Nature’s Own Holiday Light Show

IRA FLATOW: This is Science Friday. I’m Ira Flatow, wishing a Merry Christmas to those of you celebrating. The winter holidays are a time for colorful lights. And if you live in the northern latitudes, you may have been treated to nature’s own light show. I’m talking about the Aurora Borealis. Never having seen the northern lights myself, perhaps now I’ll get to see the glowing green in the sky, normally found closer to the poles. They’ve been spotted as far south as Michigan and Ohio. Sci Fri’s Charles Bergquist has more.

CHARLES BERGQUIST: The Aurora forms when streams of charged particles from the solar wind interact with gases in our upper atmosphere. It works sort of like a neon sign. The charged particles excite the gas, making it emit light. Don Hampton is a Research Associate Professor in the Geophysical Institute of the University of Alaska in Fairbanks. He studies the Aurora, including by firing rockets up into it. Welcome to Science Friday.

DON HAMPTON: Good morning.

CHARLES BERGQUIST: So people say, charged particles hitting gases? What sort of particles are we talking about, and what sort of gases?

DON HAMPTON: The solar wind is primarily electrons and protons, because the sun is mostly just hydrogen. And so when you break those apart, that’s what you get. Those are sort of captured in our Earth’s magnetic field. And so the particles that create the light that we see typically with the Aurora are primarily electrons that are accelerated down by electric fields in the magnetosphere, come down, and as you say, bump into the upper atmospheric gas molecules, and atoms and create the light.

CHARLES BERGQUIST: And generally, it’s a sort of greenish glow. But there are different colors, too. What makes the difference in colors? Is it a difference in the particles, or a difference in the gases that it’s hitting?

DON HAMPTON: It’s a difference of the gases that it’s emitting. So as you mentioned, it’s somewhat similar to a neon sign. Basically, that’s the same sort of process that happens in a neon lamp. You basically drive electrons across a gas in a tube like that. And the different gases have different configurations of the electrons in the orbitals and the outer shell. And those differences create the different colors, because the jump between the different shells of a certain energy. And that’s a certain color of light.

CHARLES BERGQUIST: I would think that if it were like a mix of gases, you’d be getting a whole bunch of different colors, all coming at once?

DON HAMPTON: Well, it turns out the different colors are more efficient or less efficient. And so the green one is very efficient. It’s easy one to produce in atomic oxygen. And that’s different oxygen that we have down here. Our oxygen we breathe is O2, which is molecular. But because the sun, ultraviolet at the upper atmospheres, can break across the oxygen, you get a lot of atomic oxygen at these altitudes, about 110 kilometers and higher.

And so it’s very efficient to create that green. So it’s the single sort of brightest emission in the visible that we can see all the time. So yeah, if you take a picture, that’s primarily what you’re going to see. But if you look carefully, you’ll see that there are some other colors in there. And especially if you take like a spectrograph, like basically a prism, and break it down into its individual colors, you’ll see the whole spectrum in there. And you get not only the oxygen– and there are a couple of different oxygen emissions as well. But because there’s a lot of nitrogen up there, you often get nitrogen emissions. If you see that sort of pinkish, reddish lower border, that’s actually nitrogen emissions, combination of some neutral and some ionized nitrogen.

And then there’s some room for more exotic things. There’s sometimes– there’s atomic nitrogen. And then there’s some O-plus emissions, atomic oxygen and ion emissions, and that sort of thing. So yeah, if you take a spectrograph, you can actually see several colors in there. But green is just the most prominent one, because it’s the most efficient one to produce.

CHARLES BERGQUIST: And so if we do see a patch that’s predominantly, say the pinkish color, does that mean oh, it’s more nitrogen rich than somewhere else? Or just that is what happened to get stimulated at that moment?

