Charged particles from the Sun

Last night people in the north of Scotland were treated to displays of the Northern Lights (aurora borealis). For this beautiful phenomenon, the further north you are the better - although it also helps that the north of Scotland is sparsely populated so there's very little light pollution. It was great to see them eulogised on BBC Breakfast this morning, by the weather forecaster Matt Taylor who also shared some images and videos on Twitter.

Why do I care? Well, I'm an astronomer so I'm interested in everything you see in the sky. Living in Scotland I have more chance than many to see the aurora with my own eyes and I have seen it on a few occasions, the first time in, maybe 1968 when my parents woke me to marvel at it (they knew their already geeky wee boy would be excited!). The spectacular glows, arcs that reached the zenith, were particularly memorable because the August sky was not yet dark - must have been a really big one. But since my research has focused on solar flares and phenomena of the Sun's outer atmosphere I also have a detailed, professional level interest in the drivers of the aurora. The outer atmospheres of both Sun and Earth involve gas in the state known as plasma, where many atoms are ionised and electric and magnetic forces play fundamental roles. Plasma physics underlies both topics and researchers in both fields can and do learn from each other.

One often hears people say, as Matt Taylor did this morning, that the aurora is caused by "charged particles from the Sun". This isn't quite right. Let me try to explain why not, and maybe what we could say if we just wanted to use a quick sentence.

In fact "charged particles" arrive at Earth from the Sun all the time. The solar wind is a steady flow of gas away from the Sun, out into the solar system. It's very tenuous gas but also at a very high temperature, in the region of a million degrees C. At these high temperatures, every collision between two atoms involves enough energy to ionise and the gas in the Sun's hot outer layers is almost completely ionised: the gas is made up of positively charged ions and negative electrons. The gas is still neutral, overall, but its constituents are indeed "charged particles". It's a plasma. One of the many remarkable features of a plasma is that gas and magnetic field become tied together. Moving gas can carry magnetic field with it, and changes in magnetic fields can make the gas move. This is quite different from the situation, e.g. in your sitting room where the air around you is made up of electrically neutral atoms and waving a bar magnet about doesn't make it move. So the solar wind has an effect on the magnetic field near Earth, squashing it up on the day side and dragging it out into a long tail, the magnetotail on the night side.

However this happens in a fairly steady way all the time but we don't see aurora all the time (actually there is a faint, steady glow called the polar aurora but that's only visible further north than the UK and it's not what we recognise as aurora borealis). Displays of the aurora are driven by some disturbance in the solar wind. There are a couple of ways this can happen. The solar wind may be either "fast" or "slow", travelling at about 800 or 400 km per second respectively, depending on what's happening magnetically where it leaves the Sun. Crossing a boundary between fast and slow wind is one occurrence that can trigger the aurora. The Sun's outer atmosphere also sees explosive phenomena, Coronal Mass Ejections (CME's), that send clouds of magnetised gas out into space. The picture at left, from the LASCO experiment on the ESA/NASA SoHO spacecraft, shows one particular CME leaving the Sun on 2 December 2002. Aurorae can also be triggered if a CME arrives at Earth so it's useful to keep an eye on the Sun for early warning this might happen - see e.g. the Space Influences Data Analysis Center in Brussels.

Either of these sorts of disturbances can trigger sudden readjustments of the magnetic field in space near the Earth (in the region called the magnetosphere where the behaviour of the plasma is dominated by magnetic forces). Rapidly changing magnetic fields mean strong electric fields which accelerate electrons in the magnetosphere to high energies. They follow the magnetic field lines and hit the atoms of the upper atmosphere above the polar regions, causing the glow of the aurora. So the actual energetic particles, that are responsile for the glows of the aurora, have been energised close to the Earth, even although the disturbances in the solar wind that caused them in the first place started at the Sun.

People studying the aurora particularly with spacecraft, exploring the relevant regions of space and actually measuring the particles, electric and magnetic fields, have known all this for a few decades now. I've seen Lucie Green, for instance, speak about the aurora on TV and give a clear and correct description of what happens. I'm slightly surprised the "charged particles from the Sun" words are still widely used. I know there isn't always time to delve into the ins and outs and it's great to see this beautiful science discussed alongside all the other news of the day, and indeed the weather forecast. Saying "charged particles accelerated in Earth's magnetic fields when disturbances arrive from the Sun" would be slightly longer, OK, but also more correct.