Generation of lightning

Spark discharge in the atmosphere. It usually happens in the case of thunderstorm clouds, but occasionally it happens in nimbostratus, snowstorms, sandstorms and volcanic eruptions. Lightning can be divided into intracloud discharge, cloud discharge and cloud-to-ground discharge according to the location. The first two are collectively called cloud lightning, and the third is called ground lightning (Figure 1[ ground lightning]). Most lightning in nature is cloud flash. The ratio of lightning to total lightning is 1/3 ~ 1/6 (the ratio in temperate zone is higher than that in tropical zone). Because lightning is a great threat to human activities and life safety, there are many studies.

A brief history of lightning phenomenon and its influence on human activities have long attracted people's attention. Lightning was recorded in China as early as14th century BC (see A Brief History of the Development of Atmospheric Science). Later, in Huai Nan Zi written by Liu An and others in the Western Han Dynasty, the viewpoint that "Yin and Yang are thin for thunder and excitement for electricity" was put forward. Wang Chong in the Eastern Han Dynasty pointed out in his book Lun Heng: "Those who thunder are also excited by the sun." He also summed up the seasonality of lightning activities: "thunder in the first month", "thunder in May" and "thunder in autumn and winter". In the18th century, in order to reveal the nature of lightning, many scientists conducted detection experiments. For example, in 1750, American scholar B. Franklin put forward the idea of measuring by lightning rods installed on towers and drawing electricity from the clouds. 1752 In June, he took the risk of lightning strike and conducted a famous kite experiment to detect lightning in Philadelphia. He observed the electric spark generated by thunderstorm clouds through kite leads, which confirmed the consistency of the nature of electricity generated by natural lightning and friction. At the same time, Soviet scholars μ β lomonosov and γ β Rikhman used self-made lightning detectors to detect the electric sparks generated during the transit of thunderstorms. Unfortunately, rickman was killed by lightning. Since then, modern lightning research based on electricity has begun.

The lightning structure consists of lightning that winds through the clouds and reaches the ground. It seems to be an instantaneous flash to human eyes, but it is often composed of multiple continuous discharges in the same channel, with an interval of about a few seconds (Figure 2 [Lightning structure shown in high-speed photography]). Every discharge in the whole lightning process is called lightning strike. A lightning strike can often record multiple lightning strikes, some as many as 10.

Blinking generally includes two processes: leading and counterattacking. Pilot is the preparation process of establishing lightning discharge ionization channel, which is divided into cascade pilot and linear pilot. The cascade leader is a dim light beam that extends to the ground step by step like a ladder. Its diameter is about 5 meters and the length of each stage is about 50 meters. The leader crosses this distance at a speed of about 10 m/s, then stops for about 30 ~ 100 microseconds, and then continues to extend forward. Therefore, the whole cascade pilot quickly extends to the ground at an average speed of about 1.5× 10 m/s, and the cascade pilot establishes an ionization channel for the return journey. When the pilot is 5 ~ 50 meters away from the ground, a counterattack will occur at a certain point along the ionization channel on the ground. The brightness of the return stroke is much stronger than that of the pilot. The flash visible to the naked eye is a counterattack with a speed of about 5× 10 m/s and a duration of about 40 microseconds. The current passing through is about 10 A, and occasionally it can reach10 a. The average diameter of the reflux channel is only a few centimeters. After the cascade pilot and the first return pass, there may be a time interval of several hundredths of a second, followed by the second pilot and return. Pilots after the second time usually go straight down from the clouds to the ground, which is called direct guidance. Because it has no step, its moving speed is about 10 times higher than the step guide. The whole duration of a lightning bolt is about 0.2 seconds, and dozens of negative charges are transported from the cloud to the ground.

Because lightning is not a steady-state process and is not affected by the discharge electrode, it is difficult to explain the lightning process with the discharge theory of electron avalanche in strong electric field (the electron concentration increases sharply due to the chain reaction of electron collision). Therefore, some people put forward the streamer theory, thinking that when the electron avalanche is strong, it will produce photon emission, which will produce photoionization and form a new derivative electron avalanche. This developing strong ionization region is called streamer, and the region extending to the anode (cathode) is called positive (negative) streamer. According to streamer theory, lightning discharge can be well explained. According to this theory, the cascade leader is a negative streamer that advances in cascade form, and its overall speed is similar to the speed of laboratory sparks in air. The direct channel pilot is a negative streamer advancing along the previous ionization channel, while the return stroke is a positive streamer advancing upward from the ground.

The electric field generated by lightning at the change distance L contains three components: electrostatic field, induced field and radiation field. The electrostatic field component is directly proportional to the discharge electric moment and inversely proportional to it. The electromagnetic induction component depends on the discharge current and is inversely proportional to; The component of electromagnetic radiation field depends on the acceleration of charge movement during discharge and is inversely proportional to L, so the electrostatic field plays a big role in near discharge and the radiation field plays a big role in far discharge (see Heaven). At first, the electric field change of lightning stroke is a slow change part (L change) related to the pilot discharge, lasting about 0. 1 sec, followed by a rapid step change (R change) related to the return stroke, lasting less than 1 ms, and finally a slow change (S change) lasting about 0. 1 sec. The electric field changes of multiple lightning strikes all start from the L change again and end with the S change, during which there is a slow change (J change) between the corresponding R change of lightning strikes and the two R changes, and the latter is formed by the intermittent non-luminous discharge process during the two lightning strikes (Figure 3 [One Lightning Strike]).

