Lightning Talks

Transcript

Ken Kingery:

This is Rate of Change, a podcast from Duke University dedicated to the ingenious ways that engineers are solving society’s toughest problems. I’m Ken Kingery.

Waves are all around us. Light waves illuminate our world. Wi-Fi and radio waves carry information. and sound waves let us hear one another.

Today I’m talking with Professor Steve Cummer, the William H. Younger Distinguished Professor of Electrical and Computer Engineering here at Duke. Professor Cummer is an expert in understanding how all sorts of waves interact with the world as they travel through it. This understanding allows him to design materials and objects that can control the waves in useful ways.

It also gives them the ability to figure out where certain waves came from and what happened to them during their journey, and those details can provide a ton of information

In today’s conversation, we’ll hear all about how looking at waves is transforming our understanding of one of the flashiest phenomena of all time.

[thunder sounds]

Lightning.

Let’s jump into the conversation with Professor Cummer, speaking about how he first got started with lightning coming from a background of wave processing.

Professor Cummer:

There was a research group at Stanford at the time that was accepting students that was doing that kind of electromagnetics research in the context of geophysics and using electromagnetic fields as a tool for remote sensing lightning, remote sensing Earth’s upper atmosphere at satellite altitudes, and sort of exploring what was happening there. And so that was a great fit, for sort of curiosity-driven research that was aligned with what I wanted to do.

And then the extra good thing that happened around that time is that, there were a lot of discoveries that were made around lightning, documented things that people had maybe seen anecdotally but had never been really captured on film.

Ken:

Were those, like the new discoveries were they like, enabled by new technologies? Or there’s just new questions that were being answered at the time, or what questions that were being posed at the time?

Professor Cummer:

Yeah. They actually were partly enabled by new science capabilities. The first one that happened was, sort of now a classic story of and this what was observed was something called a sprite, which is a really big, very brief, high-altitude optical emission, bright flash that happens above thunderstorms in response to a very big lightning discharge that happened below it.

And, there’s some interesting historical articles that describe how, for instance, pilots, airline pilots had seen these things for many years, just flashes out of the corner of their eyes that they felt pretty sure were real. But it was never captured on video. And pilots don’t like to talk about that sort of thing because it gets them in a difficult situation.

Are they suitable to fly if they’re seeing flashes out of the corners of their eyes? But it was a researcher at the University of Minnesota who had a new low-light camera, like with sort of night vision technology built into the camera, that was going to be used to image the Aurora. And he was testing it by pointing it at a distant thunderstorm, just to see if he could see the flashes of light that were being produced by lightning.

And by complete luck, he happened to capture this high altitude flash of light above that thunderstorm. Got written up into a paper in Science, and then suddenly, now that we are sure these things exist, lots of research groups quickly got low-light cameras and just started recording about thunderstorms. And sure enough, these things happen all the time.

So they’re kind of at that edge between what you think of as the atmosphere. And in space. They happen between maybe, 40 and 60 miles high. So pretty high up, but not at satellite altitudes well below where satellites tend to go. And they fill a huge volume of space because they really are 10 or 20 miles in all dimensions, like they fill a cube that is 10 or 20 miles on a side.

And when there’s a really, really powerful lightning discharge in a thunderstorm below, that creates a strong electric field that is then able to generate sort of this, the sprite, which is almost a secondary lightning discharge that happens at these high altitudes because of the relatively thin air that is up there. So people had kind of seen these, didn’t really know if they were real.

Then somebody catches that on camera accidentally. All of a sudden people start seeing it a lot. And then that means that there’s something there to research. And that’s exactly. And there were like, it’s something that nobody had seen before. And so the obvious questions of what is it? What’s making it? and yeah, why does it happen?

Some of those questions were best addressed by looking at the radio emissions from distant lightning to get a sense of how big those lightning flashes actually are that are connected to these sprites.

Ken:

Let’s dig into that a little bit. Tell me, you see a flash of lightning and obviously there’s photons coming out of it. What are all the different other things that are coming out of a lightning strike or a bolt that people don’t see?

Professor Cummer:

Well, from, an electromagnetic standpoint, like what kinds of, radio and light waves does it produce? You see the you see the light that you can detect with your eyes. But it’s generating very powerful radio waves at the same time. Just sort of across that entire electromagnetic spectrum. There are so many different processes in lightning that are produced using radio waves.

