Physicists filmed the collision of a positive lightning leader with a negative one

Summary

Physicists have captured the precise moment when positive and negative lightning leaders collide, revealing the intricate process of lightning formation.

Abstract

A team of geophysicists from China and India has successfully recorded the moment a lightning strike forms, using high-speed video at 380,000 frames per second. The footage, taken during a thunderstorm, shows the merging of a negative leader from the cloud with a positive one from the ground, occurring when they are approximately 20 meters apart. This event marks the formation of a single ionized air channel where the main lightning discharge takes place. The study, published in Geophysical Research Letters, provides new insights into the poorly understood final stages of lightning strikes, including the birth of the ground leader, the formation of a unified streamer zone, and the exact nature of the leader channels' collision. The research supports the hypothesis that the collision of leaders involves the expansion and brightening of canaliculi from both ends, which merge to form a single channel, rather than multiple streamer channels merging into one. The findings suggest that the trajectory of the lightning strike is predetermined by the high-impedance streamer zone that forms prior to the collision, leading to the creation of a hot plasma channel for the main discharge.

Opinions

The researchers believe that the second hypothesis, where canaliculi branch from the leader channels and merge, is confirmed by their observations.
The study indicates that the final path of the lightning strike is influenced by the properties and composition of the air, including the presence of aerosol particles from ship exhaust, which can affect the frequency of lightning strikes over oceans.
The authors emphasize the need for further observations and experiments to validate their conclusions and to enhance the understanding of lightning behavior.

Physicists filmed the collision of a positive lightning leader with a negative one

Geophysicists from China and India captured lightning striking a meteorological tower standing on the ground from a cloud in a video with a frequency of 380 thousand frames per second. The resulting footage showed how exactly before the main flare a single channel with ionized air is formed, in which the main discharge then occurs. The unification of two leader channels: a negative one from the cloud to the ground, and a positive one in the opposite direction, begins when the distance between their ends is about twenty meters, scientists write in Geophysical Research Letters.

The final shots of the lightning strike before the main discharge. Rubin Jiang et al. / Geophysical Research Letters, 2021 / AGU

A lightning strike that strikes objects on the ground from a cloud consists of several stages. First, as a result of atmospheric convection during a thunderstorm, charge separation occurs and highly charged regions appear in the cloud. After this, streamers begin to branch from the cloud towards the ground — thin, dim, weakly conducting channels filled with ionized gas and electrons split off from atoms. When a negative streamer, growing down from the cloud, gets quite close to the ground, a response positive channel begins to grow towards it from the side of the ground. At some point, these two channels meet and merge, forming a common streamer zone. Simultaneously with the growth of streamer channels, lightning leaders — highly conductive thermally ionized channels, the current in which is about 1–2 kiloamperes — begin to spread through them in steps of several tens of meters. The whole process ends with the meeting of positive and negative leaders and the subsequent reverse discharge — the brightest discharge during lightning with a current of several tens of kiloamperes, which moves from bottom to top.

Although the general sequence of stages during a lightning strike is fairly well known, many important details are still poorly understood, primarily due to technical difficulties: a lightning strike is a very fast process that occurs over long distances, and in environments with very high temperatures, potential differences and currents. In particular, not everything is clear about the final stages of this process: when the response leader is born on the ground, at what moment a single lightning streamer zone is formed, and how exactly the collision of two leader channels occurs.

Geophysicists from China and India, led by Rubin Jiang and Abhay Srivastava from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, managed to study the fine structure of streamer and leader channels during the final stages of a lightning strike. To do this, scientists filmed a lightning strike on a 235-meter meteorological tower standing on the ground in Beijing. To see the direct merger of the two streamer zones and the collision of the positive and negative leaders, which occurred at an altitude of 145 meters above the top of the tower, scientists filmed at a frequency of 380 thousand frames per second.

Sequential frames of the collision of two leader channels during the end-to-end phase of lightning. Below are color maps of the successive stages of merging leader channels. Rubin Jiang et al. / Geophysical Research Letters, 2021

As a result, the authors of the work managed to obtain two consecutive frames, which are separated by only one 380-thousandth of a second — in the first of them, the two leaders have not yet collided, and in the second, a flash has already occurred and a reverse discharge has arisen. According to these frames, the merging of the positive and negative streamer zones and the formation of a common streamer channel occurred when the ends of the positive and negative leader channels were more than 23 meters apart.

There are two main hypotheses about how, after the formation of a common streamer zone, the breakthrough phase occurs, when the positive and negative leaders are already very close to each other. The first hypothesis assumes the emergence of several thin streamer channels connecting the positive and negative leaders, which then merge into a common channel. According to the second hypothesis, thin canaliculi begin to branch from the ends of both leader channels, two of which meet and expand, and the rest then fade.

Two main hypotheses about the mechanism of the collision of two lightning leaders: on the left is the scenario of the merger of several parallel streamer channels, and on the right is the selection of one of the branches of the leader channel. Rubin Jiang et al. / Geophysical Research Letters, 2021

According to scientists, their survey data confirms the second scenario. In the through phase, the leader channels expanded (the expansion speed was about a thousand kilometers per second) and became much brighter. As a result, the gap between the leaders was reduced: in the last frame, preceding the flash of the reverse discharge, the ends of the two leader channels were separated by only 16 meters. Scientists believe that the trajectory of the collision of two lightning leaders was determined already at the moment when a common high-impedance streamer zone arose, and during the collision itself along the intended trajectory, a hot plasma channel was formed in this zone.

Scientists note that their results clearly demonstrate the mechanism of formation of the channel in which the main lightning discharge occurs. However, according to geophysicists, video footage alone is not enough to draw general conclusions, so further observations and experiments are needed to confirm these conclusions.

The probability of lightning strikes on objects on the surface of the earth or sea depends not only on their shape but also on the properties and composition of the air. For example, a few years ago, geophysicists discovered that due to the high concentration of aerosol particles in ship exhaust, the frequency of ocean lightning strikes is about twice as high in areas traversed by shipping lines.