…which explains why there’s so much cloud about at the moment 😡
Aficionados of spaceweather.com and other similar websites will be familiar with a yellow circle it displays on the left hand side of its main page. This is a suggested view of what the Sun looks like in visible light – so if you project its disk onto a piece of card. Recently, this has been a very blank affair as the Sun settled into quite a deep minimum. However, the sunspots are building up once again, so the disk has started to sport little black dots labelled with identifying numbers (which should not appear if you project the Sun’s disk onto a bit of card).
Sunspots are areas of high concentrations of magnetic field sticking out of the solar surface, sometimes described as a solar hernia of internal field strength. Where a loop of magnetic flux protrudes, two dark spots are seen – one corresponding to each foot of the loop. These are areas where the magnetic field constrains movement of solar plasma in a given direction. Because heat manifests itself as movement, this means the dark areas are a little cooler than the rest of the solar surface, so a little less bright, but still blindingly bright – they appear dark because the surrounding material is a little brighter still.
Over the course of a solar cycle, more and more of these loops start to pop out of the solar surface and the Sun gets spottier and spottier. The spots, which appear midway between the solar equator and the poles, start to form in greater numbers and closer to the equator until the day comes when they get too close and then stop appearing. Then the next cycle starts, with a few spots at mid latitudes again.
Cycles last between 7-14 years on average and are actually only half cycles. What happens is the magnetic field, which is anchored in the electrically conducting plasma within the body of the Sun, gets twisted by the Sun’s differential rotation (the fact that different latitudes of the Sun spin round at different velocities). Magnetic field lines cannot break as they are contours of magnetic force – it’d be like the line between wet and dry at the beach breaking. But they can reorganise, which is what happens over the sunspot cycles. By the time they do this, the magnetic field ends up on its head – north flips over to south – and everything becomes very calm and similar to a bar magnet’s field again for a short while before the twisting gets it all going again.
The sunspot cycle has been recorded since the time of ancient China. But it wasn’t until the Carrington event in 1859, when an enormous impacting solar flare was spotted by the English Astronomer just before a massive auroral storm, that the phenomena was linked to the Northern and Southern lights.
The Earth has a region of space around it that is entirely controlled by the strength of its own magnetic field – the magnetosphere. Particles within the magnetosphere bounce to and fro, trapped until something disturbs them. The surface of the Sun essentially boils off all the time – it is often described as a big ball of gas and what happens when gas is heated up – it expands. This is the solar wind, electrons and protons travelling radially outward, carrying the imprint of the solar magnetic field with them. When they meet the Earth’s own magnetic field, the two have to reorganise so one can pass the other. The sunward part of Earth’s field is compressed toward the planet. The antisunward part is extended outward and known as the magnetotail. This stretches away from the Sun and it can carry a lot of particles.
The solar wind is not constant because the Sun’s surface isn’t constant. A fast solar wind blows at latitudes closer to the polar regions and the Sun is tilted towards us, meaning we get this blast once a solar rotation, roughly every three days. In fact when the fast wind is blowing toward us, it scoops up the slow wind that preceded it and so delivers an enhanced punch to the planets it encounters, causing magnetotails to stretch out and retreat back again. This action causes particles within the magnetotail to be catapulted inwards, which in turn causes them to emit radio waves (as lightning does, causing pops and crackles on the car radio). These radio waves are capable of doing the opposite to particles they meet and so accelerate them into the atmosphere. Meanwhile the plasma coming directly in cause a great stirring around the field lines, leading to enormous plasma vortexes, which act as auroral dynamos. The two actions together mean, contrary to popular belief, particles causing the aurorae come at us from the anti-sunward position, which in turn makes midnight in winter the best time to look for them if you’re in the right latitude.
As well as the general three day emissions, the sunspots themselves emit disturbed solar wind flows due to solar flares and coronal mass ejections, which add enormous amounts of extremely fast solar wind to the mix. These do the same job as the fast solar wind, but can also inject suitably fast protons directly into the magnetosphere from the front, which then precipitate out again round the back and sides. This means at the maximum spottiness of the solar cycle, or when we’re in just the right place for a spot to point at us, there are more aurorae.
As it happens, we’re in the latter case at the moment, a spot has spat out a load of plasma and put all aurora alert services (such as this one on twitter) on their toes. But as the spots have returned to the Sun, we’re also in for a general increase in the amount of auroral signals seen for the next few years.
Of course, other planets get aurorae too, though not necessarily through the same methods. Mars and Venus get blasted directly by the solar wind and merely channel it a bit. Jupiter has an enormous magnetic field that restricts the weakened wind’s effects to close to the poles, but it has an internal dynamo system that outshine those aurorae with the brightest auroral lights of all the planets. Saturn has a moderate field, far larger than Earth’s, but smaller than Jupiter’s, and a weak internal dynamo system due to the rings, but add the two together and roughly once every three days there is a burst of particles injected into the dynamo that then sets off short lived bright lights.
At the moment, Earth looks at Saturn’s rings pretty much edge on. This produces a number of startling effects (they’re all but invisible to ordinary scopes and are producing ring bows on the moons of Saturn as seen by Cassini) but as the rotational north and south of the rings, the rotational north and south of Saturn and the magnetic north and south of Saturn are all pretty much aligned, Hubble has been able to watch both poles at once. The result – aurorae seen at both poles at once (known as conjugate aurorae). This is interesting as the battering of the planet’s magnetotail and the orientation of the magnetic field alters the direction of the particle streams that hit the rings and spin up to become the aurorae, and that is reflected in the relative brightness of the two polar lights. See the video here.
Auroral physics helps with a lot of things – knowing when to shield spacecraft, helping to work out when the ionosphere, and hence radio communications, is disturbed and avoiding blackouts due to electrical induction on the ground. But it also feeds into other areas of physics, with the solar system acting as a magnetohydrodynamical (good scrabble word, just means treating a plasma as a sort of magnetic gas) laboratory and the interaction between the Earth’s magnetosphere and the solar wind’s magnetic field tells us about how the heliosphere – the extent of the Sun’s influence – reacts to the magnetic field of the Local Fluff – seen by the Voyager probes and in signals measured by IBEX. Not to mention general atmospheric physics of the Earth, other planets and even exoplanets, which may interact with their stars in a similar way.