Auroras.

Step 1 of 8

Lights over the poles

On a clear night in northern Norway, Alaska or Tasmania the sky sometimes fills with green curtains of light, edged with pink below and red above: the northern lights and the southern lights. From space they appear as a glowing ring round each pole, the auroral oval. In the Scene the Sun is far off to the right, and the Solar wind (Kp index) sets how stormy it is.

Step 2 of 8

The magnetic bubble

The Sun constantly blows out a thin stream of charged particles, mostly protons and electrons: the solar wind. It crosses the 150 million kilometres to the Earth in a few days, at about 400 km/s. In the Unit on magnetism we saw that a charged particle moving in a magnetic field circles round the field lines. So the wind does not easily cross the Earth’s field, and flows round a magnetic bubble, the magnetosphere. Ahead of it, where the wind first slows, stands a bow shock, like the wave in front of a boat’s bow.

Show the magnetic field is On. Distances from the Earth are drawn on a square-root scale, so that the whole bubble fits in the picture.

Step 3 of 8

A solar storm

Sometimes the Sun hurls out a huge cloud of gas and magnetic field, a coronal mass ejection. When it arrives the solar wind is several times stronger, and the field it carries can point south, against the Earth’s. Scientists measure how disturbed the field is with the Kp index, from $0$⁠ for quiet to $9$⁠ for an extreme storm.

Step 4 of 8

The oval moves

With every Kp step the auroral oval grows brighter and wider and moves towards the equator. In the Carrington storm of 1859 auroras were seen in the Caribbean.

Step 5 of 8

Down the field lines

Show the magnetic field is On. On the day side the bubble is squeezed, and on the night side it is stretched into a long tail, far beyond the Moon. The dots of electrons slide along the night-side field lines into the ovals.

Step 6 of 8

Green and red

Each colour in an aurora comes from a particular gas giving off light. The common green, at a wavelength of 557.7 nm, comes from oxygen atoms 100–200 km up. Oxygen also glows deep red, at 630.0 nm, and nitrogen adds violet and blue at the bottom edge. It is now an extreme storm, $\mathrm{Kp} = 9$⁠.

Step 7 of 8

How hard the wind pushes

The solar wind is very thin, a few particles in each cubic centimetre, but it is fast, so it presses on the magnetic bubble. Show the magnetic field shows how the bubble is squeezed in a storm.

Step 8 of 8

Where the oval goes

Every big storm brings reports of auroras in places where they are hardly ever seen.