Intense auroral sub-storm, with aurora over the Great Lakes. Image from the POLAR VIS instrument.
When the solar wind is calm, the aurora might only be at high latitudes and might be faint, but there is still aurora. In order to see aurora, however, the sky must be dark and clear. Sunlight and clouds are the biggest obstacle to auroral observations. If you have a camera on a satellite you can look down on the aurora, and you'll find an oval shaped ring of brightness crowning Earth at all times. When the solar wind is perturbed from a recent flare or other event on the sun, we might get very strong aurora.
After the solar wind has transferred a lot of energy into the magnetosphere, a sudden release of this built-up tension can cause an explosive auroral display. These large events are called sub-storms. A sub-storm usually starts with a slow expansion of the auroral oval followed by a sudden brightening of a small spot, called the auroral breakup. This spot usually is near that place of the auroral oval that is on the opposite side of the sun, which means near the place where midnight is. This brightening rapidly grows until the entire auroral oval is affected. An observer on the ground where this breakup occurs will see a sudden brightening of the aurora which may fill almost the entire sky within tens of seconds. This aurora will be in the shape of rapidly moving curtains.
