Runaway breakdown

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The runaway breakdown , also called runaway discharge in German , is a physical effect of the electrical discharge that probably plays a role in the development of lightning . The resistance that electrons accelerated in air experience as a result of collisions decreases from a certain speed instead of increasing.

Problems investigating lightning generation

Ever since Benjamin Franklin first investigated lightning , it was assumed that lightning should be treated like spark discharges. These express themselves z. B. by small electric shocks that you get when you have electrostatically charged and then touch a door handle. In order for such a sparkover to occur, electrical field strengths of around three million volts per meter must occur in the air . In thunderclouds , however, these field strengths were not even rudimentarily measured. Obviously, the mechanism that leads to lightning is not a simple discharge.

The runaway breakdown

One solution to the problem could be runaway breakdown, an unusual type of electrical breakdown. This theory was first put forward in 1961 by Alexander Gurevich of the Lebedev Institute in Moscow .

In a conventional discharge, electrons move relatively slowly because they constantly collide with molecules in the air. This resistance is greater, the faster the electrons are. Above speeds of 6 million meters per second (about two percent of the speed of light ), however, the resistance drops again despite increasing speed. The reason for this is that the measure of the probability of electron impact ionization, the so-called cross-section , initially increases with the electron energy above the ionization threshold, until a maximum is reached at around three times the threshold ( Lotz formula ). Above this, the cross-section decreases again ( Bethe-Heitler formula ). Electrons that are accelerated beyond this point in a strong electric field therefore become faster and faster and can almost reach the speed of light. They are known as runaway electrons (also known as runaway electrons in German ). This creates high energies, which some researchers hold responsible for lightning. Assuming a runaway breakdown, 150,000 volts per meter of electrical field strength are sufficient to cause lightning. This value is actually measured in thunderclouds.

Proof of the runaway breakdown

When the highly accelerated runaway electrons collide with gas molecules in the air, braking radiation in the form of X-ray and gamma radiation emitted. The best evidence to date for the correctness of the runaway breakdown theory is that in 2001 it was actually possible to measure this radiation in lightning.