12/24/2021

Surfing Particles Can Supercharge Northern Lights

16:42 minutes

Northern lights over a lake
Northern lights over a lake. Credit: Shutterstock

For thousands of years, humans have been observing and studying the Northern lights, aurora borealis, and their southern hemisphere counterpart, aurora australis. The simplest explanation for how these aurora form has been unchanged for decades: Charged particles, energized by the sun, bounce off the Earth’s protective magnetic field and create flashes of light in the process.

But for a long time, scientists have known it was more complicated than that. What exactly gives those incoming particles the energy they need to create the patterns we see? And why are some aurora more dramatic and distinct, while others are subtle and hazier?

Aurora researcher Jim Schroeder explains new work published in Nature Communications that suggests that in more vivid aurora, electrons may “surf” waves of energy from space into our atmosphere. The waves, called Alfvén waves, are a side effect of the solar wind warping the Earth’s magnetic field. Schroeder explains the weird physics of our aurora, and what we could learn about other objects in the universe as a result. 


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Segment Guests

Jim Schroeder

Jim Schroeder is an experimental plasma physicist and Assistant Professor at Wheaton College in Illinois. In a universe where over 99% of what we see is made of plasma, he focuses on plasma waves that transport energy and their interactions with electrons and ions that lead to northern lights and radiation belts.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. One of the best light displays of the winter season is the northern lights– the aurora borealis. Or if the southern hemisphere is your preference, try aurora australis.

The long nights and clear, cloudless skies of winter make it easier to catch a glimpse of the fantastic greens, pinks, purples of the polar auroras. And if you’re lucky enough to see this sky show this winter, you might notice that not every aurora is created equal. No, some may look blurry and diffuse while others shine in distinct ever-moving bands of color. I have to say I have never seen one, but I’m always hopeful.

Well, believe it or not, with all that we know about Earth science, the reason for this variable display has been a long-standing mystery in the astronomical community. And joining me now is someone whose research may have finally helped us know why the aurora comes in such different flavors. Here with me is Dr. Jim Schroeder, assistant professor of physics at Wheaton College in Wheaton, Illinois. Welcome to Science Friday.

JIM SCHROEDER: Thank you, Ira. Pleasure to be with you.

IRA FLATOW: So what is known about how the northern lights and their southern hemisphere cousins happen?

JIM SCHROEDER: Yeah, so it’s known that auroras are produced when energetic particles from space, like electrons come raining down into the atmosphere. And they’ll strike atoms and molecules in the upper atmosphere. And actually, these energetic particles will give their energy to the atoms and molecules of the atmosphere. And then eventually those atoms and molecules give up their energy in the form of a little flash of light; a photon is given off.

And when a bunch of photons are given off over a region of the sky, that’s what makes up an aurora. But as you said, there are some mysteries here because auroras have all sorts of different appearances, suggesting that what’s actually pushing these electrons towards Earth varies. And there’s different types of things that can push electrons towards Earth.

IRA FLATOW: Such as I know the sun is a major player here, right?

JIM SCHROEDER: That’s right. So the sun is sending a constant stream of something called plasma. And so plasma is actually– it’s the fourth state of matter. We often learn about solids, liquids, and gases, but we don’t always learn about plasma on the same footing.

Plasma is an ionized gas. So if you take a gas and heat it up, you get ions and electrons. And the sun is sending the stream of plasma called the solar wind that flows past the Earth. And some of those particles get funneled down towards the polar regions and cause auroras.

IRA FLATOW: And so what are the other things you mentioned? There may be different things– other things that are pushing the electrons.

JIM SCHROEDER: It turns out that the direct streaming of solar particles towards the Earth is not sufficient to actually produce visible auroras. And so there has to be some extra energy boost given to these particles before they crash into the atmosphere. And so there’s various things that are believed to cause this energy boost that’s needed for particles streaming downward. And there are things like actually particles catching waves and riding waves and being accelerated on waves as they come down towards the Earth.

And then there’s also currents that flow in the magnetic field around the Earth and help to sustain the magnetic field around the Earth. And those currents can carry particles down towards the Earth as well.

