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 powerful ones this summer.  I've always liked a good storm, as long as the lightning stays away from my house.  A 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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The electric landscape

In his remarkable TED Talk "Can We Create New Senses for Humans?," neuroscientist David Eagleman describes the concept of the umwelt -- the part of the available stimulus space sampled by a particular animal's senses.  A simple example is the thin slice of the electromagnetic spectrum our eyes are sensitive to -- the familiar ROYGBIV of the rainbow.  There's plenty of electromagnetic radiation outside of that slice; gamma rays, x-rays, ultraviolet light, infrared light, microwaves, and radio waves are all ordinary photons, just like visible light is.  It's just that our eyes aren't sensitive to those frequencies, so they're outside of our umwelt.

The umwelt also has to do with the relative weighting of senses; how big a part of our sensory world a particular experience constitutes.  Most humans have a sense of smell, but my dogs live in a far richer olfactory world than I do.  But even how those inputs are utilized -- i.e., what kind of information they provide for making sense of the world -- can vary greatly.  Bats and dolphins use hearing in much the same way as we use our eyes, creating "sonic landscapes" of the objects around them.  What's kind of amazing, though -- and one of the main points of Eagleman's talk -- is that humans can train their brains to use other "peripherals" (as he calls them) to learn about the world, such as blind people who have learned to navigate the space around them by making clicking noises and listening for echoes from nearby obstacles.

It's always been fascinating to me to consider how the world would look to a night-flying echolocating bat.  Do they "see" their world through their ears and auditory cortex?

The topic of how other animals perceive their worlds -- and how different it could be from what we experience -- comes up because of a paper this week in the journal Nature about how elephantnose fish (Gnathonemus petersii), which live in murky streams in west and central Africa where eyesight doesn't serve much purpose, develop their visual picture of the world (including locating prey) using electric fields.  And not only do they gain information by creating and sensing electrical signals, they enhance those pictures using the signals created by nearby members of their species, making them one of the only known animals that relies on collective signal production and sensing.

Gnathonemus petersii [Image is in the Public Domain]

"Think of these external signals as electric images of the objects that nearby electric fish automatically produce and beam to nearby fish at the speed of light," said Federico Pedraja of Columbia University, who headed the study. "Our work suggests that three fish in a group would each receive three different "electrical views" of the same scene at virtually the same time."

The elephantnose fish's capacity for working in groups is a little like humans out on a search at night with flashlights.  One person with one flashlight would have a small illuminated field of view, but if there were twenty people it would go much faster, not only because of greater manpower, but because each person wouldn't be restricted to what is revealed by only their own flashlight beam.  Just as with twenty different flashlights in the night rather than a single one, in the case of elephantnose fish, the electrical fields produced by their neighbors clarify the picture they all receive.

"In engineering it is common that groups of emitters and receivers work together to improve sensing, for example in sonar and radar," said Nathaniel Sawtell, who co-authored the study.  "We showed that something similar may be happening in groups of fish that sense their environment using electrical pulses.  These fish seem to 'see' much better in small groups...  [They] have some of the biggest brain-to-body mass ratios of any animal on the planet.  Perhaps these enormous brains are needed for rapid and highly sophisticated social sensing and collective behavior."

To return to my original point -- how would the world look to an elephantnose fish?  Surely nothing like what we see.  Some sort of topography of electrical field strength, perhaps, creating an image of the obstacles they have to maneuver around, the prey they seek, and the predators they need to avoid.  But really, there's no way to know.  We're all trapped within our own umwelt.  I can't even imagine what the world is like for my dogs, who are a great deal more similar to me than these fish are.

To perceive the world like another living being does, you'd not only have to come equipped with their sensory systems, but put the information together using their brains.  We can only speculate, with all the inevitable biases that come from being locked in our own ways of knowing.  But this study did at least give us a hint of how different the world could appear -- if we were odd little fish living in muddy African rivers.

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