What the aurora is and why it has colours

1The aurora starts on the Sun, 150 million km away

The Sun does not only shine. It also leaks. A thin stream of charged particles, mostly protons and electrons, flows away from it in every direction all the time. NOAA calls it the solar wind. Near Earth the slow, everyday version moves at about 400 km per second. Faster streams from dark patches on the Sun called coronal holes run at 500 to 800 km per second, and an eruption called a coronal mass ejection can throw a dense cloud of it straight at Earth.

None of that would matter for the aurora if Earth had no magnetic field. The field acts like a shield with two funnels. Most of the solar wind flows around the shield. Some of its energy gets into the tail of the field on the night side, and from there electrons are accelerated down the field lines toward the two magnetic poles. That is the part NOAA describes as the engine of the aurora: electrons, energised on the night side, following the field down to the polar regions.

2Why the air glows, and why only up there

Between 80 and 500 km above the ground the air is thin but not empty. When a fast electron hits an oxygen atom or a nitrogen molecule it hands over energy, the atom jumps to an excited state, and a fraction of a second later it relaxes and gives the energy back as light. NOAA's own comparison is a neon sign: a gas, a current, a glow. The colour depends on which gas was hit and at what height.

The electrons arrive along the magnetic field, so the glow forms where the field lines reach the atmosphere: two rings, one around each magnetic pole, which NOAA calls the auroral ovals. The best seats are under the ovals, between about 60 and 75 degrees of latitude, where the aurora can be seen on more than half of the nights of the year according to NOAA. That band is Tromsø, Iceland, Lapland, Yellowknife and Fairbanks. It is why those names fill the tours page and why Madrid does not.

3The colours: green is common, red is rare, pink is low

The Canadian Space Agency gives the cleanest list. Green is the most common colour from the ground: oxygen hit at 100 to 300 km. Pink or dark red fringes on the lower edge of a curtain are nitrogen at around 100 km, where the air is denser and the electrons run out of energy. Pure red auroras come from oxygen higher up, at 300 to 400 km, where collisions are rarer and the atom has time to emit its slow red light. Blue and purple come from hydrogen and helium and are hard for the human eye to catch against a dark sky.

Height is also why the aurora seen from far away looks red. A person in Spain or southern Germany during a storm is not under the oval but hundreds of kilometres south of it. The green part sits low and hides below the horizon. The red part is high enough to be seen over the curve of the Earth, so the camera on a Spanish hillside in May 2024 recorded a red and magenta sky, not green curtains.

4What a storm does: the oval grows

The ovals are not fixed. When the solar wind gets faster, denser or carries a magnetic field pointing the right way, more energy enters Earth's field and the ovals expand toward the equator. NOAA notes that during major geomagnetic storms the aurora can be seen over most of the United States, and during very large events further still. That expansion is what the Kp index measures, and it is the whole subject of chapter 2. For now one rule is enough: the further a place is from the magnetic pole, the stronger the storm it needs.

Two things never change, storm or not. The sky has to be dark, so the bright nights of the Arctic summer switch the aurora off from mid April to late August at Tromsø's latitude, and the midnight sun itself runs there from 20 May to 22 July. And the sky has to be clear, because the glow happens above the clouds and no Kp value can push it through them.

5The Sun has moods: the 11-year cycle

The number of sunspots, flares and eruptions rises and falls over roughly eleven years. NOAA tracks it on its solar cycle progression page, and the current cycle, number 25, reached its maximum around 2024 to 2025. That is why the last two years brought storms strong enough to light up Spain three times, and why the years after a maximum usually keep a few big storms before activity fades toward the minimum.

A maximum does not mean every night is good. It means strong nights are more frequent. The everyday aurora under the oval does not need a maximum at all, which is the reassuring part for a first trip: pick a place inside the oval, go in the dark season, and the odds depend on clouds far more than on the Sun's mood.

6What chapter 2 adds

Chapter 1 explains where the light comes from. Chapter 2 turns that into numbers: what Kp 3 means for Rovaniemi and Kp 6 for Kiel, what the coloured bar on this site's dashboard says, and why the solar wind density, speed and Bz are shown next to the Kp. The chapter on reading the Kp index is free with an account, and the data and method page has the formulas for readers who want them now.