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What Creates the Aurora Borealis?

The Earth's "magnet" is deep in the core. Since we can't see the magnetic field, we draw lines to represent it. The field lines go into and out of the Earth around the Earth's magnetic poles. Where the lines are closest together the field is strongest. Where they are furthest apart it is weakest.

There are always electrons and positive ions in the surrounding magnetic field. Charged particles in a magnetic field move in a special way: they are guided by the field. The particles travel along magnetic field lines as if they were wires, circling around the lines in a long spiral as they go. Charged particles are the "ammunition" of an aurora.
How the aurora happens is that energetic electrically charged particles (mostly electrons) accelerate along the magnetic field lines into the upper atmosphere, where they collide with gas atoms, causing the atoms to give off light.
The particles strike the gases in the ionosphere, causing them to glow, the same way electrons passing through the gases in a neon tube make a neon sign light up. The colors correspond to the different gases in the ionosphere. Oxygen atoms give off red and green light, depending on how high they are in the ionosphere. Nitrogen molecules give off blue and violet light.

The Sun also has an atmosphere and a magnetic field that extend into space. The Sun's atmosphere is made of hydrogen, which is itself made of protons and electrons. These particles are constantly boiling off the Sun and streaming outward at very high speeds. Together, the Sun's magnetic field and particles are called the solar wind.
This wind is always pushing on the Earth's magnetic field, changing its shape. We call this compressed field around the earth the magnetosphere . The Earth's field is compressed on the day side, where the solar wind flows over it. It is also stretched into a long tail like the wake of a ship, which is called the magnetotail , and points away from the Sun.
Squeezing the earth's magnetic field takes energy. The whole process is still not fully understood, but energy from the solar wind is consistently building up in the magnetosphere, and this energy is what powers auroras.

Solar particles are always entering the tail of the magnetosphere from the solar wind and moving toward the Sun. Now and then, when conditions are right, the build-up of pressure from the solar wind creates an electric voltage between the magnetotail and the poles, like the voltage between the two terminals of a battery. It can reach about 10,000 volts!
The voltage pushes electrons toward the magnetic poles, accelerating them to high speeds. They zoom along the field lines towards the ground to the north and south, until huge numbers of electrons are pushed down into the upper layer of the atmosphere, called the ionosphere.
In the ionosphere, the speeding electrons collide aggressively with gas atoms. This gives the gas atoms energy, which causes them to release both light and more electrons. In this way, the gases of the ionosphere glow and conduct flowing electric currents into and out of the polar region. The electrons flowing back out don't have as much energy as the speedy arriving ones had - that energy went into creating the aurora!