[00:00.00] W: For centuries, people have told stories to explain the moving lights in the night sky—the curtains of greenish-white light with pink fringe. [00:10.27]People described these lights as the breath of the Earth, powerful spirits, or angel light. [00:17.37] An early twentieth- century explorer wrote about the “bloody red” and “ghostly green” lights. [00:23.39]These lights, of course, are the aurora borealis—the northern lights—and, in the south, the aurora australis. [00:32.57]Most of the time they’re greenish-yellow, but sometimes they take colors from violet to red. [00:39.77]The auroras can be seen at any time of the year, with the right atmospheric conditions. [00:45.27] They’re most often seen near the North and South Poles, during times of maximum solar activity. [00:52.27]The closer to the North or South Pole you are, the better you can see the lights.
[00:57.57]The auroras occur in the ionosphere.[01:00.87]The ionosphere is the layer of the upper atmosphere where high- energy solar radiation strips electrons from oxygen and nitrogen atoms, and leaves them as positively charged ions. [01:14.27]The auroras are the result of a complex interaction between the solar wind and the Earth’s magnetic field. [01:23.27]Here’s what happens. [01:24.27]The sun’s heat charges the particles in the solar wind, a stream of electrically charged subatomic particles that continually emanates from the sun. [01:35.57]As the solar wind approaches Earth, it’s deflected by Earth’s magnetic field and diverted north and south toward the magnetic poles. [01:45.27]The interaction between the solar wind and the magnetosphere generates beams of electrons. [01:52.77]These electrons collide with atoms and molecules within the ionosphere near Earth’s magnetic poles. [02:00.27]The collisions rip apart molecules and excite atoms. [02:04.77]Thus, oxygen and nitrogen atoms in the ionosphere become “excited,” or ionized. [02:11.27] The auroras happen when these ionized atoms return to their normal state from their excited, energized states. [02:18.67]The ions combine with free electrons—as they do so, they emit radiation. [02:24.77]Part of this radiation is visible light: the aurora borealis and aurora australis.Yes, Simon?
[02:33.27]M: Uh ... it sounds kind of like electricity.
[02:36.77]W: Yes, that’s right. The auroras are an electricalphenomenon. [02:41.27]As you know, an electrical generator has two components: a conductor and a magnetic field. [02:47.57]To generate electricity, the conductor has to move across the field to produce a force. [02:53.47]With the auroras, the conductor is the solar wind carrying a stream of charged particles.
[02:59.27]M: So, what happens is, when, uh, when the charged particles reach Earth’s magnetic field, they, uh, move along in the field towards the north and south magnetic poles.
[03:10.67]W: Exactly. And then the particles collide with gases in the atmosphere—oxygen and nitrogen—and the oxygen and nitrogen atoms get excited. [03:21.17]And then, when the particles get de-excited and return to their normal state, they emit the auroras by releasing energy in the form of light.[03:30.67]Oxygen releases either dark red or ghostly green. [03:34.87]Nitrogen emits rosy pink or magenta. [03:38.67]The activity of the auroras varies with the sun’s activity. [03:42.97]When the sun is quiet, the auroras can be seen only in a small area.[03:47.67]When the sun is active, however, the aurora borealis can be seen across southern Canada and the northern United States.
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