On a clear, dark night at high latitude, the sky can suddenly start to move: ribbons of green, sometimes edged in pink or purple, rippling overhead like a curtain in the wind. The aurora borealis looks otherworldly because, in a sense, it is. What you are seeing is the tail end of a journey that starts on the surface of the sun, crosses 93 million miles of space, and ends with subatomic particles slamming into Earth’s atmosphere at a meaningful fraction of the speed of light.

It Starts With the Sun

The sun is constantly shedding charged particles into space in a stream called the solar wind, and it periodically hurls out bigger bursts through solar flares and coronal mass ejections. According to NASA, when that stream of particles reaches Earth, it interacts with the planet’s protective magnetic field, and energy from the collision builds up in our surrounding magnetosphere. NASA describes the whole system as an intricate dance of particles and magnetism between the sun and Earth known as space weather.

Earth’s Magnetic Field Funnels the Energy to the Poles

That stored magnetic energy does not release evenly. NOAA’s Space Weather Prediction Center explains that when the solar wind’s magnetic field tips southward, it links up with Earth’s own field lines, dumping energy into the magnetosphere and accelerating electrons downward along those lines toward the polar regions. This is why auroras form in rings, called auroral ovals, centered on the magnetic north and south poles rather than showing up randomly around the globe.

A Genuinely Wild Speed

Those accelerated electrons are not moving gently. NOAA’s tutorial puts their speed at roughly 20,000 kilometers per second, about one-tenth the speed of light, or 44 million miles per hour, as they slam into oxygen and nitrogen atoms and molecules more than 60 miles above Earth’s surface. That impact knocks electrons in those atmospheric atoms into a higher-energy state. As the atoms relax back down, they release the extra energy as photons, and a coordinated shower of trillions of those photons is what registers as a glowing aurora.

Why the Colors Change

The color depends on which gas gets hit and how high up the collision happens. NASA and NOAA both point to atomic oxygen as the source of the most common aurora color, a yellow-green glow produced roughly 60 to 120 miles up. Higher altitude collisions with oxygen, above about 120 miles, tend to produce a rarer red glow, since that particular excited state takes longer to release its energy and needs the sparser air up there to survive long enough to do it. Nitrogen molecules, meanwhile, are responsible for the blue and purple fringes sometimes visible along the lower edges of a display.

Why Some Nights Are Better Than Others

Aurora activity tracks the sun’s roughly 11-year solar cycle. Near solar maximum, when the sun produces more frequent flares and coronal mass ejections, geomagnetic storms are stronger and more frequent, pushing the auroral oval further from the poles and making northern lights visible at lower latitudes than usual. NOAA’s Space Weather Prediction Center issues real-time aurora forecasts for exactly this reason, giving skywatchers advance notice of when a strong solar wind stream might paint the sky.