What causes the northern lights? The aurora explained

The northern lights, or aurora borealis, are caused by charged particles that Earth's magnetic field steers down into the upper atmosphere, where they collide with oxygen and nitrogen and make them glow. The energy comes from the Sun, carried by the solar wind. The same thing happens around the South Pole, where it is called the aurora australis, or southern lights.

In short

• The Sun's wind of charged particles energizes Earth's magnetic field, which drives electrons down onto the upper atmosphere near the poles.
• Oxygen glows green or red and nitrogen glows blue or pink, depending on the gas and how high up the collision is.
• Big solar storms push the glow toward the equator, which is why people in places like Texas or Italy sometimes see it.

How does the Sun make the sky glow?

The chain has four links.

1. The Sun sends out particles. It constantly blows off a thin stream of charged particles, electrons and protons, called the solar wind. Sometimes it also throws off a huge cloud of them, a coronal mass ejection.
2. Earth's magnetic field catches the energy. The field, which reaches far into space, deflects most of the solar wind. But when the wind's own magnetic field points the opposite way to Earth's, the two link up and energy pours into the region around Earth, called the magnetosphere.
3. Electrons race down the field lines. According to NOAA's Space Weather Prediction Center, electrons accelerated in the magnetosphere follow magnetic field lines down toward the polar regions.
4. Air atoms light up. The electrons hit oxygen atoms and nitrogen molecules typically 80 to 500 kilometers up. Each collision gives an atom extra energy, and the atom releases it as a tiny flash of light of a particular color. Billions of flashes make the curtains you see.

It works like a neon sign: electricity passes through a gas and the gas glows its own color. The aurora is a neon sign the size of a continent, powered by the Sun.

Why are the northern lights green, red or purple?

The color depends on which gas is hit and at what height, as NASA's aurora guide sets out:

Color	Gas	Typical height
Green, the most common	oxygen	about 100 to 200 km
Red	oxygen	above about 200 km
Blue	nitrogen	about 100 to 200 km
Pink	nitrogen	below about 100 km

Where colors overlap, the aurora can look purple or white. Red is often seen at the top of tall curtains, or alone during big storms far from the poles, because those observers see only the highest part of an aurora that is overhead much farther north.

Phone cameras often show stronger colors than the eye does. In the dark, the eye relies on cells that sense light but not color well, while a camera sensor collects light over a long exposure.

Why do auroras happen near the poles?

Earth's magnetic field lines bunch together and dip into the atmosphere near the magnetic poles, so that is where the particles arrive. The result is not a cap over the pole but a ring around it, called the auroral oval. NOAA says the best viewing is usually between about 60 and 75 degrees latitude: northern Norway, Iceland, Alaska and northern Canada in the north, Antarctica in the south. The northern and southern lights usually happen at the same time, as mirror images.

Why do solar storms push the lights south?

When a fast coronal mass ejection hits Earth, it shakes the magnetic field hard, the auroral oval grows, and it moves toward the equator. Forecasters rate these geomagnetic storms on NOAA's G scale, from G1, minor, to G5, extreme, tied to an index of magnetic disturbance called Kp. NOAA's own scale gives a rough guide to how far south the aurora can reach in the United States:

Storm level	Kp	Aurora seen as far south as	How often per 11-year solar cycle
G1 minor	5	northern Michigan and Maine	about 900 days
G2 moderate	6	New York and Idaho	about 360 days
G3 strong	7	Illinois and Oregon	about 130 days
G4 severe	8	Alabama and northern California	about 60 days
G5 extreme	9	Florida and southern Texas	about 4 days

The strongest storm in more than two decades came on 10 and 11 May 2024: a G5, the first since 2003, according to NASA's Earth Observatory. Auroras were seen across much of the United States and Europe and, in the south, over Australia and New Zealand. The largest storm on record, the Carrington Event of 1859, produced auroras seen in the tropics and disrupted telegraph systems around the world.

Worked example: how much warning you get

The Sun is about 150 million kilometers away. How long a storm takes to arrive depends on the speed of the coronal mass ejection.

• A slow one at 500 kilometers per second: 150,000,000 ÷ 500 = 300,000 seconds, about 3.5 days.
• A typical fast one at 1,000 kilometers per second: 150,000 seconds, about 42 hours.
• One of the fastest, near 2,800 kilometers per second: about 54,000 seconds, roughly 15 hours. NOAA's guide to coronal mass ejections says the fastest Earth-directed ones arrive in as little as 15 to 18 hours.

That is why space-weather forecasters can issue a storm watch a day or two ahead, but cannot say exactly how strong a storm will be until it reaches a spacecraft about 1.5 million kilometers upstream of Earth, roughly 15 to 60 minutes before it hits.

When are the northern lights most likely?

Auroras follow the Sun's roughly 11-year cycle of activity, with more sunspots, flares and storms near its peak. In October 2024, NASA and NOAA announced that the Sun had reached the maximum period of Solar Cycle 25. A maximum is a stretch of months, not a single day, and large storms can also come in the years after a peak, so a quieter Sun does not rule them out.

A checklist for a good chance of seeing one:

• Dark skies: away from city light, with the Moon low or new.
• Clear weather: auroras are 100 kilometers up, far above the clouds that block them.
• Long nights: around the equinoxes and in winter at high latitudes; summer nights in the far north are too bright.
• The right hour: often around local midnight, though storms can bring them earlier.
• A forecast: the Kp index and storm watches from a space-weather service such as NOAA's.

What else does this tell us?

Earth's magnetic field is a shield as well as a light show, steering away most of the particles that would otherwise strip the atmosphere over time, which is thought to be part of what happened to Mars. Other planets with magnetic fields and atmospheres, Jupiter and Saturn among them, have auroras of their own. The source of it all is the Sun, a ball of hot, electrically charged gas whose tangled magnetic field reverses over each cycle. The colors of the sky by day have a different cause entirely, explained in why the sky is blue. And the aurora is one small sign of how connected the bodies of the solar system are, in a universe full of magnetized stars.

Check yourself

• Without looking back: trace the path of energy from the Sun to a green curtain over Norway.
• Why is the aurora a ring around the pole and not a cap over it?
• Why might someone in Texas see only red during a big storm?

Sources

• Aurora — NOAA Space Weather Prediction Center
• Auroras — NASA Science
• NOAA space weather scales — NOAA Space Weather Prediction Center
• Historic geomagnetic storm dazzles — NASA Earth Observatory, 2024
• Joint solar maximum announcement from NASA and NOAA — NOAA Space Weather Prediction Center, 2024
• Coronal mass ejections — NOAA Space Weather Prediction Center