Aurora lights happen when electrons carried by the solar wind are steered by Earth’s magnetic field into the upper atmosphere, where they hit oxygen and nitrogen between roughly 80 and 500 km up and make those gases glow.

The color you see depends on which gas gets hit and how high the collision happens: oxygen produces green and red, nitrogen produces blue, pink and purple.

That is the whole mechanism in two sentences. The rest of this page follows the chain link by link, as NOAA‘s Space Weather Prediction Center and NASA describe it, and ends with the Kp index so the forecast numbers finally mean something.

From the Sun to the poles: solar wind and the magnetosphere

The Sun constantly sheds a thin plasma of protons and electrons called the solar wind. Near Earth it usually moves at about 400 km per second, while streams pouring out of coronal holes run faster, between 500 and 800 km per second.

Speed alone does not light the sky. The solar wind carries a piece of the Sun’s magnetic field, and when that field points south, opposite to Earth’s own, energy pours into our magnetosphere far more efficiently. NOAA names that southward field as the most important ingredient for a geomagnetic storm.

The biggest storms ride in on coronal mass ejections, clouds of around a billion tons of plasma that typically take several days to reach Earth. The fastest on record arrived in about 18 hours.

Earth’s magnetic field carves out a protected bubble called the magnetosphere. The solar wind squashes it on the dayside to about 10 Earth radii and stretches it into a tail on the nightside that reaches hundreds of Earth radii, well past the Moon at 60.

Electrons are accelerated in that nightside tail and then follow field lines down toward the poles. Because the lines converge there, the glow forms two rings, one around each magnetic pole, roughly between 60 and 75 degrees latitude. That is why Alaska, northern Canada and Scandinavia see the lights on an ordinary night.

At altitude the electrons collide with oxygen atoms and nitrogen molecules, kicking them into excited energy states. As each one relaxes it releases a photon of a specific color. NOAA compares the process to a neon sign: a gas, an energy input, and a color fixed by the gas itself.

Video: The Aurora Chasers

Why green dominates and red sits on top

Green is the color nearly everyone sees first, and it comes from oxygen excited around 100 to 200 km up, brightest near 100 km. Oxygen is plentiful at that height and the green transition is a fast one, so the glow is strong and sharp-edged.

Red also comes from oxygen, but from an energy level that only shows itself above about 200 km. Up there the air is so thin that an excited atom can wait a long time before emitting without another molecule bumping it first.

Lower down, that collision would steal the energy before a red photon ever appeared.

That is why red appears as a fringe along the top of green curtains, and why a sky turning deep red from horizon to horizon signals a major storm pushing electrons high and far south.

Source: NASA, Auroras and NOAA SWPC, Aurora phenomena page, both opened October 2026.

Where purple, pink and blue come from

Nitrogen handles the rest of the palette. Between 100 and 200 km, excited nitrogen glows blue, though the blue is faint and usually drowned by the surrounding green. Below 100 km, where only the most energetic electrons reach, nitrogen gives off a pink to reddish-purple light.

That pink rim hugs the bottom of fast, rippling curtains because only a hard-driving burst of electrons punches that deep. When red oxygen above and pink nitrogen below overlap with green between them, your eye blends the three and reads purple or even white.

Cameras change this picture. A long exposure collects far more red and blue than your retina does at night, so a display that looked pale green to you can come out magenta on the camera screen. The color is real, but the photo is not showing what a bystander saw.

Why the lights move, pulse and fade in one night

NOAA describes a typical active night as a sequence. In the evening, tall rays form quiet arcs from horizon to horizon. Near midnight the arcs begin to twist and sway, then can suddenly expand to fill the sky and brighten sharply. That peak is an auroral substorm, a release of energy stored in the magnetotail.

In the early morning the forms turn cloud-like and patchy, blinking on and off for hours until dawn washes them out. If you went home at 11 pm because nothing was happening, you may have left an hour before the show.

The same field physics shapes other worlds. Saturn’s magnetic field feeds its own polar auroras, part of the context in our look at whether Saturn’s rings are disappearing, and bodies too small to hold a strong field are one thread in what makes a dwarf planet.

The Kp index in plain words

Kp is a single number from 0 to 9 that says how disturbed Earth’s magnetic field has been over the last three hours. NOAA derives it from magnetometers at 13 observatories between 44 and 60 degrees geomagnetic latitude.

The K comes from the German Kennziffer, meaning characteristic digit, and the index dates to Julius Bartels in 1938.

For a watcher, Kp is a ruler for how far south the auroral oval has slid. At Kp 0 the equatorward edge sits near 66 degrees geomagnetic latitude. Each step up moves it roughly 2 degrees toward the equator, so Kp 9 puts the edge near 48 degrees.

A reading of 5 or higher counts as a geomagnetic storm and maps onto NOAA’s G scale. The table pairs each level with where NOAA records aurora having been seen.

Source: NOAA SWPC, NOAA Space Weather Scales and Tips on Viewing the Aurora, opened October 2026. Latitude limits are averages in geomagnetic, not geographic, coordinates.

The ladder is an average, and NOAA says so plainly. There are nights where Kp 6 delivers nothing to Michigan, and nights where a Kp 4 oval sits bright on the northern horizon because a substorm hit at the right moment.

NOAA’s short-term aurora forecast runs on the OVATION model, which reads the solar wind at the L1 point about 1.6 million km upstream. That gives a lead time NOAA puts at 30 to 90 minutes, the travel time for the rest of the trip.

Anything further out is a guess about whether a storm will arrive at all.

Two more things kill a sighting that no physics can fix. A full moon washes out the apparent brightness, and at the high latitudes where aurora is most frequent, summer nights never get dark enough to see it.

If you are planning an actual night out around the Great Lakes, our Michigan northern lights viewing guide covers the dark-sky spots and timing.

Aurora lights questions readers ask

What causes aurora lights in the simplest terms?

Electrons from the solar wind are guided by Earth’s magnetic field into the upper atmosphere, where they collide with oxygen and nitrogen. Those gases absorb the energy and release it as light, the same way a neon tube glows when current runs through its gas.

Why is the aurora usually green?

Green comes from oxygen atoms excited at around 100 to 200 km, the altitude where most electrons deposit their energy and where oxygen is abundant. Red needs oxygen above 200 km and a storm strong enough to light that thin layer, so it appears less often and mostly as a fringe.

What Kp index do you need to see the aurora?

There is no single threshold. Under the oval, at 60 to 75 degrees latitude, Kp 3 or 4 gives a good display. For the northern edge of the United States NOAA puts the realistic range at Kp 6 to 7, and Kp 8 to 9 can bring the lights overhead across the northern states.