One of the Earths great natural spectacles is the aurora, which may be seen visually on clear nights from locations at high northern and southern latitudes. The auroras occur in the upper atmosphere (∼100 km altitude),and they are caused by energetic particles, primarily electrons, guided into the atmosphere by the Earth's magnetic field. Hence, structure observable in the aurora is often aligned with the geomagnetic field. Figure 1 shows an active aurora, as it is often seen in Alaska. The aurora can be very active, and perhaps most spectacular is the expansive phase of the auroral substorm, in which bright and very dynamic auroras spread over many degrees of latitude on the nightside of the Earth releasing energy at rates comparable to the U.S. electric energy consumption.