The Exploratorium:  A museum located in San Franscisco, the Exploratorium has a site that explains the Aurora Borealis in a clear, concise manner. The Aurora Explained: On the Alaska Science site Neal Brown explains demonstrates the natural forces that cause the aurora.
    

WHAT DO WE KNOW ABOUT EARTH’S AURORAS?

“The aurora is a physical manifestation of the connection between the Sun and the Earth, a solar powered light show. The Sun is an active star with a roughly 11-year long ebb and flow of storminess and quiescence that is revealed, in part, through the aurora.  The last solar max, caused by the reversal of the Sun’s magnetic fields, peaked in 1989.  We are experiencing high auroral activity again in the year 2001. Auroral scientists believe that the aurora is a universal phenomenon, which will take place wherever a high-speed plasma flow (such as the solar wind and stellar wind) interacts with a magnetized celestial body. The Earth's own magnetic field is constantly perturbed by the impinging magnetic field from the sun called the solar wind. During strong bursts (such as those caused by extraordinary solar flares) reconnection can be induced in the near-Earth magneto tail, a narrow magnetic field structure located on the night side many Earth-radii away. The tenuous plasma in that region is then accelerated down magnetic field lines into the Polar Regions, striking Earth's atmosphere and exciting nitrogen and oxygen atoms as well as other atoms present in our atmosphere. The immediate de-excitation of these atoms then emits the wonderful and often intricate display of light we know as the aurora or northern and southern lights.  Basically, the solar wind interacts with Earth’s magnetosphere, surrounds the entire Earth, and concentrates in two ovals crowning Earth’s magnetic poles like a fire in the sky.  The resulting interaction produces the beautiful polar light displays we see from Earth. Auroras occur between altitudes of 100 and 1,000 km high and one km thick, where electron energies, atmospheric densities and chemical abundance dictate the intensity and frequency of particular colors. The most common color is green.  Red emissions at high ends of auroral displays are caused by oxygen while less frequent red emissions at the lower ends of displays are caused by nitrogen.  Overlapping red and green auroras can produce a yellow glow.  Auroras can be purple or blue and pink, often too faint to be seen by the naked eye.  Science has only understood the origin of Earth’s auroras over the past 100 years with the most complete knowledge developing during the last 50 years.  The aurora disturbs the polar ionosphere and the propagation of radio waves, causing disruption of radio communication and navigational difficulties.”