Earth & climate
Auroras
Green curtains dance in the polar sky: their story begins on the Sun and ends high above your head.
On a clear winter night in the far north of Norway the sky suddenly begins to glow: green curtains drift silently across the stars, fade away and flare up again. This spectacle is called the aurora, the northern lights in the north and the southern lights in the south. People have marvelled at it for thousands of years, and for a long time nobody knew where it came from. Today we know the answer, and it is astonishing: the story of every aurora begins about 150 million kilometres away, on the Sun.
A wind from the Sun
The Sun constantly hurls tiny charged particles into space, the . It races along at about 400 kilometres per second and still needs several days for the roughly 150 million kilometres. There it meets our planet's , which steers most of the particles around the Earth like a shield. But at the magnetic poles the invisible field lines dive steeply down towards the ground. There particles slide along the lines like beads on a string and enter the high atmosphere. That is why auroras glow mainly in a ring around each magnetic pole, over Norway, Iceland, Alaska and Canada in the north and around Antarctica in the south. The magnetic poles do not sit exactly on the geographic poles, so the ring reaches further south over Canada than over Siberia.
N facing S: they attract
Where the colours come from
High above the ground, usually at 100 to 300 kilometres, the fast particles crash into the air. For comparison: an airliner flies at about 10 kilometres, so auroras glow far above that, at the edge of space. In the collisions oxygen and nitrogen receive a burst of energy, which they immediately give back as light. Each gas glows in its own colours: oxygen radiates the familiar green and, at great height, a deep red, while nitrogen adds blue and violet fringes. The colours are like a fingerprint that reveals which gas is glowing.
How far the glow spreads is decided by the Sun itself. When it is especially active and launches a strong solar storm, the magnetic field fills with so many particles that the glowing ring grows far towards the south. That is why in May 2024 auroras could be seen over all of Germany, a rare gift of space weather. And the Earth is not alone: Jupiter and Saturn also have magnetic fields. and space telescopes have photographed enormous auroras at their poles.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Where do the particles that trigger auroras come from?
What does the Earth's magnetic field do with most particles of the solar wind?
Auroras begin at a height of about 100 kilometres. An airliner flies at about 10 kilometres. How many kilometres lie between the plane and the lower edge of the auroras?
The familiar green glow of the auroras comes mainly from … in the high atmosphere.
The solar wind races through space at about 400 kilometres per second. How many kilometres does it cover in 5 seconds?
Match each term about the aurora with the description that fits.