For example, you can – not everybody has an Am radio 0 in their car or ever listens to Am radio anymore – but if you listen to an AM radio during a thunderstorm, you can hear snap, crackle, pop all the time. That’s the radio emissions. That’s for very short radio bursts that are produced by lightning. And lightning generates ultraviolet light, infrared light. There’s lots of heat being produced. There’s, of course, thunder that’s produced by very rapid heating of the air when a lot of electricity is forced into it, through it. And, then there’s even thing other things that were discovered around the same time as sprites that we didn’t know. For instance, gamma rays, super high energy photons, even more energetic than what are used for medical X-rays, thunderstorms generate these, which was another complete surprise that happened in that, early-to-mid-1990s window, that gamma rays…

Ken:

Isn’t that what the Hulk is made out of, too?

Professor Cummer:

That is what I think made the Hulk into the Hulk. But, yeah. So that’s interesting.

Ken:

Other than just, like, pure scientific curiosity, is there a reason that everybody wants to know what these things are and how they work? What’s the idea behind all of that?

Professor Cummer:

Yeah, well, some of it definitely is just that curiosity driven, research that there’s this spectacular, spectacular phenomenon that’s been happening on Earth for billions of years. And, something that I’ve learned, you know, being in this research field over 30 years is every question you ask leads to like five more. And the real nature of lightning is so, so, so complicated. Like the classic question that I would like raise in this context is that, or really just sort of unknown – nobody really knows the physics of how lightning is able to start. We know thunderstorms have large electric fields in them, which is sort of the energy source for lightning. We know where that comes from. It comes from ice particles and water particles in the thunderstorm colliding with each other.

And like friction charging them up, like literally the same way that when you rub your feet on a wool carpet, you can build up static electricity that way or balloon on your shirt. The hair is exactly the same thing. It’s ice and water droplets in the thundercloud, and then winds blowing certain kinds of particles, lighter particles higher, and leaving the heavier ones at the bottom.

So that’s what creates this giant electric field. But then where do the little sparks start that then can build into this gigantic many miles long conducting channel is unbelievably complicated.

Ken:

You mentioned that there’s a bunch of other phenomenon other than sprites, other than, how it propagates, how it sparks. What are some of the other, like, cool questions that scientists are looking for in lightning?

Professor Cummer:

Well, let me let me jump off to something that I alluded to before. This question of, gamma rays again because that just is the story that keeps on giving. So this was another accidental discovery reported first in a paper in 1994. And it was a NASA gamma ray astronomy satellite that did the detection. So this is a satellite that has gamma ray sensors. That is actually out in space in order to detect things like supernovas that are, you know, among the most energetic phenomena in the entire universe, and blast all kinds of electromagnetic energy all the way up to gamma rays.

And you can tell a lot about what’s happening in something like a supernova from the details of the gamma rays that are being produced by it. And purely by accident, a gamma ray instrument doesn’t really have directional sensitivity because gamma rays kind of go through anything. So they just have a detector. And what they saw completely unexpectedly, were these very short bursts of gamma rays, way shorter than should be coming from any astrophysical source, like, supernova.

And when they looked at the timing of those and were able to tease out a little bit about the direction that they arrived at the sensor, turns out they were coming up from Earth, in locations where there were thunderstorms. So thunderstorms are shooting out jets of very high energy gamma rays out into space. Nobody knew that. And they’ve been doing it for a billion years.

Which was also super exciting. And over the last 30 years, people have sort of built up the theory of how gamma rays are generated inside thunderstorms in these strong electric fields. But what has been interesting, I would say, and this is, just over the last 12 months these short gamma ray bursts may actually be playing a role in the initiation of lightning.

Ken:

Is lightning the same anywhere you go? Like is this lightning here the same as, like, in Montana or across the country or across the world?

Professor Cummer:

Yes and no. I mean, like what you would see. Yeah. I mean, it pretty much is the is the same thing in terms of, you know, long electrically conductive channels coming out of the cloud, hitting the ground. But, there are a lot of different contexts for lightning that depend on the meteorology of the location. As an example, this is something that’s actually been known for a long time, but there are certain thunderstorms on the west coast of Japan that are very unique, and the lightning is quite a bit different in terms of how powerful it is.

And those unusual storms are produced in wintertime when super cold air is coming out of Siberia and like leaving the mainland of Asia and crossing over the Sea of Japan, which is that body of water between the west coast of Japan and the rest of Asia. And in the winter, it’s not really that warm, but at the Sea of Japan, the water is way warmer than the super cold air that’s coming over out of Siberia.

