What Causes the Northern Lights?

The aurora borealis is one of the most spectacular things you can witness on Earth. But what actually causes it? The answer involves a star, a magnetic field, and the thin shell of gas keeping us all alive.

The Short Answer

The northern lights happen when charged particles from the sun slam into gases in Earth's upper atmosphere. Earth's magnetic field steers those particles toward the poles, they collide with oxygen and nitrogen molecules between 60 and 200 miles up, and those molecules release energy as colored light. That's the aurora.

Three ingredients, every time: solar wind, magnetic field, atmosphere. Remove any one of them and there are no northern lights. Mars has an atmosphere and gets hit by solar wind, but it lost its global magnetic field billions of years ago — so no aurora. Venus has a thick atmosphere and no magnetic field — no aurora. Earth has all three, and we get the show.

How It Actually Works

The sun doesn't just produce light and heat. It continuously blasts a stream of charged particles — mostly electrons and protons — outward in every direction at speeds between 250 and 500 miles per second. This is the solar wind, and it's been flowing since the sun ignited 4.6 billion years ago.

When that stream of particles reaches Earth (a journey of about 93 million miles that takes 2–4 days), it encounters Earth's magnetosphere — the magnetic bubble generated by our planet's molten iron core. The magnetosphere deflects most of the solar wind around Earth like water flowing around a rock in a stream. But it's not a perfect shield.

At the north and south magnetic poles, the field lines converge and dip down toward the surface, creating openings where solar wind particles can funnel inward along the field lines. These particles spiral down into the upper atmosphere at tremendous speeds — up to 45 million miles per hour — and begin colliding with the gas molecules that make up the thin air at those altitudes.

Each collision transfers energy to the gas molecule, bumping its electrons to a higher energy state. This excited state is unstable. Within fractions of a second, the electron drops back to its normal energy level and releases the excess energy as a photon — a tiny packet of visible light. Multiply that by trillions of collisions happening simultaneously across a curtain of atmosphere hundreds of miles wide, and you get the aurora.

Why the Poles?

If you could look down at Earth from directly above the North Pole during a geomagnetic storm, you'd see a glowing ring of light surrounding the pole. This is the auroral oval — a permanent, shifting band of aurora activity centered at roughly 65 to 70 degrees magnetic latitude, typically about 10 to 20 degrees from the magnetic pole.

The oval exists because of the geometry of Earth's magnetic field. Near the equator, field lines run roughly parallel to the surface — solar wind particles can't follow them down. Near the poles, the field lines angle steeply toward the ground, and charged particles spiral along them like beads on a wire, reaching the atmosphere where collisions happen.

During quiet solar conditions, the oval is narrow and sits far north — mostly over the Arctic Ocean and the northernmost fringes of land. When a strong burst of solar wind hits, the oval expands southward and grows wider. During severe geomagnetic storms (Kp 8–9), it can expand far enough that aurora becomes visible from the southern United States.

What Creates the Colors?

The aurora's colors aren't random. Each one is a direct signature of which gas got hit and how high up the collision happened.

Green is the most common aurora color by far. It comes from oxygen molecules being struck at altitudes between roughly 60 and 150 miles (100–250 km). The specific wavelength is 557.7 nanometers — you could identify an aurora on another planet just from this number. Green aurora is what most people see on a typical active night in Alaska.

Red aurora also comes from oxygen, but at higher altitudes — above 150 miles (250 km). Up there, the atmosphere is so thin that oxygen atoms are struck less frequently, and they emit light at 630 nanometers instead. Red aurora often appears as a diffuse glow above the green curtains. It's less common and usually requires stronger geomagnetic activity to become visible.

Blue and purple come from nitrogen molecules at lower altitudes, below about 60 miles (100 km). Ionized nitrogen produces blue (around 427.8 nm), while neutral nitrogen can add purplish-red hues. These colors often appear at the lower edges of bright auroral curtains during strong storms.

Pink and white aurora result from a mix of these emissions happening simultaneously at overlapping altitudes. A bright pink lower edge on a green curtain is a sign of a particularly energetic display — the particles are penetrating deep enough to excite nitrogen alongside the oxygen above.

Why Some Nights Are Better Than Others

The sun's output isn't constant. Several factors control how much charged material reaches Earth and how effectively it triggers aurora.

Coronal mass ejections (CMEs) are the big ones. These are massive eruptions of magnetized plasma from the sun's surface, hurling billions of tons of material into space. When a CME is aimed at Earth, it arrives in 1–3 days and can trigger geomagnetic storms lasting hours or even days. The strongest aurora displays — the ones that light up the sky across the entire state — are almost always CME-driven.

Solar wind speed and density matter even without CMEs. The sun has regions called coronal holes that release faster-than-normal solar wind streams (600–800 km/s instead of the usual 400 km/s). These high-speed streams create recurring aurora activity that can be predicted weeks in advance as the sun rotates.

Bz direction is the sleeper factor most people overlook. The interplanetary magnetic field (IMF) carried by the solar wind has a north-south component called Bz. When Bz points southward (negative values), it connects efficiently with Earth's northward-pointing magnetic field, opening the door for solar wind to pour in. A Kp 3 night with strongly negative Bz can produce better aurora than a Kp 5 night with northward Bz. This is why experienced aurora chasers watch real-time solar wind data, not just the Kp number.

Solar cycle sets the overall baseline. The sun follows an approximately 11-year cycle between periods of minimum and maximum activity. We're currently in Solar Cycle 25, which is tracking above initial predictions and is expected to maintain peak activity through 2026. This means more frequent CMEs, more geomagnetic storms, and more nights with visible aurora — a genuinely excellent time to plan an Alaska trip.

Why Alaska Is Special

Plenty of places on Earth can see the northern lights. But Alaska — and Fairbanks in particular — has a combination of advantages that's hard to beat.

Fairbanks sits at 64.8 degrees north latitude, placing it directly beneath the auroral oval. This matters enormously. In Fairbanks, the aurora appears overhead and across the entire sky, not just as a faint glow on the northern horizon. On an active night with Kp 3 or higher, the lights can stretch from horizon to horizon, with curtains and bands directly above you.

The second advantage is weather. Interior Alaska has a continental subarctic climate, which means cold, dry winters with frequent clear skies. Fairbanks averages about 240 days per year with at least some sunshine. Compare that to Tromso, Norway (which is famously cloudy) or Reykjavik, Iceland (overcast roughly 80% of winter). Clear skies are non-negotiable for aurora viewing, and Alaska delivers them more reliably than most high-latitude destinations.

Finally, there's infrastructure. Fairbanks has an international airport, heated lodges, aurora tour operators, and a road system that lets you reach dark-sky locations in minutes. You don't need a dogsledding expedition to see the northern lights here — though that's available too, if you're into it.

Aurora Borealis vs. Aurora Australis

The northern lights have a twin. The aurora australis — the southern lights — occurs simultaneously at the opposite magnetic pole, mirroring what happens in the north. The same CME that produces a Kp 7 storm over Alaska also lights up the skies over Antarctica, southern New Zealand, and Tasmania.

The physics is identical. The only practical difference is geography: the southern auroral oval passes mostly over open ocean and Antarctica, with only the southern tips of New Zealand, Australia, and South America catching occasional displays. That's why the northern lights get all the attention — far more people live under the northern oval, and far more destinations are accessible for viewing.