IRA FLATOW: Waves? What kind of waves are you talking about?

JIM SCHROEDER: There’s waves that exist in plasma that are not waves we experience in our everyday life, right? When we think about waves and our everyday life, we think about light waves. We think about sound waves. We think about radio waves or x-rays.

But actually, in plasmas, there’s a whole bunch of other types of waves. And one of them– it’s known to exist in the plasma around the Earth– is alfvén waves, named for a Swedish physicist Hannes Alfvén who won the Nobel Prize. And this is a type of vibration in plasma.

And so it’s known that there are alfvén waves around the Earth. And actually, it’s known that there are alfvén waves above auroras traveling down towards the Earth in the regions above auroras. And so there’s been a hypothesis for a long time that these waves are a part of pushing electrons toward Earth, and actually causing those electrons to have enough energy to produce visible auroras.

IRA FLATOW: Wow, the geek in me wants to know more about these alfvén waves. How do they start? Where do they end? Give me some more details on that, please.

JIM SCHROEDER: Yeah. So alfvén waves are a disturbance of magnetized plasma. So I was talking about plasma being ions and electrons. And so to make it magnetized, there has to be a magnetic field running through it as well.

And that’s exactly what we have around the Earth. Earth has its magnetic field that extends out into space. And so when the ions and electrons and magnetic field get together, there can be vibrations of that magnetic field if you picture kind of waves on a string. If you take an instrument and you pluck the string with a high speed camera, you could see waves traveling along the string.

And actually, it’s the same, very similar mathematics behind alfvén waves. There’s vibrations that carry along the magnetic field lines, very analogously to how waves travel along strings.

IRA FLATOW: Wow. And so your research has been trying to connect these waves to the electrons that give us the most distinct, the dramatic auroras?

JIM SCHROEDER: That’s right. We have all sorts of survey data and case studies from satellites and rockets that have shown us for decades that alfvén waves are really common above auroras. And they’re actually really common above what are called discrete auroras– these really bright bands of light across the sky. The sort of iconic auroras that you think of when you picture an aurora in your mind is a discrete aurora.

And so we know that alfvén waves are common above those auroras. And actually, as the auroras get to be more active and there’s more auroras in the sky, there’s more alfvén waves present during those times. And so there’s this really suggestive correlation. But we’ve never actually been able to say definitively if the alfvén waves are a part of causing the auroras, or if simply the alfvén waves happen alongside them.

And so our goal was to try to do a really detailed study and create alfvén waves in a laboratory with conditions relevant to where auroras are produced, and monitor those alfvén waves to see if they can give their energy to electrons. As I was saying, electrons need a boost in energy in order to actually produce auroras.

IRA FLATOW: And what did you find in your lab? Drum roll, please.

JIM SCHROEDER: That’s right, drum roll, please. We found alfvén waves do transfer energy to electrons in conditions relevant to where auroras are produced. And so that means we have a definitive test showing that alfvén waves can participate in the production of auroras.

And actually, the way that this process would unfold is pretty striking. If you picture surfing– I’ve actually never been surfing, but what I’m told–

IRA FLATOW: Me neither, so we’re on the same page.

JIM SCHROEDER: –OK, perfect. What I’m told is that you have to paddle up to the right speed. And if you watch surfing videos, this is what you see. You don’t see the surfers just sitting out there in the middle of the ocean. And so once you’re at the right speed, then you can be picked up by an ocean wave.

And what we found in the plasma, in our laboratory experiments is something similar– that electrons have to be going at the right speed in order to be picked up by the alfvén waves. So they’re surfing on alfvén waves. So next time you see an aurora, you could think about electrons surfing on alfvén waves out in space.

IRA FLATOW: Now that this, how does it go from the lab to space?

JIM SCHROEDER: That’s right. Of course, we would love to do a demonstration, actually an in-situ measurement of this process, right? You notice I said alfvén waves can cause auroras in conditions relevant to where auroras are formed.

