Chapter 4 · From the Sun to the sky at 100–300 km
The aurora
A wind from the Sun
Imagine standing on the Sun and feeling a wind that never stops. It is the solar wind, a thin stream of charged particles, mostly protons and electrons. It usually flows at about 400 km/s, and in fast streams reaches 800 km/s. The Sun is 150 million kilometres away. Worked example: 150,000,000 divided by 400 is 375,000 seconds, a little over 4 days, and a fast wind at 800 km/s arrives in about 2 days. Big eruptions on the Sun, called coronal mass ejections, can reach us in one to three days. In the picture the particles pour outward in thin streams.
A shield with two doors
The Earth is a magnet, and its magnetic field forms a bubble called the magnetosphere. It bends most of the solar wind around the planet like a rock in a stream. But the field lines run into the Earth near the two magnetic poles, and some particles slide down them. They land in a ring around each pole, the auroral oval. The stronger the solar wind pushes, the wider the oval grows. Scientists measure this with the Kp index, from 0 to 9. As a rough rule the edge of the oval moves two to three degrees of latitude towards the equator for each step of Kp. Slide Kp in the sandbox.
Colours from the air
Particles hit atoms of the atmosphere at heights of about 100 to 300 kilometres and give them energy, which the atoms release as light. Oxygen gives green at a wavelength of 557.7 nanometres, usually at 100 to 150 km, and red above about 200 km. The red glow takes about two minutes to emit, so it appears only where the air is so thin that atoms rarely collide first. Nitrogen gives blue and purple, mostly at the lower edge. The same process lights a neon sign. The aurora is therefore a glow of the air itself, and whether it can be heard is debated, because some observers report faint crackles.
The storm of 1859
On 1 September 1859 the English astronomer Richard Carrington saw a bright flash on the Sun. About 17 hours later, on 2 September, the biggest geomagnetic storm in recorded history reached the Earth. Auroras were reported as far south as the Caribbean, and telegraph lines failed across Europe and North America, with reports of sparks and shocks to operators. A similar storm today could damage satellites and power grids, which is why NOAA and other agencies watch the Sun all the time. In March 1989 a smaller storm switched off the power in Quebec for about 9 hours.
May 2024
On 10 and 11 May 2024 an extreme storm, rated G5 by NOAA, was the strongest in more than twenty years. Kp reached the top of the scale at 9. People saw red and pink aurora in places where it is almost never seen, including southern Europe, Florida, Mexico, and in the Southern Hemisphere in Australia and New Zealand. Phone cameras caught colours that the eye saw only faintly, because the camera collects light for several seconds. The storm was dangerous only for technology. On the ground an aurora is harmless to people.
Light on other worlds
Earth is not the only planet with an aurora. Jupiter has the strongest, powered by its very strong magnetic field, fast rotation and the volcanic gas thrown out by its moon Io. The Hubble telescope photographs its glowing rings in ultraviolet light. Saturn has them too, and so do Uranus and Neptune. Mars has a weaker, patchy aurora because it has no global magnetic field. To see our own, get far from city lights, look towards the pole and check the forecast of your national weather service or NOAA, then wait, because the show can change in minutes.
In numbers
- ≈ 400 km/s
- typical speed of the solar wind
- 557.7 nm
- wavelength of the green light of oxygen
- 1859
- the year of the Carrington storm