Why don't auroras happen near the equator?

The above picture, taken with the DES satellite on March 13, 1989 during a Great Aurora, shows that some aurora can be seen very far south. The southern edge of this auroral oval extended to the Great Lakes and could be seen almost directly over head. Further south, in Florida, observers saw a bright red glow in the northern horizon, but close to the horizon.

Aurora are commonly seen only at latitudes near 60 degrees, however, a few rare aurora have been seen near the equator, like the 1909 storm seen in Japan. The reason they don't happen in the equatorial regions is that the flows of energetic electrons and protons that trigger aurora travel along magnetic field lines that connect the distant geomagnetic tail region with the Earth's surface field. These field lines reach the Earth only in the polar cap areas. In the equatorial zone, the only field lines there connect the two poles via magnetic field lines that are much closer to the Earth and do not each out into the geotail. Some aurora, during exceptional geomagnetic and solar storms, are seen in the equatorial zone, but not very close to the zenith. You still have to look directly north or south to see the auroral glow, so they are still a product of geotail 'field aligned' current flows, although over a greatly expanded range of magnetic field lines.

Return to Dr. Odenwald's FAQ page at the Astronomy Cafe Blog.