Curtains in the Sky

A Wind From the Sun

The Sun constantly flings out charged particles, a stream called the solar wind. The Earth’s magnetic field acts like an invisible shield and keeps most of them out.

But on the side facing away from the Sun, the field is drawn out into a long tail. Energy builds up there and speeds some electrons back toward the Earth along the field lines.

Earth and its magnetic field, schematic · not to scale

Only in Two Rings

Electrons slide easily along magnetic field lines but can hardly cross them. The lines that come back from the tail plunge into the atmosphere near the two magnetic poles, so the aurora appears in two oval rings around them, roughly between 60 and 75 degrees magnetic latitude.

In the north it is the aurora borealis, in the south the aurora australis, and the two usually glow at the same time, like near mirror images.

A Valley Under the Oval

This is Abisko, in Swedish Lapland, from satellite elevation data: Lake Torneträsk frozen for the winter, birch woods along its southern shore and, to the south-east, the U-shaped notch of Lapporten between two mountains. Abisko lies at 68.4° north, about 200 kilometers beyond the Arctic Circle.

Measured from the magnetic pole rather than the geographic one, Abisko sits at about 66°, inside the band from 60° to 75° where the ovals lie. Ringed by mountains, it is also in a rain shadow: from 1991 to 2020 it averaged about 350 millimeters of rain and snow a year, the least of any Swedish weather station, so its winter nights are often clear.

Abisko, Sweden · 68.35°N 18.82°E · heights × 2 · trees not to scale · the aurora is drawn far lower than its real 100 km

Curtains a Hundred Kilometers High

The electrons strike oxygen atoms and nitrogen molecules high in the air and hand over their energy, which the atoms and molecules give off again as light. Aurora usually glows 80 to 500 kilometers up — far above any airplane.

Because the electrons fall along field lines, the aurora often hangs in tall curtains that ripple and fold as the magnetic field shifts.

Auroral curtains · color changes with height

Green, Red and Pink

The color depends on which gas is struck, and how high. Between about 100 and 200 kilometers, oxygen gives off the familiar green; above about 200 kilometers, thin oxygen glows red.

Lower down, nitrogen trims the bottom of the curtain with pink. Mixed together, the colors can look purple, pink or even white.

When the Sun Is Busy

The harder the Sun’s bursts of particles strike, the more the magnetic field is disturbed, and the further the rings spread toward the equator.

Aurora is usually seen only at high latitudes, near the Arctic or Antarctic. In the most extreme geomagnetic storms it reaches much farther: in May 2024 it was seen across much of the United States and Europe, and in parts of Asia.

The Aurora Right Now

The aurora on this globe is not drawn by hand. It comes from the forecast model of NOAA’s Space Weather Prediction Center, which shows where the aurora will be, and how bright, 30 to 90 minutes ahead.

The brighter the glow, the better your chance of seeing an aurora there in the next hour or so. The forecast’s date and time are shown at top right.

Aurora forecast: NOAA Space Weather Prediction Center, OVATION model