If you live in Canada, Alaska, Greenland, Iceland, Scandinavia or Northern Russia, you’re probably familiar with the Aurora Borealis, or Northern Lights. They are streaks of multi-colored light that illuminate the sky mostly at night, sometimes appearing as an all-over glow, and sometimes as discrete patterns with sharp boundaries including spirals or folds/lines that are a bit like curtains. It’s thought that the term “aurora” was first used to describe this phenomenon by the astronomer Galileo Galilei in the early 17th century. (Aurora was the Roman goddess of the dawn.) The Aurora Borealis has its counterpart in the Southern hemisphere, where it is known as the Aurora Australis.
The Northern Lights are caused by the interaction of solar wind with the earth’s ionosphere. Solar wind is a hot plasma consisting of electrons, protons and a few heavier ions, and blows continuously from the surface of the sun at a speed of about 400 km/second. When the plasma particles reach the earth, they are trapped in the earth’s magnetic field, and start to spiral back and forth along the field lines. (This interaction between the charged particles and the earth’s magnetic field is known as the magnetosphere.) The lines of the earth’s magnetic field are high above the surface of the earth at the equator, but they disappear into the surface at the poles. The points where the electrons in the magnetosphere hit the atoms present in the earth’s ionosphere are the points where the aurora is generated (more on this below). The area covered by these points is known as the auroral oval, which is shaped like a Frisbee with the center cut out; the center of the oval coincides with the geomagnetic pole. This is why it’s actually rare to observe an aurora directly over the geomagnetic poles.
The auroral oval can extend much further away from the poles during high periods of solar activity, which occur on an eleven-year cycle. These high levels of solar activity cause the magnetosphere to experience a disturbance, or geomagnetic storm. During such periods the Aurora Borealis can be seen at latitudes as far south as Florida.
Why the different colors? The electrons that form part of the solar wind collide with nitrogen and oxygen high in the earth’s ionosphere, between 90 and 250 km above the earth’s surface. At this height, most of the nitrogen and oxygen is in atomic rather than molecular form. This is because the sun’s electromagnetic radiation (not to be confused with the solar wind) breaks the molecules apart. At this height, they are so thinly distributed that they stay in atomic form for long enough to be “excited” when hit by the electrons of the solar wind. The excitation is caused by electrons in the nitrogen or oxygen atoms jumping up to a higher orbit. Eventually they “decay”, i.e. they go back to their original orbit. When they do this, they emit light in the visible spectrum. The color of this light varies depending on whether the atom is oxygen or nitrogen, and on the initial level of excitation. For example, two commonly-excited states of atomic oxygen result in the production of green and red light. The red light is produced at a height of 200 km or more above the earth’s surface, while green light can be produced at lower altitudes. Reddish-purple and blue are other colors that can also be seen.