Reykjavik aurora borealis tour: what it is and what you will actually see

The sun throws off a constant stream of charged particles. When that stream is disturbed — by a solar flare, or by a fast stream coming out of a coronal hole — more of it reaches the Earth.

The Earth’s magnetic field deflects most of it, but funnels some down towards the poles. Where those particles collide with atoms in the upper atmosphere, the atoms are excited and then release that energy as light. That light is the aurora.

The colour depends on which gas and at what height. Oxygen at around a hundred kilometres gives the familiar green. Oxygen much higher up gives red, which needs a stronger event. Nitrogen contributes blue and purple, usually at the lower edge of a curtain and usually only visible in a photograph.

This all happens in a ring around the magnetic pole called the auroral oval, and Iceland sits under it. That is the country’s advantage: you do not need an unusual solar event here, you need the sky to be open.

Photographs of the aurora are not lies, but they are not what you see. A camera on a three-second exposure collects light your eye collects in a fraction of a second, so the greens come out stronger and the purples appear at all.

To the eye on an ordinary night: a pale band low to the north, greyish at first, which sharpens into unmistakable green once your eyes have adapted. It moves slowly. It brightens at one end. Somebody points a phone at it and everybody realises it is stronger than it looked.

On a strong night it stops being a band and becomes curtains — vertical structures that ripple along their length, fill half the sky, and move fast enough to follow. That is the night everyone hopes for and it does not come every week.

The single most common reason people say they saw nothing is that they looked for five minutes. Eyes take fifteen to twenty minutes to adapt to the dark, and a phone screen resets the clock.

“It has to be freezing.” Temperature has nothing to do with it. Clear nights in Iceland are often cold, which is where the idea comes from.

“You need a big solar storm.” Not in Iceland. Under the auroral oval a quiet night with an open sky produces a perfectly good display.

“A full moon ruins it.” It washes out a weak display and barely touches a strong one. Guides do not cancel for the moon.

“You can hear it.” Reports exist and the physics is disputed. Do not plan your evening around it.