The Sun has been increasingly active over the last few years, far more so than astronomers predicted. Ezzy Pearson investigates
Things have been hotting up on the Sun over the last few years. In December 2019, its surface was a very quiet place, a time known as the solar minimum. In the years since, it has been gradually waking up, with sunspots and flares being sighted across its surface. This activity is expected to reach its peak in the coming year or so, after which it will fall back into slumber once more, heading towards a new minimum. This pattern of rising and falling activity is known as the solar cycle.
“The solar cycle is driven by the magnetic field of the Sun,” says Stephanie Yardley, a solar scientist from the University of Reading. “Approximately every 11 years the Sun’s polar magnetic field reverses polarity – it swaps direction.”
This swapping is a chaotic process, with magnetic field lines becoming tangled and churning up the plasma the Sun is made of, which we see as an increase in solar activity around the solar maximum. When the poles are holding steadily in place, there is little solar activity and we have a solar minimum.
As the Sun’s magnetic field is difficult to measure, astronomers instead track the solar cycle using something much easier to see: sunspots. They occur when a magnetic field line breaks through the visible surface of the Sun, preventing the hot plasma in a specific spot from mixing properly. This creates a cool patch, which we see as a dark blemish on the visible surface of the Sun. Astronomers track solar activity using a value called the sunspot number, which takes into account not just the number of individual spots, but how they are grouped together.
“We’re currently in Solar Cycle 25, which is the 25th cycle since consistent records began in 1755, when extensive sunspot observations started,” says Yardley.
Eccentric behaviour
In that time, the cycles have been far from consistent. Some solar cycles are as short as eight years between minima, others stretched out to 14 years. Solar Cycle 19 reached an all-time high sunspot number of 285 in one day in March 1958, while historical records suggest there may have been virtually no sunspots seen between 1645 and 1715.
Since the 1980s, though, solar activity has been decreasing, with sunspot numbers dropping around 20 per cent every cycle. The previous cycle, Solar Cycle 24, was the lowest in a century
