What Causes the Northern Lights? The Science of Aurora Borealis

The short version

The northern lights — formally aurora borealis — happen when charged particles streaming from the Sun crash into atoms in the upper atmosphere. The atoms get briefly excited, then release that energy as light. It's the same principle as a fluorescent tube or a neon sign: pump electrons into a gas, the gas glows. The colour depends on which gas you're hitting and how high up the collision happens. The shape, brightness and movement depend on what the Sun is doing on any given night.

Everything else in this article is filling in that loop in more detail — what the Sun does, how the particles get from there to the air above Norway, and why oxygen specifically is responsible for that signature green colour.

Step 1: It starts on the Sun

The Sun is constantly leaking. Its outer atmosphere — the corona — is so hot (over a million degrees Celsius) that hydrogen and helium ions can escape the Sun's gravity entirely. This stream of charged particles flows outward in every direction at speeds of 300 to 800 kilometres per second. We call it the solar wind, and Earth is bathed in it constantly.

On top of this baseline solar wind, the Sun occasionally throws out massive bursts of plasma called coronal mass ejections (CMEs). A big CME can launch a billion tonnes of solar material straight at Earth at over 2,000 km/s. The Sun also has darker, cooler regions called coronal holes that release especially fast solar wind streams that can produce recurring aurora as they rotate around with the Sun every 27 days.

Step 2: Solar wind reaches Earth

The solar wind takes between 18 hours and four days to cross the 150 million kilometres between the Sun and Earth, depending on how fast it's moving. Spacecraft like NOAA's DSCOVR sit at the L1 Lagrange point about 1.5 million kilometres ahead of Earth and measure the solar wind in real time, giving us a 15–60 minute warning before it actually slams into our magnetosphere.

The most important thing those spacecraft measure is the orientation of the magnetic field embedded in the solar wind — what scientists call the "Bz" component. When Bz points south, it's opposite to Earth's magnetic field, which lets the two fields connect, like a circuit closing. That's when energy flows efficiently from the solar wind into Earth's magnetosphere — and that's when the aurora explodes. A high-speed solar wind stream with Bz pointing south is the recipe for a great aurora night.

Step 3: The magnetosphere funnels particles to the poles

Earth has a magnetic field that extends out into space, forming a teardrop-shaped bubble around the planet called the magnetosphere. The field lines emerge from near the magnetic south pole, loop out into space, and dive back down at the magnetic north pole. When solar wind energy gets transferred into the magnetosphere, charged particles get accelerated along these field lines and funnelled toward the polar regions.

This is the key reason the aurora is concentrated at the poles. The magnetic field acts like a giant invisible chimney. Particles can't easily cross magnetic field lines, so they spiral along them. Every field line that the solar wind energizes ends in a polar region. The set of field lines that map back to a circular ring around each magnetic pole is what creates the auroral oval — the band where the aurora is brightest at any given moment.

Step 4: Particles collide with the atmosphere

As the accelerated particles spiral down the field lines, they eventually reach the upper atmosphere — typically between 100 and 300 kilometres above the ground. Here they start colliding with atoms and molecules of oxygen and nitrogen. Each collision transfers a tiny amount of energy from the moving particle to the stationary atom, exciting one of the atom's electrons to a higher energy level.

That excited state isn't stable. Within a fraction of a second, the electron drops back down to its ground state — and the energy difference comes out as a single photon of light. Multiply that by trillions of collisions per second across hundreds of thousands of square kilometres, and you get a glowing curtain visible across the entire night sky. That's the aurora.

Why the aurora is green (and sometimes red, blue, or purple)

The colour depends on which atom is being hit and how high up the collision happens, because each gas releases a specific wavelength when it relaxes back from an excited state.

• Bright green (557.7 nanometres): Atomic oxygen at altitudes around 100–150 km. This is the dominant aurora colour because oxygen is abundant at this altitude and the green emission line is very efficient. It's the colour you'll see most nights from Tromsø.
• Deep red (630.0 nanometres): Atomic oxygen at much higher altitudes, around 200–300 km. The red line takes about 110 seconds for an oxygen atom to emit, so it can only happen high up where collisions with other atoms are rare enough not to interrupt the process. You see red during stronger storms and at the top of the aurora curtain.
• Blue and violet (391.4 and 427.8 nm): Molecular nitrogen ions, especially during very energetic events when particles penetrate to lower altitudes (below 100 km).
• Pink and magenta: A mixture — green from oxygen at the bottom of the curtain combined with red or blue at the top. The pink fringes you see in strong displays are this layering effect.

This is why a Kp 7 storm in Tromsø can produce ribbons that are green at the bottom, pink in the middle and deep red at the top — you're literally seeing different altitudes of the atmosphere lit up by different physics.

Why the aurora moves and dances

The aurora is rarely a static curtain. It twists, ripples, brightens suddenly and then fades — a phenomenon aurora chasers call "dancing." The motion happens because the field lines themselves are constantly being stretched and reconnected by the changing flow of solar wind. When a particularly large reconnection event happens in Earth's magnetic tail (the side facing away from the Sun), it dumps a huge pulse of particles into the polar atmosphere all at once. That's a substorm, and it's what produces the most spectacular dancing displays.

A typical substorm cycle lasts 1–3 hours: build-up, sudden onset (the aurora brightens dramatically and starts moving fast), and gradual recovery. If you've been waiting outside for an hour with only a faint green glow on the horizon and then suddenly the entire sky erupts — you've just witnessed a substorm.

Why the aurora only happens near the poles

The whole reason the aurora is a polar phenomenon is the geometry of Earth's magnetic field. Charged particles can only easily move along field lines, not across them, and the field lines that carry the most solar wind energy all converge near the magnetic poles. The auroral oval is essentially the footprint of the magnetic field lines that map back to the energized region of the magnetosphere.

This is also why high-latitude places like Tromsø are so reliable: even a quiet Kp 1 night still has the auroral oval sitting right above them. As the Kp index grows, the oval expands outward toward the equator, allowing places further south to catch the same aurora that's been visible in the north all along. There's a corresponding aurora near the south magnetic pole called the aurora australis, but with much less land underneath it (just Antarctica and the southern tips of New Zealand and Tasmania), it gets far less attention.

Solar cycle: why some years are better than others

The Sun has an 11-year activity cycle that ramps up to a peak called solar maximum, then winds back down to solar minimum. Around solar maximum the Sun has many more sunspots, more flares, more coronal mass ejections — and therefore much more aurora. Solar Cycle 25, which we're in right now, is expected to peak around 2024–2025, which is why so many aurora displays have been reported this past year, including some that reached as far south as Spain and Mexico in May 2024.

That doesn't mean aurora vanishes during solar minimum. Coronal holes still produce high-speed solar wind streams during the quieter parts of the cycle, and Northern Norway sits inside the auroral oval most clear nights regardless of where we are in the cycle. But statistically, the next 2–3 years are going to be the best aurora viewing opportunity for another decade.