The observation and study of lightning spectrum and lightning emission spectrum of lightning current provide the earliest clues of lightning physical properties. When investigating the lightning spectrum of 3000 ~ 10000 angstrom, it is found that it is composed of atomic and ion emission lines and molecular emission and absorption bands on the background of weak continuous spectrum. Nearly 200 spectral lines have been published, most of which belong to neutral and ionizing emission lines of nitrogen and oxygen. From the study of lightning spectrum, it can be inferred that the temperature of lightning channel reaches the peak of 25000 ~ 30000 K in a few microseconds, but drops to the half peak in about 30 microseconds. At the peak temperature, the electron concentration in the channel is 10 ~ 10/cm, which is greater than or equal to the sum of the concentrations of molecules, atoms and ions in the channel. The pressure reaches 10 atmospheric pressure. Some research results of lightning spectrum are consistent with the lightning electrical parameters observed by electromagnetic method. From the lightning protection design of electric power, construction and other departments, lightning current is one of the most important parameters of lightning discharge, from which parameters such as charge, energy and electric moment can be inferred. The current of lightning generally reaches its peak at about 10 microsecond (10 ~ 100 kA), and the current rise rate reaches the maximum (about 10 kA/ microsecond) before the peak current. The potential difference between the cloud and the ground is generally 10 ~ 10 volts, and about 20 charges are transported at a time, so the energy of a lightning strike is about 2× (10 ~ 10) Joules. Such a powerful lightning current instantly passes through a channel with a diameter of several centimeters, producing a shock wave, which propagates for a certain distance and degenerates into sound waves, that is, lightning we hear.

According to the different forms and characteristics of lightning, lightning can be divided into different types, such as linear lightning, ribbon lightning, rocket lightning, sheet lightning, thermal lightning, bead lightning and spherical lightning. Linear lightning is the most common in nature, and its main feature is a thin and bright luminous column. If the light beam is straight and does not diverge, like a tree trunk, it is called dendritic lightning; If the beam twists and turns, it is called forked lightning (see color picture [forked lightning]). Banded lightning is a cloud-to-ground lightning with a width of about ten meters (Figure 4 [Banded lightning]). It is because the channel of linear lightning moves under the action of strong wind, so that the spatial position of lightning is separated in the horizontal direction and becomes a band. Rocket lightning is a long-distance air discharge. The naked eye can directly observe that the discharge moves slowly along the lightning path like an arrow, and the duration of the whole discharge is about 1 second. Flake lightning is lightning used to make clouds glow. Hot lightning is used to refer to lightning that is too far away to hear thunder and only see flash. Flake lightning and thermal lightning are usually difficult to distinguish. Beaded lightning means that the lightning channel looks like lightning broken into many small pieces. Each section is about tens of meters long and looks like a string of beads hanging in the sky from a distance. Spherical lightning is a kind of unusual lightning that can cause certain harm (see color picture [spherical lightning]), which usually appears in strong thunderstorm weather. The shape is spherical, with a diameter of 10 ~ 20cm (some less than 1cm, or even as large as 10m), red, orange or yellow, and its existence time is less than 5s (some more than 1min). Its horizontal moving speed is usually several meters per second, and sometimes it can stop in mid-air or land from the air. Spherical lightning tends to drill cracks. When disappearing, it is often accompanied by explosions and loud noises, or it disappears silently. The place where it disappears often has an odor similar to ozone or nitric oxide. There have been many hypotheses about the cause of spherical lightning, but no consensus has been reached. Some people think that spherical lightning is a heated air ball, while others think that it is a plasma with extremely high density (the electron concentration is about 10/m). There are also two opinions about the energy source of spherical lightning: ① It is believed that the energy of spherical lightning is stored in the sphere, such as the chemical energy of gas reaction, the composite energy of charged particles, and the radiation energy of excited atoms or molecules. ② It is considered that the energy of spherical lightning comes from the outside of the sphere, for example, the radio frequency radiation energy of hundreds of megahertz emitted by thunderstorm clouds or radioactive cosmic ray particles gathers in the strong electric field of thunderstorm clouds to produce nuclear fission reaction, which provides enough energy for the generation of spherical lightning. The above theories are still controversial, and none of them can explain the motion characteristics of spherical lightning.

philology

Translated by R.H. Gold and Zhou Shijian. : Lightning, Volume I, Electric Power Industry Press, Beijing, 1982. (Gao De, edited by Li Hening, vol. 1, Academic Press, London, 1977.

Mi (abbreviation of meter)) new york McGraw Hill Wright Ningwuman, 1969.