Northern lights in South Dakota: tonight’s chance

East river, west river

South Dakotans divide their state at the Missouri, and the river also separates two aurora landscapes. East river is farm country with many small towns, lakes left by the glaciers and the larger population of Sioux Falls, Brookings and Watertown. The northeast corner, around Aberdeen and the glacial lakes of the Coteau des Prairies, is the best placed part of the state magnetically, at Kp 5.8. West river is ranch land, with fewer roads, fewer lights and drier air, from the breaks of the Missouri to the Black Hills.

The thresholds differ by only about 0.6 Kp across the state, much less than the difference in sky quality between a farmyard light and a remote pasture. On a storm night the decision is mostly about where the darkness and the clear air are.

The Missouri reservoirs

A string of large reservoirs follows the Missouri through the middle of the state, Lake Oahe above Pierre among them. Their shores face long distances of open water and grassland, and the country around them is very thinly settled. Looking north up a reservoir from a high bank gives the kind of flat, dark horizon that an aurora at the edge of the range requires. Pierre itself is small enough that a few minutes of driving takes you out of its light.

What a G2 night looks like here

At the threshold, the aurora over South Dakota is a pale arch close to the northern horizon, steady for long minutes and then suddenly brighter when a substorm sends rays up from it. The eye often reads it as grey or white; a phone camera shows green at the bottom and, in stronger storms, red above. Above Kp 7 the rays climb toward the zenith and the red glow can spread across the northern half of the sky. Keep watching after the first peak: substorms often come in a series an hour or two apart on a storm night.

Seasons on the plains

Summer thunderstorms and short nights limit June and July, with about 3.6 hours of full darkness on June 15 in Pierre. Autumn brings long, often clear nights before the winter cold; winter adds hours of darkness but also cloud from passing systems and dangerous wind chill. The Rapid City page covers the Black Hills side of the state and the Sioux Falls page the southeast. For lower thresholds, the North Dakota page is the neighbor to check.