Fritz and Eather estimated that, given clear skies, aurora could be observed about 100 nights a year in the region where it was most frequent. However, imaging cameras aboard satellites, more sensitive than the eye, observe a ring of diffuse aurora around the polar cap at most times. In magnetic coordinates--z along the dipole axis, the sun's direction in the x-z plane--the region where aurora is likely to occur forms a fixed pattern around the magnetic pole, known as the auroral oval.
That pattern is approximately circular, centred about 5 degrees night ward of the magnetic pole, and the Earth rotates beneath it. The oval also expands and contracts with magnetic activity: its typical radius equals 17 degrees of latitude. The reason the aurora is a rare sight at lower latitudes is that it only appears there when the oval is grossly expanded.