DON HAMPTON: Well, that’s a great question. Because the ratio of these different emissions actually tell us the energetics of those particles coming down. Because the atmosphere’s somewhat layered, the oxygen is predominant at higher altitudes, and the nitrogen becomes more predominant, like it is down here on the troposphere. At lower altitudes, that means if we see more nitrogen emissions, that means probably, those particles are more energetic. In fact, we’ve got sort of a recipe for being able to figure that out.

So if we can take images, or use a spectrograph to look at the ratio of the colors, we can actually sort of tell from the ground what the energy of this particles is coming down, to at least to rough order.

CHARLES BERGQUIST: Why does it look sort of wispy, like clouds? It’s not an overall glowing background. There’s lines and streaks and shapes.

DON HAMPTON: That’s right. There’s a lot of morphology. And that’s kind of the $64 question is, why do we get the morphology and the dynamics that we’re getting? And it has a lot to do with processes that happen the magnetosphere. The region that will accelerate the particles to come down and create the Aurora are sort of localized, based on the sort of shape of the magnetosphere.

CHARLES BERGQUIST: Are changes to that shape why sometimes people are able to see it far south of the poles?

DON HAMPTON: That’s right. So the solar wind not only has these charged particles, but it also brings along with it a magnetic field. Because they’re charged particles, they’ve got currents flowing around. You get magnetic fields. And then you also, because it’s basically a gas as well, it has pressure. And that pressure bumps into our magnetic field of the Earth. This whole region is called the magnetosphere. And it’ll actually make that magnetosphere ring, or it’ll actually compress it or turn it more into a teardrop shape.

And as those processes happen, that changes the configuration of the magnetosphere. And so that will change where the Aurora’s happening in one place. And it’ll also sort of change the actual specific arcs and patches that you see in the Aurora.

CHARLES BERGQUIST: So in recent weeks, when we’ve been seeing reports of it as far south as Michigan, Ohio, what does that tell us about the current state of what’s going on in space?

DON HAMPTON: That means that the solar wind has gotten much more energetic. So the latitude of the Aurora that you see as a sort of direct response to that sort of total energy in the solar wind at that time. So the sun goes through an 11 year cycle. And during sort of the solar minimum, we call it, there are fewer sunspots, and it seems to be more sort of stable. The solar wind comes out at sort of a standard speed. And you get some Aurora, but you don’t get quite as big of the storms.

During solar maximums, we get all these sunspots. They can sort of– instead of having the solar wind come out sort of at a regular speed, sometimes these sunspots will sort of produce these bubbles of charged particles that sort of build up. And then they come out as a big burst. And that’s called a coronal mass ejection. When that happens, you get a much denser, and maybe a faster stream of particles in the solar wind.

And when that happens and happens to bump into Earth’s magnetic field, that’s when you get these very large storms. And all that energy goes into sort of expanding that [INAUDIBLE], and making a much more impactful storm on Earth’s upper atmosphere.

CHARLES BERGQUIST: It seems to me like it would take a lot of energy to get the sky, glowing so to speak. Is there any kind of analogy that we can use for human scale energy, that we would know how much energy that is?

DON HAMPTON: Sure. If you go and look at the NOAA website, there’s a space environment center there. And they actually have a sort of a real time estimate of how much energy is being dumped in the polar regions. And it’s in terms of gigawatts. And that sounds like a lot. But you have to remember, that’s gigawatts over an area the size of the pole, which is a very large area. So the power density is pretty low. But it still is a lot of energy going on.

CHARLES BERGQUIST: Is it possible to make an artificial Aurora? If I put up a powerful enough radio transmitter or dropped a microwave oven out of the back of an airplane, could I achieve this?

DON HAMPTON: Yeah, you could do it a couple of ways. There have been– some of the sounding rocket experiments have actually put on board, basically a particle accelerator, or a way to produce these high-energy electrons, and then looked with sensitive cameras to see if they could see the particles, both on one side or the other side of the magnetic field, and we’re successful in doing that.

