The earth’s atmosphere often puts on real “show” called the northern lights (aurora borealis) or southern lights (aurora australis). On such occasions, regions of the sky near the north or south magnetic poles glow with a light that may take on several forms: a general glow, a fanlike shape, curtains hanging in the sky, or spikes shooting upward [Fig. 3.16 (a and b)]. Such a display originates from the flow of charged particles from the sun, the solar wind. These particles interact with the earth’s magnetic field, and some are accelerated along the magnetic field lines toward the north or south magnetic poles. As they enter the earth’s atmosphere, at a height of 300 km, they bombard atoms which make up that atmosphere and excite those atoms. As you will recall from Chapter 2, downward electron transitions that stop at least temporarily at energy level 2 produce visible light. In this process, certain colors of light are more probable because of the kinds of atom present and the degree to which they are excited. For instance, oxygen molecules (O_2) will tend to produce red and/ or yellow light, individual oxygen atom will tend to produce green light, and and nitrogen atom will tend to produce violet light, each color being related to the dominant lines in the particular spectrum.
We have evidence now that when an aurora occurs at one pole, a very similarly shaped aurora occurs at the other pole as well. Electrons are thought to travel along magnetic lines from one pole to the other in less than one second, and thus a means is created by which the two poles virtually communicate with one another in forming auroras.