What we’d like to do next is actually go out into space and perform a measurement that they would definitively show this happening out in-situ. That’s been a really tricky task. And that’s actually initially why we turned to the laboratory. But our ability to take space-based measurements is always improving. That door is not closed.

IRA FLATOW: OK, well, we’ll wait for that to happen. And all these surfing electrons relate to these sharp distinct auroras. Now, what about when auroras are very blurry looking? Do we have no surfing electrons there?

JIM SCHROEDER: That’s right. So when auroras are more blurry looking, those are called diffuse auroras. And it’s believed the source of those electrons– what’s actually pumping up the energy of those electrons– is something different in that case.

Earth has this band of energetic particles around it called the radiation belts. Actually, there’s a couple of belts around the Earth, and some of them come and go. So there’s some belts that are there and then and then they vanish.

But it’s believed that diffuse auroras are produced by particles leaking out of that energetic belt of particles around the Earth. And so there would be some trapped particles that make their way down to the poles from those radiation belts. And so that would not be electrons surfing on alfvén waves. That would be some other scattering process.

IRA FLATOW: That’s terrific. So that’s it. You solved the 40-year mystery.

JIM SCHROEDER: That’s right, although I have to say there are a lot of open questions about auroras. There’s auroras at Earth. There are auroras elsewhere. This isn’t to be interpreted as meaning that we’ve solved everything there is to know about auroras, but we have answered a long-standing question.

IRA FLATOW: So if you’re looking out at an aurora– I’m outside in the cold. I’m noticing its color and its shape. What are all the things I can tell about it just from the naked eye?

JIM SCHROEDER: Yeah, that’s a great question. So as you’re looking at an aurora, the first thing you could do is look to see if it’s a discrete aurora or a diffuse aurora; if it has well-defined bands of light or if it’s just kind of hazy. That would give you a hint about if the electrons were being pushed down towards Earth by alfvén waves, by surfing, or if they were leaking out of the radiation belts instead.

Another thing you might notice is actually the different colors of auroras. And so auroras are often green, but sometimes they’re reddish and purplish. And that would indicate actually where the light is being given off at different altitudes.

IRA FLATOW: Is that right? Well, it’s like a rainbow then.

JIM SCHROEDER: Yeah. So the atomic transitions that make up different colors require different amounts of time in order to occur. And so if an atom getting ready to have an atomic transition and give off, let’s say, purple light, if it bumps into something else in the meantime before that transition can occur, then the energy is stolen away, and it won’t actually produce its little flash of light.

And so as you go up into the atmosphere, things are more spread out. The density goes down. And that allows more time for the transitions to happen, and we can see different colors.

IRA FLATOW: Hmm. Can the colors tell me anything about what elements, what molecules are being activated here?

JIM SCHROEDER: That’s right. So oxygen is known for giving off it kind of yellow-green light and also some red light. But the red light given off by oxygen is much, much higher in altitude because those transitions require a lot more time to occur. And then nitrogen molecules can give off kind of dark reddish light.

IRA FLATOW: We’ve been talking about the role of the sun in bringing us these aurora displays. Can understanding the sun better help us predict when we might see a lot of auroras or what the nature of them might be?

JIM SCHROEDER: Yeah. So the sun is, of course, the ultimate driver of all auroral activity. If we didn’t have the sun sending out its solar plasma, the solar wind, then we wouldn’t have auroras. Knowing something about the sun, especially the variations of that solar wind as it’s coming out, helps us to predict when there might be more or less auroras.

And so the sun goes through an 11-year cycle where it’s more or less active, and the solar wind is more or less variable as it’s streaming outward. And that is a good indication of a sort of day-to-day basis when you’re likely to see more auroras or less auroras. But there are a lot of mysteries that remain.

Like, why does the sun have its 11-year cycle? And why do certain things that we can see on the sun actually correlate to the variability of the solar wind that’s streaming past the Earth?

IRA FLATOW: Really cool. This is Science Friday from WNYC Studios. In case you’re just joining us, we’re talking with Jim Schroeder, assistant professor of physics at Wheaton College in Wheaton, Illinois.