And you get these thunderstorms that are really low altitude, like the clouds themselves are maybe only 1000 meters above the ground. More typically they would be like summer thunderstorms in the US. It’s probably more like 5 to 10km. So these are really low clouds. And you get some unusual circumstances where, mountains, even modest mountains are inside the thunderstorm, not below it.

And when you’ve got this pointy section of ground that is actually extended into the high electric field region in the thunderstorm, all kinds of crazy stuff happens, including gamma rays being generated. But there’s some really powerful, very fast lightning strokes that develop.

Ken:

Speaking of differences in lightning and all that type of stuff makes me wonder, what’s the magnitude of difference between what you would consider a small lightning bolt versus a really large one.

Professor Cummer:

Yeah, that’s an interesting question, and sort of dovetails to one that I’m kind of interested in, which is just like, how strong can lightning get? Like, that’s always just a fun search in the data. Can you find, like, something bigger than anybody else has seen before?

So for context, I mean, there’s a few different ways to measure how powerful lightning is. One of the most common is, the peak electric current, like how much electric current is flowing in that lightning channel and in a pretty brief amount of time, small fraction of a second when it hits the ground and you see the gigantic flash. Typical lightning flashes are maybe 10 to 20 kiloamps of electric current.

Ken:

Can you put that into context with some other phenomenon?

Professor Cummer:

I mean, it’s so hard to because it’s so much bigger than anything that you would encounter normally and, I mean, it just blows everything else out of the water. Yeah. I mean, when you’re, when you’re starting, your – if you don’t have an EV – like starting your car, your car battery puts out maybe a few hundred amps and already that’s a pretty big battery and a pretty big system. And so lightning is at least 100 times more powerful than that. Which is why, by the way, you just never want to be that close to lightning. That is always a bad idea. But the most powerful lightning strokes in terms of electric current are maybe a thousand kiloamps, so a hundred times more. And, you know, that’s a big difference.

I’ve heard some anecdotal reports of, like, just tremendous destruction when lightning hits a building or a house and just very unusual levels of how much damage was done. Like big concrete pads exploding, everything melting and burning. And the few times I know when people have been able to look into what lightning stroke was actually responsible for that, it tends to be these really, really big ones that caused the massive, outsized damage.

Ken:

So then, you know, lightning strikes and the currents, amps, whatever you were just referencing. So does it even make sense to think of lightning in terms of gigawatts?

Back to the Future clip:

1.21 Gigawatts!

Professor Cummer:

Yes. You can definitely measure the peak power that’s, you know, gigawatts is power and you can measure the peak power that’s being produced by a lightning stroke hitting the ground. I mean, that power is what’s, for instance, generating the heating of the air that’s generating the thunder, that’s the acoustic wave that you can hear.

Ken:

So Back to the Future 1.21 gigawatts. Is that a reasonable number for, like, did they just pull that out of the air or is that something that’s actually accurate?

Professor Cummer:

Yeah. You know what? Because I’ve done this calculation a few times and that’s actually pretty close to the peak power you would get in a lightning flash hitting an object or hitting the ground. Now that power exists, or at that level, for a tiny fraction of a second. But no, it really is that high.

Ken:

They must have had some sort of consultant.

Professor Cummer:

Yeah, I’m sure they did. And then, a related question that I get asked, is, you know, given that peak power, that’s so much, shouldn’t we be like, harnessing lightning’s energy in order to do something useful with it? Is that a form of, sort of sustainable power generation?

Ken:

I could see that being a common question.

Professor Cummer:

Oh, that’s a totally reasonable question. Sadly, the answer is definitively no. It’s not useful. and the reasons behind that one is how short lightning actually is. You know, that 1.21 gigawatts is less than a thousandth of a second. And so the total amount of power that comes from that, is not at all that much.

And then the real challenge is that lightning just doesn’t occur where you want it to occur in a way that you could capture it. You know, in the very highest lightning rate parts of the United States, most of which are in Florida, if you take one square kilometer, which is close to one square mile, and ask, how many times does lightning hit that one square kilometer in a year, it might be as many as 20, which is a lot.

But that would mean if you wanted to harness this energy, you have to build some kind of giant lightning collector. That’s one kilometer by one kilometer. And even then, when it hits that, when lightning hits that 20 times, you would be getting enough power to run a 100 watt light bulb for, like, a month. And so it’s just not worth it.

Ken:

That’s our show for today. You can learn more about Professor Cummer’s research into lightning at our website pratt.duke.edu. And while you’re there, check out his research into designing metamaterials, which can control the movements and behaviors of the waves all around us.