If you have an interest in aurora watching you may have heard that we’re entering solar minimum: but what does this mean ?
The sun doesn’t provide a constant stream of energy but exhibits phases of higher and lower intensity. It tends to operate on an 11-year cycle and it is this cycle which has a significant bearing on the chances of seeing an aurora on earth. Sunspot cycles have been observed since 1755 and we’re currently in Cycle 24, which started in early 2008 and peaked in April 2014.
When the sun’s activity is at its highest, sunspots on its surface spew out plasma – charged protons and electrons – which escape at several hundreds of km/h. The number of sunspots on the sun’s surface is a pretty good guide to the sun’s relative activity. Some sunspots are relatively stable but others erupt violently, emitting plasma containing incredible amounts of energy. As the sun rotates, so too do the sunspots. When sunspots are facing earth it is these ‘coronal mass ejections’ (CMEs) that create the aurora, typically taking about three days for the strong solar winds to reach earth. The charged particles interact with the gases in the earth’s atmosphere, following the magnetic lines around its poles and creating light shows in the polar skies at night.
I’m frequently asked by people visiting Scotland what are their chances of seeing the northern lights. There are lots of ‘ifs’ and ‘buts’. Quite apart from the local weather, the phases of the moon and whether there have been any recent solar flares (CMEs), the phase of the solar cycle also makes a big difference.
In simple terms, solar minimum refers to the phase of the solar cycle when there are fewer sunspots. As you can see in the diagram below, we’re currently on the ‘downward slope’ of Cycle 24, expected to reach its ebb around 2019-20. At that point we would expect Cycle 25 to begin. However, you can also see from the diagram that Cycle 24 was much weaker in terms of solar activity (as indicated by the number of sunspots) than the previous two cycles.
Looking at the solar cycles over a much longer timeframe you can also see (below) that there is significant variation in the relative intensity across cycles. So while solar activity peaked in the 1950s it’s been much lower in recent cycles. Some commentators are even suggesting that Cycle 24 could see a ‘Grand Solar Minimum’ of sustained low activity, where sunspots may be absent for months and possibly a year or two.
But before you give up hope of seeing an aurora over the next few years, it’s not quite as straightforward as this. During solar minima, gaps in the sun’s atmosphere – called coronal holes – tend to drift towards the sun’s equator from its poles and become larger. These coronal holes then allow solar flares to escape and when they’re pointing towards earth, they can create geomagnetic storms and auroras.
So all is not lost. Even in the current period of lower solar activity there’s still a chance of seeing an aurora – just keep an eye on aurora forecasts to identify whether there are coronal holes facing earth. But as with CMEs, the further north you are the better the chance of seeing an aurora and so those skywatchers in the Arctic and Antarctic zones are best placed.
In the meantime, there’s plenty time to save up for that trip of a lifetime to see the northern lights during the next solar maximum in 2023.