Science, Science and Tech

Colours of the solar wind

It was a dark, clear sky, the sun was hurling particles into space, and colours danced over the UK.

The aurora borealis, predominantly visible from very northern latitudes including Scandinavia and Canada, made a special appearance over Scotland, Northern Island and parts of England, including the Isle of Man, and even venturing as far south as east Sussex on the night of 8 October 2012.

The northern lights are usually only glimpsed in the UK a few nights each year, as it is rare for all the conditions to be favourable. Sarah Reay of the British Geological Survey’s geomagnetism team explained that “the magnetic storm lasted 48 hours and the peak was perfectly centred in our night-time.”

However, magnetic storms are just a small part of the story. As the original gigantic thermonuclear reactor, the sun is continuously fusing hydrogen atoms into helium, producing the heat we all know and love.

It is during this process that a steady stream of particles is ejected from the surface. This is known as solar wind, and consists mainly of electrons and protons, reaching speeds of up to 750 km/sec. As they approach earth, the particles are deflected by the planet’s internal magnetic field, forming the magnetosphere. Particles that break free of the magnetosphere accelerate along the earth’s magnetic field towards the poles.

When these particles enter the atmosphere, they collide with oxygen and nitrogen, causing them to gain or lose electrons. In order to return to their normal state, they emit photons; and then light is produced. Oxygen atoms create the most common aurora colour, green, although those at high alitudes can produce red, whilst nitrogen creates blue or purple/red. The strength of the solar wind increases with the number of sunspots (dark spots on the surface of the sun resulting from intense magnetic activity).

Every 11 years the solar activity peaks, which means more sunspots, the associated solar flares and coronal mass ejections (CME). Solar flares are large explosions in the solar atmosphere, often followed by CMEs, huge bursts of solar wind.

It was a CME that caused our special light show. When CMEs collide with the magnetosphere, the resulting disturbance, or magnetic storm, pushes more particles into the atmosphere which creates brighter, more frequent spectacles.

The more solar activity, the more magnetic storms, and further south the lights move; the solar storm of 1859 brought the aurora borealis to Rome! The next solar maximum is predicted for 2013, and although it’s expected to be quite weak, perhaps it will be worth keeping an eye out for a glow in the night sky.


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June 2021
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