THE SCIENCE
The Science Behind the Northern Lights, Explained
How solar wind, charged particles and Earth's magnetic field combine to light up the Arctic sky green, red and violet — the physics behind Tromsø's aurora.
See Northern Lights tours on GetYourGuide ↗It starts 150 million kilometres away, on the sun
The aurora begins with the solar wind — a constant stream of charged particles, mostly electrons and protons, flowing off the sun at speeds that can reach several hundred kilometres per second. When the sun is more active it also throws off coronal mass ejections: huge bursts of solar material that reach Earth in one to three days. Both the steady wind and the occasional eruption supply the raw material for a display, but neither guarantees one — what happens next, when that material meets Earth's magnetic field, decides whether the sky over Tromsø actually lights up on any given night.
Earth's magnetic field funnels the particles to the poles
Earth's magnetosphere deflects most of the solar wind around the planet, which is why the aurora is rare near the equator — the same protective bubble that shields the atmosphere over geological time. But the field lines converge at the north and south magnetic poles, channelling charged particles down into the upper atmosphere in two ring-shaped zones a few thousand kilometres across — one over the Arctic, one over the Antarctic, roughly mirroring each other. Tromsø's far-north position places it almost directly beneath the northern ring, known as the auroral oval, which is the single biggest reason the city sees so many more aurora nights than places at lower latitudes.
Collision, not combustion — how the light is made
The aurora isn't fire or reflected light; it's collision, on a scale that's hard to picture. Particles funnelled down by the magnetic field slam into oxygen and nitrogen atoms roughly 100 to 300 kilometres up — vastly higher than the 10 to 12 kilometres commercial aircraft cruise at — transferring energy the atoms then release as photons, broadly the same underlying process as a neon sign, just at an enormously larger scale and altitude. The exact height and the specific gas involved determine the colour, which is why a single display can shift through several distinct shades as collisions happen at different heights through the same night.
Why green dominates almost every display
Oxygen atoms around 100 to 300 kilometres up produce the yellow-green that accounts for most of what people actually see from Tromsø on a typical night — conveniently, it's also the colour human eyes are most sensitive to in low light, which makes a moderate display look even more vivid than its actual brightness might suggest. Rarer reds come from the same oxygen higher up, above roughly 300 kilometres, where collisions happen less often and the resulting glow is fainter. Blues and purples, meanwhile, come from nitrogen lower in the atmosphere, and usually only appear during a stronger, more energetic display driven by a bigger burst of solar particles.
The auroral oval isn't fixed — it breathes
The ring of aurora activity around the magnetic pole expands and contracts continuously with geomagnetic conditions, more like a living shape than a fixed line on a map. On a quiet night it can shrink back toward the pole itself, leaving even far-north locations outside it; during a strong geomagnetic storm it swells outward, which is why an intense storm can occasionally push visible aurora as far south as central Europe. Tromsø's position almost directly under the oval means the city rarely needs much expansion at all to get a display — the scientific reason a modest KP reading can still produce a strong, colourful show directly overhead here.
The eleven-year rhythm behind the numbers
Solar activity rises and falls on a roughly eleven-year cycle known as the solar cycle, and the more sunspots and eruptions appear on the sun's surface, the more raw material there is available to drive the aurora. Solar Cycle 25 reached its maximum around 2024, arriving somewhat earlier and stronger than many forecasters had expected, and elevated activity typically continues for a few years past the peak before the sun gradually quietens toward the next solar minimum. None of this changes Tromsø's underlying geographic advantage — its position beneath the oval matters in every part of the cycle — but a more active sun generally does mean more frequent, brighter displays.
More things to do nearby
Not sure which night, or which kind of aurora tour?
Leave your email and roughly when you're travelling — we'll send a short rundown of how the season works month by month, why a chasing minibus beats a fixed camp on cloudy nights, and how to read the forecast so you book the right evenings.