Engineers across many disciplines are aware of and concerned with lightning—and for good reasons. A lightning strike can cause significant structural damage, house and forest fires, and severe electrical surges (Figure 1).
Figure 1 The intensity of a lightning strike is always awe-inspiring and represents a millisecond-level transient of hundreds of kiloamps. Source: Science Daily
Even if the strike is not directly on the equipment (in which case the unit is probably “fried”), the associated transients induced in nearby wires and paths can be damaging. Lightning can also be mystifying: some people who have been hit have no ill effects; others have some temporary or long-lasting physical and mental impairments; and for some….well, you know how it ends.
Measuring lightning
For these reasons, protection against the effects of lightning to the extent possible is an important factor in many designs. These efforts can include the use of lightning rods, which provide low-impedance paths to Earth ground functioning as a near-infinite source and sink for electrons, gas-discharge tubes (GDTs), and metal oxide varistors (MOVs), among other devices. Implementing protection is especially challenging when there are multiple strikes, as they can erode the capabilities of the protective devices.
This natural phenomenon occurs most frequently during thunderstorms, but has also been observed during volcanic eruptions, extremely intense forest fires, and surface nuclear detonations. There are many available numbers for the voltages, currents, timing, and temperature ranges associated with lightning. While there is obviously no single lightning waveform, Figure 2 shows representative data; note the maximum current of several hundred kiloamps.
Figure 2 These are representative values for lighting-stroke current versus time and current magnitudes; these are not the only ones, of course. Source: Kingsmill Industries Ltd
Researchers have studied lightning for many decades, using a variety of techniques ranging from “man-made” lightning in controlled enclosures, to field measurements in lightning-prone areas, to instigating it with a grounded wire launched into a lightning-prone cloud. There’s also the futile quest to direct and capture lightning’s energy into some sort of project store-and-use scheme. (For fictional demonstration, see the 1931 classic Frankenstein, where lightning is used to energize the doctor’s monster-like creation, or the end of the 1985 classic Back to the Future, where lightning is captured by a rod on the clock tower and used to recharge the flux capacitor of the DeLorean time-travel vehicle 
.
The standard explanations for lightning and its initiation are like this one from Wikipedia: “Lightning is a powerful natural electrical discharge caused by a buildup of static electricity within storm clouds. This buildup occurs when ice crystals and water droplets collide in the turbulent environment of a cumulonimbus cloud, separating charges within the cloud. When the electrical potential becomes too great, it discharges, creating a bright flash of light and a loud sound known as thunder.”
But what really happens inside the cloud?
Well, maybe that’s only a partial answer, or perhaps it’s misleading. Why so? For decades, scientists have understood the mechanics of a lightning strike, but exactly what sets it off inside thunderclouds remained a lingering mystery. Apparently, it’s much more than static electricity potential finally reaching a “flashover” level.
That mystery may now be solved, as a team at Pennsylvania State University (Penn State) has produced what they say is the complete story. It’s far more complicated than just a huge static-electricity burst; it’s really a mixture of cosmic rays, X-rays, and high-energy electrons.
Their work involves some deep physics and complex analysis. It also introduced me to some new acronyms: initial breakdown pulses (IBPs), narrow bipolar events (NBEs), energetic in cloud pulses (EIPs), and terrestrial gamma ray flashes (TGFs), flickering gamma ray flashes (FGFs), and Initial Electric Field Change (IEC).
They have taken both historical lighting-related data (and there is a lot of that available from multiple sources) with current measurements, presented a hypothesis, correlated the data, developed models, and ran simulations, and put it all together. The result is a plausible explanation that seems to fit the facts, although with natural events such as lightning, you can never be completely sure.
The Penn State research team, led by professor of electrical engineering Victor Pasko, explained how intense electric fields within thunderclouds accelerate electrons. These fast-moving electrons collide with molecules such as nitrogen and oxygen, generating X-rays and sparking a rapid surge of new electrons and high-energy photons. This chain reaction then creates the necessary conditions for a lightning bolt to form, showing the link between X-rays, electric fields, and the physics of electron avalanches.
These electrons radiate energetic photons (X-rays) as they scatter by the nuclei of nitrogen and oxygen atoms in air. These X-rays radiate in all directions, and some fractions are radiated in the opposite direction of electron motion. These particular X-rays lead to the seeding of new relativistic seed electrons due to the photoelectric effect and thus a strong amplification of the original avalanche.
To validate their explanation, the team used mathematical modeling to simulate atmospheric events that match what scientists have observed in the field. These observations involve photoelectric processes in Earth’s atmosphere, where high-energy electrons—triggered by cosmic rays from space—multiply within the electric fields of thunderstorms and release short bursts of high-energy photons. This process, known as a terrestrial gamma-ray flash, consists of invisible but naturally occurring bursts of X-rays and associated very high frequency (VHF) radiation pulses, Figure 3.
Figure 3 A conceptual representation of conditions required for transition from fast positive breakdown (FPB) to fast negative breakdown (FNB) based on relationship between the relativistic feedback threshold E0/δ and the minimum negative streamer propagation fields E—cr/δ. Source: Pennsylvania State University
They 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. They 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.
I’ll admit: it’s pretty intense stuff, as demonstrated by a read-through of their paper “Photoelectric Effect in Air Explains Lightning Initiation and Terrestrial Gamma Ray Flashes” published in the Journal of Geophysical Research. (I do have one minor objection: I wish they did not use the term “photoelectric effect” in the title or body of the paper. Although that phrase is technically correct as they use it, I associate it with Einstein’s groundbreaking 1905 paper, which resolved all the contradictions of the data of this phenomenon and instead proposed photons as energy quanta, for which he received the Nobel Prize.)
While the root causes of lightning, as delineated in the work of the Penn State team, are not directly relevant to engineers whose designs must tolerate nearby lightning strikes, it’s still interesting to see what is going on and how even our modern science may still not have all the answers to such a common occurrence. In other words, there’s still a lot to learn about basic natural events.
Have you ever been involved with a design that had to be lightning-tolerant? What standards did you try to follow? What techniques and components did you use? How did you test it to verify the performance?
Related content
- Lightning as an energy harvesting source?
- When Lightning Strikes, Will a Surge Protector Help?
- Pulse power and transient loads: a very different world
References
- Kingsmill Industries Ltd, Characteristics of Lightning Discharges
- Wikipedia, Lightning
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