How Do Auroras Form? Why the Sky Glows Near the Poles

The Sun constantly releases a stream of charged particles called the solar wind. Earth is surrounded by a magnetic field that interacts with this flow and helps deflect many particles. During active space weather, energy and particles can be transferred into Earth's magnetic environment and directed toward regions around the magnetic poles.

When these particles enter the upper atmosphere, they collide with oxygen and nitrogen. The collisions can give those atoms and molecules extra energy. As they return toward lower-energy states, they emit photons, which we see as light. This chain of particle motion, collision, excitation, and light emission is the core of how auroras form.

The northern display is called the aurora borealis, while the southern display is called the aurora australis. Both come from the same general physics. They usually appear in oval-shaped regions around the magnetic poles rather than directly above the geographic poles because Earth's magnetic and rotational axes are not perfectly aligned.

Green is one of the most familiar auroral colours and is commonly produced by excited oxygen at certain altitudes. Oxygen higher in the atmosphere can also contribute red light. Nitrogen and ionised nitrogen can add blue, purple, or reddish tones. The exact colour depends on the gas, its energy state, altitude, and the energy of the incoming particles.

This makes how auroras form a useful lesson in atomic physics as well as Earth science. Atoms and molecules can absorb energy only in particular ways, then release characteristic wavelengths of light. The glowing sky is therefore not painted by sunlight in the ordinary sense. It is light produced after energetic collisions in the thin upper atmosphere.

Because the atmosphere becomes thinner with height, the same gas can emit differently depending on altitude and collision frequency. That is one reason auroral colour can change within a single display.

Solar eruptions can disturb Earth's magnetic environment and produce geomagnetic storms. During stronger storms, the auroral ovals can expand, allowing displays to be seen farther from the poles than usual. The brightness and shape can also change rapidly as electric currents and magnetic fields move energy through near-Earth space.

When learning how auroras form, remember that a dramatic solar eruption does not guarantee a bright display at every location. The eruption's direction, magnetic orientation, arrival time, local darkness, cloud cover, and your position relative to the auroral zone all matter. Space-weather forecasts estimate the chances, but the sky still has to cooperate.

Auroras can form arcs, rays, bands, patches, and curtain-like shapes. These structures trace changing magnetic and electrical conditions far above the surface. What looks like a soft moving ribbon from the ground is part of a much larger interaction linking the Sun, Earth's magnetic field, and the upper atmosphere.

The same space weather that produces beautiful displays can also affect technology. In 1859 a powerful solar eruption, now called the Carrington Event after the astronomer who saw the flare, triggered auroras visible unusually far from the poles. Telegraph systems in Europe and North America were disrupted, with reports of sparks from equipment and some lines carrying messages even after their batteries were disconnected. Rapidly changing magnetic fields had driven electric currents through the long wires.

Modern infrastructure is more exposed. In March 1989 a geomagnetic storm caused a blackout across the Canadian province of Quebec lasting several hours, because currents induced in the ground overloaded parts of the power grid. Strong storms can also disturb satellite electronics, degrade GPS accuracy and increase drag on low orbiting spacecraft by heating the upper atmosphere. This is why space agencies monitor the Sun continuously and issue storm warnings, which aurora watchers also find useful.

A common picture is of solar wind particles pouring straight down into the atmosphere near the poles. The reality is more indirect. The solar wind stretches Earth's magnetic field into a long tail on the night side, and energy builds up there. When that stored energy is suddenly released, an event called a substorm, electrons already in Earth's magnetic environment are accelerated down the field lines into the upper atmosphere. That is why displays often brighten abruptly around local midnight and then break into rapidly moving rays and patches.

How you see an aurora also depends on your eyes. In dim light, vision relies mainly on rod cells, which are sensitive but cannot distinguish colour, so a faint aurora often looks like a pale grey or whitish cloud. A phone or camera can gather light over a second or more and record the green and red that your eyes cannot pick out. During the strong geomagnetic storm of May 2024, many people far from the usual auroral zone only realised a display was happening when they pointed a phone at the sky.

Much of the modern explanation of auroras was anticipated by the Norwegian physicist Kristian Birkeland around the start of the twentieth century. He led expeditions to the far north to record auroras alongside careful measurements of the magnetic field, and he built a laboratory model called a terrella, a magnetised metal sphere inside a vacuum chamber. When he fired beams of electrons at it, glowing rings formed around its magnetic poles, strikingly like the auroral ovals that satellites would photograph from space many decades later.

Birkeland also proposed that strong electric currents flow into and out of the upper atmosphere along magnetic field lines, carried by charged particles from space. Many scientists rejected the idea, partly because such currents could not be measured directly from the ground, and for decades leading models assumed the currents stayed within the atmosphere. Satellite measurements in the 1960s and 1970s settled the argument by detecting currents running along the field lines above the auroral zones. They are now called Birkeland currents, and they link the glow you see to processes far out in Earth's magnetic environment.

Some of the most interesting recent findings about the night sky came from amateur photographers. Around 2015 and 2016, aurora watchers in western Canada began sharing pictures of a narrow mauve ribbon stretching from east to west, sometimes accompanied by green streaks like the posts of a picket fence. Nobody had a scientific name for it, so they called it Steve, borrowing a joke from the animated film Over the Hedge, in which the characters name a mysterious hedge Steve. Scientists later turned the name into an acronym, Strong Thermal Emission Velocity Enhancement.

In 2016 a European Space Agency Swarm satellite flew through the region above one display. It measured a narrow stream of extremely hot gas flowing westwards at several kilometres per second, with temperatures thousands of degrees higher than its surroundings. Studies published from 2018 concluded that STEVE is not an aurora in the strict sense, because it is not produced by charged particles raining down along field lines. Instead it seems to be gas heated by fast-flowing currents in the upper atmosphere. It appears closer to the equator than the main auroral oval, and older photographs suggest it had been seen for decades without being recognised as something distinct.

Charged particles from Earth's space environment are guided toward high latitudes, collide with atmospheric gases, and transfer energy that is released as visible light. The gas type, altitude, and particle energy influence the colour, while solar activity helps determine how intense and widespread the display becomes.