Everything you’ve ever wanted to know about auroras. And Jim, what do you not know that you would like to know? I’m going to give you the blank check question, which I give a lot of scientists. If you could build a device or study something more and it would cover that cost, what would you like to know? How would you do that?

JIM SCHROEDER: Oh, I would love to have a fleets of satellites at every planet where we know auroras exist so that we can get really complete data about what’s creating those auroras in different scenarios. Like, we see auroras at Jupiter that look much different from what we see here at Earth. And there’s also evidence of ultraviolet auroras appearing at Mars.

IRA FLATOW: No kidding.

JIM SCHROEDER: And so I would love to know more about that.

IRA FLATOW: Ultraviolet. Auroras at Mars. So if there are ultraviolet, and you were on Mars looking up, you might not see them because we can’t see ultraviolet natural light.

JIM SCHROEDER: That’s right, you need an ultraviolet camera. And that’s, in fact, how they have been seen with a UV camera on the Maven Spacecraft.

IRA FLATOW: Do these electrons, as they’re traveling up and down, do they create any sounds as they travel.

JIM SCHROEDER: So the electrons don’t create sounds themselves, but they do have vibrations that map directly to the audible spectrum– so sounds that we can hear. So if you take the vibrations of electrons traveling around the Earth, and you just transform that into an audio file, you can hear it.

And there are some great videos out there on YouTube of these types of noises– what are called electron whistler waves. And it’s this really kind of eerie noise where it’s a signal that is transformed into an eerie noise of kind of chirping– the frequency going up and the frequency coming down. And it has to do with waves and plasma actually being separated by different frequencies.

IRA FLATOW: And I know there are a lot of ham radio enthusiasts that try to listen to these waves.

JIM SCHROEDER: That’s right. So they’re picking up those vibrations and then translating them into an audio signal using their receiver.

IRA FLATOW: Are there any other phenomena in the universe that this research into auroras might help us better understand stuff?

JIM SCHROEDER: Yeah. So I often get kind of the so what question. Why do we care about how electrons gain the energy that’s needed to produce auroras? And so close to home, the reason why we care about something like this is because we’re more dependent than we’ve ever been on the space around Earth, what’s called geospace.

We have all sorts of assets and satellites out there that help us to communicate and navigate and monitor the Earth. And so we care about the dynamics of what’s happening around the Earth. But then in terms of just kind of pure science questions– what’s left out there that we don’t understand– there’s all sorts of energetic particles out in the universe.

So if you look even just a little bit further out from the auroras, there’s the radiation belts and the energetic particles of the radiation belts. We don’t really understand how those particles get to be so energetic. And we’d love to know that, again, because we depend on geospace.

IRA FLATOW: You know, I heard this year earlier in the year– as I say, I have never managed to see an aurora. But I heard that the aurora was moving up and down the hemisphere a little bit for certain reasons. Why is that? Why do we see that– hey, some people a little further south might be able to see the aurora at this time of the year.

JIM SCHROEDER: That’s right. So when there are geomagnetic storms that are more severe– like there’s a larger disturbance of Earth’s magnetic field by the variable flow of the solar wind passed the Earth– that tends to create auroras that are visible at lower latitudes. So we don’t often get to see them here in Illinois, but there’s been a couple nights this year where we’ve been warned that it might be a good time to go outside and look. And so that has to do with actual the disturbances of the magnetic field kind of penetrating deeper into Earth’s magnetic field and being visible at lower latitudes.

IRA FLATOW: Wow. Well, I’m going to keep my fingers crossed and hope we get to see one, or I get to see one, Jim. Thank you for taking time to be with us today.

JIM SCHROEDER: Absolutely. My pleasure, Ira. Happy holidays.

IRA FLATOW: Jim Schroeder, assistant professor of physics at Wheaton College in Wheaton, Illinois. But if you want to continue exploring auroras and this idea of electrons surfing on magnetic waves some more, we’ve got something special for you on our website.

It’s a fun activity you and any young person you know can try I like to think like an aurora scientist just like Jim here to predict the color and shape of Aurora in the sky. That’s on sciencefriday.com/waves. Sciencefriday.com/waves.

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