Are sunspots really in decline?
Lucie Green reveals how the science of the Sun’s magnetic heartbeat explains a drop in solar activity
Our recent solar activity has seen a tailing off of sunspot numbers. Is this the start of a new solar cycle?
The number of sunspots observed on the solar surface follows a roughly 11-year pattern of peaks and troughs known as the solar cycle. I started my career in solar physics in cycle 23 and we are currently coming to the end of cycle 24. While we still have a lot to learn to accurately estimate the patterns of future solar cycles, predictions over the past decade suggested that our current cycle would be smaller than average. Does this mean an overall waning of solar activity? New research over the coming years may help us get a better idea of what the future holds for our host star.
Sunspots have been observed for thousands of years, even before the development of the solar telescope provided a way to safely view our dazzling local star. Systematic records of sunspot numbers and their positions didn’t begin, though, until the mid-1800s. Still, these records represent the longest running dataset of any cosmic phenomenon and reveal an intriguing side to the Sun’s character.
That the solar cycle was ever noticed is thanks to astronomers spurred on by the possibility of discovering a new planet. In 1846 Neptune had been discovered in the search to understand whether irregularities in the orbit of Uranus were due to problems with Newton’s theory of gravity, or the gravitational effect of a then undiscovered planet. The latter idea won. So when irregularities were discovered in Mercury’s orbit, it seemed only sensible to take the same approach and the existence of
the planet Vulcan orbiting close to the Sun was proposed. The theory could be tested observationally by looking for the planet as it transited the Sun.
One person who took up this challenge was German pharmacist-turned- astronomer Heinrich Schwabe, who observed the Sun every clear day from 1826 to 1843. Although he found no evidence for the hypothetical planet Vulcan, his sunspot records did reveal a 10-year period or so during which their numbers rose. A repetitive process seemed to be at work. Soon astronomers across Europe were being encouraged by the director of Bern Observatory – Rudolf Wolf – to make regular observations of the Sun and record their fndings. Wolf combined the new sunspot records with those taken in the centuries before and was able to reconstruct sunspot cycles back to 1755. He called this ‘cycle 1’ and numbered all the following cycles consecutively.
What causes sunspots?
Solar cycle studies focused for many years on data collection, such as sunspot number, location and shape, until the true origin of sunspots was shown by George Ellery Hale in 1908. Hale discovered that these dark spots on the Sun are actually the intersection of colossal tubes of magnetism penetrating the photosphere. This discovery started a revolution in solar physics and showed us that the solar cycle is actually a magnetic one.
Today we understand that fows of electrically charged gas inside the Sun both sustain and evolve the global magnetic feld. The solar cycle captures all the stages of this evolution. At the start of the cycle, the Sun’s global feld is aligned in a north-south direction. The fows then drag out this feld so it becomes more aligned with the east-west direction. When portions of the interior feld grow too strong, they become buoyant and a loop rises to penetrate the Sun’s surface. Sunspots form at the two foot points of the loop: one sunspot with a south magnetic pole next to a spot with a north pole. Just like the confguration of a magnet.
The fuid nature of the Sun means sunspots don’t live forever. The solar cycle progresses because fows at the surface of the Sun start to tear apart the magnetic felds of the sunspots, spreading the feld over ever-larger areas. As the feld disperses, the sunspots disappear. Eventually, more fuid fows take the magnetic feld up towards both poles of the Sun where it gets reprocessed, ready to feed into the next cycle.
Overall, the evolution of the Sun’s magnetic feld that drives the solar cycle might seem straightforward.
But consider this: cycles can be large or small, and cycles of different sizes do not follow each other at random. Instead, a few large cycles will be followed by a few small cycles. Analysing this trend has also revealed an 80-year so-called ‘Gleissberg cycle’ on top of the 11-year cycle. And on top of this are the 200-year de Vries and the 2,300-year Hallstat cycles too. Developing a model that can explain all these details is an important area in modern solar physics.
The discovery of the Sun’s magnetism was also key to understanding the violent side of the Sun’s character: solar fares and coronal mass ejections (CMEs) driven by colossal releases of energy stored in the solar magnetic feld. The frequency of solar fares and CMEs peaks at the apex of each solar cycle, as does the amount of light that the Sun emits. At cycle maximum, with the most sunspots, the Sun is fraction of a per cent brighter.
Certain rules are obeyed during a solar cycle. Sunspots appear at high latitudes at the start, with the sunspots that form during subsequent years appearing at progressively lower latitudes. This pattern is shown wonderfully clearly in the so- called ‘butterfy’ diagram published by astronomers Annie and Walter Maunder in 1904 (see page 34). When pairs of sunspots appear, one with a north pole and one with a south, sunspots in each hemisphere will nearly always appear in the same order as the Sun rotates. For cycle 24, pairs of sunspots in the northern hemisphere that came into view as the Sun rotated had the south pole spot appear slightly ahead of the north. In the southern hemisphere, it was the opposite way round. This swaps every solar cycle, meaning you can use the magnetic feld of sunspots to determine which cycle they formed in. The rules apply to the overall global feld of the Sun too, and at the peak of each cycle the polar magnetic felds swap their magnetic polarity.
All this prompted the American National Oceanic and Atmospheric Administration and NASA to form
This montage of 365 images from ESA’s Proba-2 satellite shows the low activity of our Sun throughout 2018. At the extreme ultraviolet wavelengths pictured, active regions are shown as bright spots
Prof Lucie Green is a Professor of Physics at Mullard Space Science Laboratory. She is author of 15 Million Degrees: Journey to the Centre of the Sun
Looking back at recent solar cycles reveals a decline in sunspot numbers. Early indications suggest this trend is likely to continue in cycle 25
Solar cycle 23, captured in extreme ultraviolet light by NASA’s SOHO spacecraft, reveals maximum activity in 2001