A couple other ways you can do that– there are some ionospheric heating projects, where they take, basically high powered radios on the ground, and try to sort of match a resonant frequency in the atmosphere. When you do that, you get enough energy in the electrons locally, they can sort of bump into the atmosphere constituents, sort of like an Aurora, and produce some glow as well.

We also do sort of chemical tracer experiments sometimes. And that’s to look at some of the electric fields, and also some of the motion of the neutral particles, which is kind of hard to do from the ground. That’s typically a chemical called trimethyl aluminum, or sometimes things called barium and strontium, which sort of have the resonant emissions in the sun. And you put those up and watch how they move around. And that gives you an indication of what’s going on in the atmosphere.

CHARLES BERGQUIST: What other things are you interested in learning through the rocket experiments, or other parts of your work, about the Aurora?

DON HAMPTON: There’s the basic science question. If you took anything, or if you took any point in our solar system, you would basically drop into a thing that’s called plasma, which is basically a gas with a significant number of charged particles. It’s very few places in the solar system that are not plasma. And that one place is basically the surface of the Earth. So most of the universe, in fact, is actually plasma.

So what we’re trying to do with Aurora research is, it’s a cheap way to be able to observe plasma in its sort of natural environment. So there’s sort of the basic research question about that as well. But also, these large solar storms we talked about earlier, they produce a layer in our upper atmosphere of variation in the ionization in the ionosphere that can really wreak havoc if you’re trying to communicate with satellites, or you’re trying to figure out your position with a GPS receiver. Because those changes in the density of those charged particles changes the radio transmission through that medium.

CHARLES BERGQUIST: So here on Earth, we’ve got a certain mix of gases in the atmosphere, and a certain shape to our magnetic field. Do other planets have this? If I were standing on Venus or Mars, would I be able to see something like an Aurora?

DON HAMPTON: Absolutely. Well, Jupiter and Saturn have Aurora quite often. In fact, some of my colleagues here at the GI are studying the effects of the solar wind on Jupiter, and how it causes the Aurora as well. So what you need to create an aura, basically, is a magnetic field. Because you have to have the magnetic field to sort of capture and then redirect those particles, sort of accelerate them down into your atmosphere. And then you have to have an atmosphere.

So a planet like Mercury has neither– doesn’t have a very strong magnetic field, and has really no atmosphere. So you’d have to have a really sensitive camera to see the Aurora there. Plus, you’re right next to the sun. And Mars, they’ve seen some evidence of some accelerated particles hitting the upper atmosphere and causing what looks like an Aurora. But again, there’s a very, weak field there.

But no, so if you’ve got a strong magnetic field in an atmosphere, you’re probably going to see Aurora, if you’ve got a solar wind going by.

CHARLES BERGQUIST: Interesting. So on a practical level, you see these amazing pictures of the northern lights. But my night sky pictures never look anything near as good as the pros. Are there any tips for snapping a photo of the Aurora, should you be lucky enough to see one?

DON HAMPTON: Sure. So the best thing is a stable platform. I mean, most of us, that would be a tripod. But if even if you could just sort of clean it up against a wall or on top of your car, or something like that– and then you need to put your camera into manual mode and be able set two to four second kind of time exposure. And then it just comes down to artistry.

Just find the nice thing in the front and give it a try. So my rough rule of thumb for taking pictures is, if you have your ISO– you know what that is. That’s the sensitivity of the camera– and your exposure time in seconds, if you multiply those together, if you come up with about 10,000, that’s a good starting point. Then you can sort of adjust from there.

CHARLES BERGQUIST: Don Hampton is a Research Associate Professor in the Geophysical Institute of the University of Alaska in Fairbanks. Thanks for being with me today.

DON HAMPTON: Absolutely. You’re quite welcome.

CHARLES BERGQUIST: You can see a video of his research in action up on our website at sciencefriday.com/aurora. For Science Friday. I’m Charles Bergquist.

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