Tikalon Blog by Dev Gualtieri

Lightning Striking Again

September 17, 2015

Zeus was the king of the gods, and the symbol of his power was a thunderbolt; that is, lightning. He obtained lightning as a gift from the Cyclops, a primordial race of giants, and he was known by the epithets, Αστραπηος (Astrapios, Lightning Wielder) and Βροντηος (Brontios, Thunderer). If one of these names sounds familiar, you're likely thinking of Brontosaurus, the "Thunder Lizard."

Zeus in his war chariot wielding a thunderbolt.

(From "Stories from the Greek Tragedians," by Alfred Church, Dodd, Mead and Company, New York, 1879, Project Gutenberg, via Wikimedia Commons.)

Lightning is often used as a metaphor for the power of nature, as found in everything from Saturday morning cartoons to company logos and government emblems. This reputation is well deserved, since the electrical current in a lightning bolt is tens of thousands of amperes. Compare this with the ten amperes of a typical hair dryer. The electrical power contained in a typical bolt is nearly a million watt-hours.

Provisional Emblem of the United States Air Force Cyber Command

An iron fist would likely make a good lightning rod.

Emblem of the (Provisional) United States Air Force Cyber Command.

(Via Wikimedia Commons.)

One interesting cultural reference to lightning can be found in the 2002 film, Sweet Home Alabama. A main character creates glass sculptures by driving rebar into beach sand as lightning rods. The electrical current, flowing through the metal rod, fuses the sand into an intricate glass pattern. In the film, these fulgurites have a more artistic appearance than the typical lightning product.

As can be imagined, lightning causes problems for aviation. Many air travelers, myself included, have been delayed by a rule that aircraft can't be fueled during a lightning storm. According to the National Lightning Safety Institute,[2] nine aircraft have been lost in flight from lightning strikes, all in decades prior to the time when aircraft systems were redesigned to mitigate lightning damage. Just last month, a Boeing 737 awaiting takeoff at the Atlanta airport was struck by lightning, but with no ill affects to the passengers and crew.[3]

A high-powered laser creates a ray that's a compact path of intense electromagnetism, so the idea arose soon after its invention that laser beams could be used to direct the path of a lightning bolt to ground by generating a plasma in the air molecules.[4] The early experiments had disappointing results, since the long pulse lasers of that era produced discontinuous plasmas. The short pulse lasers of today have shown better results when special techniques are employed.[5-6] I wrote about laser lightning rods in an earlier article (Laser Lightning Rod, March 23, 2012).

The Internet has enabled many crowd-sourced projects, one of which is a very useful lightning monitoring network facilitated by Blitzortung.org.[6] Lightning is detected by its radio signature, and the time delay of the radio frequency pulse allows a determination of the stroke location. The radio receivers cost about $200, and the many volunteers make this network possible. A nice graphical view of the data can be seen at lightningmaps.org.[7] The time evolution of lightning over the course of 2014 can be seen in a YouTube video.[8]

Lightning strike probability map of the world created from space-based optical sensors. The units are flashes per square kilometer per year. Click for larger image. (NSSTC Lightning Team image, via NASA.)[9]

An international team of scientists from the University of Pennsylvania (Philadelphia, Pennsylvania), the Albert-Ludwigs-Universität (Freiburg, Germany), the GeoForschungsZentrum Potsdam (Potsdam, Germany), and the Australian Nuclear Science and Technology Organization (ANSTO, New South Wales, Australia), has found that a black patch of rock with an unusual structure in a granite outcrop in southern France is a fulgurite formed by a powerful bolt of lightning.[10-11]

The study, published in the journal, American Mineralogist, was led by Reto Gieré, chair of the University of Pennsylvania Department of Earth & Environmental Science, with collaborators Wolfhard Wimmenauer and Hiltrud Müller-Sigmund of the Albert-Ludwigs-Universität, Richard Wirth of GeoForschungsZentrum Potsdam, and Gregory R. Lumpkin and Katherine L. Smith of the Australian Nuclear Science and Technology Organization.[10-11]

The rock outcropping, located near Les Pradals in southern France, has a jagged fracture in granite that's discolored black.[11] The black layer, which is the lightning-formed fulgurite, appears under an optical microscope as a porous, shiny ceramic glaze.[11] Chemical analysis of the fulgurite layer showed elevated levels of phosphorous pentoxide and sulfur dioxide that may have come from lichen living at the rock's surface when lightning struck.[11]

The granite fulgurite found near Les Pradals, France.

(University of Pennsylvania image.)

Transmission electron microscopy showed that the fulgurite was amorphous, having formed from highly heated material that was cooled quickly.[11] However, the region adjacent to the fulgurite contained shock lamellae; that is, material appearing as straight, parallel lines. Such a feature occurs when quartz or other minerals are deformed by a large pressure wave.[11] Says Gieré,

"It's like if someone pushes you, you rearrange your body to be comfortable... The mineral does the same thing."[11]

The layer of lamellae was only about three micrometers thick. Before this study, the only known natural events producing lamellae were meteor impacts.[11] Meteor impacts form these lamellae form at pressures in the range of 10 gigapascals.[11] The fulgurite-producing lightning strike must have generated a pressure pulse in that range.[10]

A transmission electron micrograph of the parallel lines of the shock lamellae.

(University of Pennsylvania image.)

This study shows that not all shock lamellae are indicative of a meteor strike.[11] Says Gieré, "...This is a good reminder to always use multiple observations to draw big conclusions, that there are multiple mechanisms that can result in a similar effect."[11]

Rock climbers and hikers are advised that when they see a shiny black glaze on a rock, it indicates a site where lightning has struck before, and it might strike again.[11]

Lightning Striking Again

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