Thunderbolts and lightning (very very frightening)

The cause of lightning has been strangely elusive.

Oh, in the broadest-brush terms, we've understood it for a while.  The rapidly-rising column of air in a cumulonimbus cloud induces charge separation, resulting in an electric potential difference between the ground and the air.  At a potential of about three megavolts per meter, the dielectric strength of damp air is exceeded -- the maximum voltage it can withstand without the molecules ionizing, and becoming conductive to electrical current.  This creates a moving channel of ionized air called a stepped leader.  When the leader reaches the ground, the overall resistance between the ground and the cloud drops dramatically, and discharge occurs, called the return stroke.  This releases between two hundred megajoules and seven gigajoules of energy in a fraction of a second, heating the air column to around thirty thousand degrees Celsius -- five times hotter than the surface of the Sun.

That's the origin of both the flash of light and the shock wave in the air that we hear as thunder.

[Image licensed under the Creative Commons Maxime Raynal from France, Port and lighthouse overnight storm with lightning in Port-la-Nouvelle, CC BY 2.0]

The problem is, there was no consensus on what exactly caused the very first step -- the charge separation in the cloud that triggers the voltage difference.  Some scientists believed that it was friction between the air and the updrafting raindrops (and hail) characteristic of a thundercloud, similar to the way you can induce a static charge on a balloon by rubbing it against your shirt.  But experiments weren't able to confirm that, and most places you look, you'll see words like "still being investigated" and "uncertain at best" and "poorly understood process."

Until now.

A team of scientists led by Victor Pasko of Pennsylvania State University have shown that the initiation of lightning is caused by a literal perfect storm of conditions.  They found that free "seed" electrons, knocked loose by cosmic rays, are accelerating into the rapidly-rising air column at "relativistic" speeds -- i.e., a significant fraction of the speed of light -- and then ram into nitrogen and oxygen atoms.  These collisions trigger a shower of additional electrons, causing an avalanche, which is then swept upward into the upper parts of the cloud.

This is what causes the charge separation, the voltage difference between top and bottom, and the eventual discharge we see as lightning.

It also produces electromagnetic radiation across the spectrum from radio waves to gamma rays, something that had been observed but never explained.

"By simulating conditions with our model that replicated the conditions observed in the field, we offered a complete explanation for the X-rays and radio emissions that are present within thunderclouds," Pasko said.  "We 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...  [T]he high-energy X-rays produced by relativistic electron avalanches generate new seed electrons driven by the photoelectric effect in air, rapidly amplifying these avalanches.  In addition to being produced in very compact volumes, this runaway chain reaction can occur with highly variable strength, often leading to detectable levels of X-rays, while accompanied by very weak optical and radio emissions.  This explains why these gamma-ray flashes can emerge from source regions that appear optically dim and radio silent."

There's still a lot left to explain, however.  Also this week, a paper came out of Arizona State University about the astonishing "megaflash" that occurred in October 2017, where a single lightning bolt traveled over eight hundred kilometers -- from eastern Texas all the way to Kansas City.  Even though the megaflash dropped some cloud-to-ground leaders along the way, it didn't discharge completely until the very end.  Megaflashes are rare, but what conditions could lead to a main stepped leader (and the corresponding return stroke) extending that far before grounding are unknown.

So like with all good science, the new research answers some questions and raises others.  Here in upstate New York we're in thunderstorm season, and while we don't get the crazy storms they see in the southeast and midwest, we've had some good ones this summer.  I've always liked a good storm, as long as the lightning stays away from my house.  A good friend of ours had his house struck by lightning a few years ago and it fried his electrical system (including his computer) -- something that leads me to unplug my laptop and router as soon as I hear rumbling.

Even if the mechanisms of lightning are now less mysterious, it's still just as dangerous.  Very very frightening, as Freddie Mercury observed.

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