confirm our explanation on lightning initiation, I compared our results to previous modelling, observation studies and my own work on a type of lightning called compact inter cloud discharges, which usually occur in small, localized regions in thunderclouds." Pervez said. The keys, based on their research, are a very specific chain reaction and the feedback loop that results from it. As a high-energy electron from a cosmic ray impact travels through a thunderstorm cloud, it is accelerated to a significant fraction of the speed of light by the storm's electric field. When that relativistic electron eventually collides with an air molecule, it produces a burst of intense X-rays, along with lower energy radio waves, which radiate out in all directions. Through a phenomenon called the photoelectric effect, the X-rays cause surrounding air molecules to emit electrons. Those 'fresh' electrons then get accelerated by the cloud's electric field, collide with other air molecules, which releases X-rays, resulting in more electrons being emitted, in a feedback loop. The stroke of lightning begins when this loop starts up at a point back along the original electron's trajectory, just behind where the original X-ray flash occurred. There, each time the loop repeats, a fraction of the 'fresh' electrons will take the same path as the original, amplifying the electron avalanche in that direction. This 'runaway' effect only stops when the lightning finally discharges, temporarily balancing out the charge in the cloud and reducing the strength of its electric field. The next lightning flash will come once the electric field has regenerated and the feedback loop sets up yet again.

Published by Pasko and his collaborators in 2023, the model, Photoelectric Feedback Discharge, simulates physical conditions in which a lightning bolt is likely to originate. The equations used to create the model are available in the paper for other researchers to use in their own work. In addition to uncovering lightning initiation, the researchers explained why terrestrial gamma-ray flashes are often produced without flashes of light and radio bursts, which are familiar signatures of lightning during stormy weather. "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," Pasko said. "In addition to being produced in very compact volumes, this runaway chain reaction can occur with highly variable strength, often leading to detectable levels of X-rays, while accompanied by very weak optical and radio emissions. This explains why these gamma-ray flashes can emerge from source regions that appear optically dim and radio silent." This chain reaction also explains the 'terrestrial gamma-ray flashes' that we see from thunderstorm clouds. TGFs are high-energy bursts of electromagnetic radiation, which include both hard X-ray and gamma-rays, which were first observed in thunderstorms by NASA's orbiting Compton Gamma Ray Observatory.

“By simulating conditions with our model that replicated the conditions observed in the field, we offered a complete explanation for the X-rays and radio emissions that are present within thunderclouds,” Pasko explained. In addition to Pasko and Pervez, the co-authors include Sebastien Celestin, professor of physics at the University of Orléans, France; Anne Bourdon, director of research at École Polytechnique, France; Reza Janalizadeh, ionosphere scientist at NASA Goddard Space Flight Center and former postdoctoral scholar under Pasko at Penn State; Jaroslav Jansky, assistant professor of electrical engineering and communication at Brno University of Technology, Czech Republic; and Pierre Gourbin, postdoctoral scholar of astrophysics and atmospheric physics at the Technical University of Denmark.