NASA says that more geomagnetic storms occur in the fall and spring, and this is a great time for viewing the aurora borealis or australis (if you're lucky enough to be in an area where it's visible!)
According to the Geophysical Institute at the University of Alaska Fairbanks, an aurora is caused by the collision of electrons from space with atoms and molecules of gases (like oxygen and nitrogen) from the Earth's atmosphere. This collision results in a transfer of energy to the oxygen's electrons, and, as a result, quick bursts of light are emitted. A great number of these collisions create the light that's visible to the naked eye.
The aurora borealis, also known as the northern lights, is seen in the northern hemisphere, whereas the aurora australias, or southern lights, is visible in the southern latitudes.
Here's a gallery of some images of the aurora, from space and from terra firma. Be sure to vote for your favorite!