Listen to part of a lecture in an Astronomy class.

(male professor) OK, moving on. Let me ask an open question. Have any of you ever seen an aurora? Um, maybe you’ve heard it called by its other name the northern lights? Carolyn.

(female student) Yeah, when I was little, it’s the light in the sky, right? I was in Alaska on vacation.

(professor) OK, good. It’s in the very high latitudes, places like Alaska, which is near the North Pole of Earth or areas

near the South Pole, where you’d be able to, uh, see an aurora best. So, what did you think?

(female student) Oh, um, I don’t remember that well. I think it was like just this big stripe of light in the sky, sort of,um, like a big greenish band type of thing.

(professor) A band, huh? Uh, sort of like this?

(female student) Yeah, I think so.

(professor) OK, great. Yeah, looks like a band, or curtain, and the greenish color is quite common, but you’d see other shapes and colors, too, for example, arcs, um, or a red color, and you’d be able to see them when the sky’s clear and dark and, like I said, you’d need to be in higher latitudes. Mat.

(male student) I have to say this is the first I’ve ever heard of something like this. Is it caused by, like, light pollution?Like too much light shining up from some human source or something?

(professor) Like I said, you need a dark and clear sky to experience this, and light pollution, why don’t we take a closer look at these auroras.

(male student) All right.

(professor) Um, let’s start with solar activity, and its interactions with Earth’s magnetic field. See, the Sun, as you know, on the Sun you have constant activity, like explosions and plasma, the Sun’s matter just sort of seething all the time. The explosions from the Sun, they blow tons of superhot, electrically charged particles out into space. These particles break off from the Sun and blow away in all directions. This movement of charged particles is called solar wind.

(male student) I see. So, you’re saying the solar wind’s the cause, but then, um, if the solar wind is blowing from the Sun at ever which way, how come you could only see an aurora in polar regions, or near polar regions?

(professor) Think of Earth as one big giant magnet. So, Earth is surrounded by a magnetic field and we know that every magnet has a north and a south pole, and things get oriented toward those poles. Earth is no exception, right?

(male student) Right.

(professor) OK. Keep in mind that solar wind is electrically charged, which means that it interacts with the magnetic field as electromagnetic theory tells us. OK. How does it interact? Earth’s magnetic field deflects most of the solar wind, makes it flow around Earth without getting very close and that’s actually quite important because these highenergy particles could be dangerous for living organisms like us if they ever reached Earth’s surface. Some of the charged particles derived from solar wind, however, do get closer, but those are channeled by the magnetic field toward Earth’s poles or polar regions. That’s where they hit the upper atmosphere and create light, the aurora.

(female student) Hum, how do the particles create light?

(professor) Good. Let’s talk about the atmosphere. It’s mostly made up of nitrogen and oxygen, right? It’s thickest closer to the Earth’s surface; gets thinner as you go up. That’s why it’s easier to breathe at lower altitudes, and we get most of our weather in the lower atmosphere, in the first ten miles or so of it. As for the aurora, its lowest edge is about sixty miles above Earth’s surface, and can go up hundreds of miles from there. OK? So, you have those charged solar wind particles hitting the upper atmosphere near the poles, right? These are high-energy electrons. And what happens is these electrons collide, way up in the atmosphere, with the molecules of the gases that are there. Mat.

(male student) You mean oxygen and nitrogen.

(professor) Yes, the gases that make up the atmosphere. So, some of the energy from the charged electrons get transferred to oxygen and nitrogen molecules, which causes the molecules to get into an excited states, which is a technical term for molecules with extra energy, but these molecules hold onto that energy just briefly, and then let it go, and the energy they let go comes out as light. So, when this happens to a lot of molecules at once, they can produce enough light that is visible to people.