Lightning is mysterious. We know a lot about it, but there are still some basic features scientists still don’t grasp. Perhaps the biggest is the most basic: what triggers it?

Lightning starts inside a thundercloud, where rising and falling winds create currents of air. It’s thought that ice crystals carried inside the cloud by these currents rub against each other as they move in opposite directions, moving electrons around and creating the separation of charges that leads to the huge voltages needed for lightning. But what lights that match, so to speak? 

A new paper posits an extraterrestrial source for the trigger: cosmic rays, subatomic particles zipping around space at very nearly the speed of light. This has been proposed before as a trigger, but the new paper puts the math behind it [link to journal paper].

In BAN Issue #884 I talked about terrestrial gamma-ray flashes, or TGFs. These are very brief flashes, lasting just microseconds, that blast out high-energy gamma rays a tiny fraction of a second before a lightning bolt erupts. I wrote about how huge flows of electrons through the powerful electric field of a thundercloud can create the flash. 

The new work looks at what happens just before that, what starts that flow of electrons in the first place. Their answer is cosmic rays.

When a cosmic ray hits our atmosphere, it has so much energy that it can strip the electrons away from the molecules of nitrogen and oxygen in the air. That subatomic shrapnel is blasted away at high speed, which can cause a cascade of even more electrons moving downward at high speed, which strip off more electrons, etc. These air showers have been detected for decades. They happen all the time, all over the planet.

But if they happen over a thundercloud the situation is different. The cloud has a very powerful electric field in it, and when the electrons are blasted off molecules by cosmic rays at the top of the cloud, these electrons are accelerated even harder. They achieve incredible speeds, and slam into more molecules in the cloud. This is another cascade, creating a flood of electrons that generates the TGF I discussed in the earlier issue, and these occur right before the actual lightning bolt is initiated. 

What’s new here is that the mathematical model in the research explains how this works, and also explains the X-ray and radio emission seen. They find the region emitting the TGFs can be very compact, and the strength of the runaway cascade varies considerably, explaining why some TGFs are not seen to have accompanying radio or visible light flashes as well.

OK, great! Let’s assume this all works. If cosmic rays trigger lightning, what generates cosmic rays?

Supernovae.

Yup, exploding stars. The process is complicated, because of course it is. But, when a star explodes, the material making up the star is flung outwards at very high speeds, many millions of kilometers per hour. This slams into the material outside the star, predominantly interstellar gas (or circumstellar gas, gas literally around the star much closer in), compressing it. The gas can have weak magnetic fields in it, but when the shock wave hits the compression makes magnetic fields get stronger.

Electrons and other charged particles in the gas get accelerated by these fields to high speeds. In fact the subatomic particles bounce around inside the gas multiple times, accelerating each time until they are relativistic, approaching the speed of light. At some point the magnetic field can no longer hold them in, and they fly away into deep space. They can travel for a long way, hundreds of light years or more, before hitting our atmosphere.

And, apparently, triggering lightning.

Oh I should also note that some cosmic rays may come from black holes gobbling down matter, which heats up to ridiculous temperatures on its way down, swirls around, generates fantastically powerful magnetic fields, which then launch twin jets of matter and energy out at soul-crushing speeds that also bounce electrons around and then send them flying through the cosmos at near light speed which can then reach Earth and spark a lightning bolt. So, yeah.

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