What causes the northern lights
The northern lights happen when charged particles streaming from the Sun are funnelled by Earth's magnetic field toward the poles, where they collide with oxygen and nitrogen between roughly 100 and 300 kilometres up. Those collisions leave the gas atoms briefly excited, and the light they give off as they settle is the aurora.
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The chain, from the Sun to your sky
- The Sun constantly sheds charged particles, mostly electrons and protons. This flow is the solar wind.
- Earth's magnetic field deflects most of it, but the field is not a sealed shell. It is stretched into a long teardrop by the wind itself.
- When the wind arrives carrying a southward magnetic field of its own, it links up with Earth's field and pours energy into it.
- That energy accelerates particles down the field lines toward both magnetic poles.
- Between roughly 100 and 300 kilometres up, those particles hit oxygen and nitrogen atoms and knock their electrons into higher energy states.
- The atoms settle back almost immediately, and each one releases a photon as it does. Enough of them at once is a visible curtain of light.
Why the colours differ
The colour is set by which gas was hit and how high up it was, because each gas releases a specific wavelength when it settles. This is why aurora photographs show bands of colour stacked by altitude rather than mixed together.
| Colour | Gas | Altitude | How often you see it |
|---|---|---|---|
| Green | Atomic oxygen | 100 to 150 km | The most common by far, and the brightest |
| Red | Atomic oxygen | Above 200 km | Common in strong storms, often above the green |
| Purple and blue | Molecular nitrogen | Below 100 km | Usually at the lower fringe during strong activity |
| Pink | Nitrogen and oxygen together | Around 100 km | Seen along the bottom edge of bright curtains |
Your eyes are poor at colour in low light, so a display that photographs a vivid green often looks grey or pale white to the naked eye. This is not the camera lying. A three to five second exposure simply gathers more light than an eye can.
Why it forms an oval, not a cap
The aurora traces a ring around each magnetic pole rather than a disc centred on it, because the particles arrive along magnetic field lines that converge in a ring. That ring is the auroral oval, and it sits offset from the geographic pole because the magnetic pole does.
This offset is why North America gets a better deal than its map latitude suggests. The magnetic pole currently sits over the Canadian Arctic, which pulls the oval further south over the United States than over Europe. It is also why WeatherTotals works out visibility from geomagnetic latitude rather than the latitude on a map.
Why some years are better than others
The Sun runs through a cycle of roughly eleven years, from quiet to active and back. Near the peak it carries far more sunspots, and with them more flares and more coronal mass ejections, which are the huge eruptions of solar material that drive the strongest auroral displays. Solar Cycle 25 reached its maximum in the mid 2020s, so the current period is an unusually good one for aurora at mid latitudes.
Two different things produce a display. A coronal mass ejection is a single eruption that arrives one to three days later and can produce a severe storm with little notice. A coronal hole is a gap in the Sun's atmosphere that lets a fast stream escape, and because the Sun rotates about every 27 days, these tend to return on a schedule.