Caltech Question of the Month: What causes the auroral lights?

Answered by Paul Wennberg, associate professor of atmospheric chemistry and environmental engineering science, Caltech.

Ions and electrons produced in the sun's atmosphere form the "solar wind." This stream of charged particles interacts with Earth's magnetic field and impacts the atmosphere in the region called the aurora oval. In the northern hemisphere, the collisions produce a wonderful light show called the aurora borealis. In the southern hemisphere the phenomenon is known as the aurora australis. The light, best observed near the fall or spring equinox, is produced when atmospheric gases that have been excited by these collisions, relax back to their normal states in a process known as fluorescence.

The color and altitude of the aurora tell us which atmospheric gases are being excited. Below 100 kilometers in altitude, nitrogen is responsible for blue and red auroral light. Between 100 and 200 kilometers, green light is produced by oxygen atoms, while above 200 kilometers, red light from oxygen dominates the auroral light.

Much more information, and beautiful photos, can be found at "The Aurora Page," http://www.geo.mtu.edu/weather/aurora/