Predicting the Sun’s solar cycle
With the Sun slowly awakening from a recent minimum, Stephanie Yardley investigates the process of forecasting solar activity
Just how do experts forecast the Sun’s activity in Solar Cycle 25?
The Sun is an active place; sometimes it’s calm, while at other times it’s far stormier. But is there any way that we can forecast our nearest star’s activity? Our current understanding of the Sun comes from centuries of observations. Naked-eye observations of sunspots were first made in ancient times. either during cloudy conditions, after sunrise or before sunset – a dangerous practice that risked serious damage to eyesight, even blindness, despite reducing the Sun’s glare enough to make sunspots visible.
Since the mid-1800s, long-term records of sunspots have been made by many scientists and amateur astronomers, revealing that the number of sunspots varies cyclically over an 11-year period known as the solar cycle.
Sighting sunspots
There is still some controversy surrounding who first observed sunspots, but the German medical student Johannes Fabricius was the first to publish his scientific findings. In 1611, after taking his first telescopic observations of the Sun, Fabricius teamed up with his father to track the motions of sunspots across the Sun’s surface.
Despite the invention of the telescope, sunspots could still only be viewed shortly after sunrise or before sunset, and the considerable risk of eye damage remained. (Today we know never to look directly at the Sun without proper protection – eclipse glasses or a certified solar filter on optical equipment – no matter what time of day.)
The Fabricius father-son duo adopted a safer technique, previously devised by Johnnes Kepler (1571–1630) to observe what he mistakenly thought was the transit of Mercury. They used a camera obscura, a pinhole opening that projects an image of the Sun – still often used during solar eclipses today.
The Fabricius team correctly suggested that sunspots were features of the Sun itself, rather than clouds in the Sun’s atmosphere or transiting
planets, and that these features moved in the same direction, eventually disappearing off the Sun’s surface to reappear a few weeks later.
The discovery of the solar cycle came later, thanks to pharmacist, amateur astronomer and botanist Heinrich Schwabe. By continuously monitoring the number and location of sunspots between 1825 and 1843 he noticed a cyclic variation. Schwabe’s findings encouraged Swiss astronomer and mathematician Rudolf Wolf to go one step further. Wolf combined his own regular sunspot observations with past records and reconstructed the sunspot cycle back to 1755. This became known as Solar Cycle 1 with all successive cycles being numbered accordingly.
For years scientists routinely recorded the size, number and position of sunspots without understanding their true origin. Then in 1908 George Ellery Hale, founder of the Mount Wilson Observatory, used a 60ft (18m) solar telescope to determine their magnetic nature. Hale made the first detection of magnetic fields beyond Earth by using the Zeeman effect – the splitting of several spectral lines caused by the presence of strong magnetic fields. This was the first indication that the Sun’s magnetic field is the powerful driving force behind the solar cycle.
Powering the Sun
The mechanism that describes the appearance, evolution and destruction of these magnetic fields is known as the solar dynamo. It describes how the Sun modulates its large-scale magnetic field over astronomical timescales due to fluid motions.
The dynamo operates in the Sun’s convection zone and can account for all stages of the solar cycle.
At the beginning of a solar cycle, during solar minimum, the Sun’s global magnetic field is aligned
north–south. As the Sun’s rotation varies with latitude, a faster rotation rate near the equator drags the magnetic field until it becomes increasingly aligned east–west.
All this twisting and stretching strengthens the magnetic field, and once it is strong enough, sections of it become buoyant and emerge as loops through the Sun’s surface. It is at the footpoints of these magnetic loops that sunspots appear, in pairs, one representing the north and the other the south magnetic pole like a bar magnet.
Sunspots first appear at high latitudes at solar minimum, when the new cycle is starting, forming more frequently and progressively lower as the solar cycle advances. Sunspot pairs are also tilted – the leading sunspot of the pair is located at a lower latitude, with a polarity matching the direction of the closest pole. The partner spot meanwhile follows behind and is polarised in the opposite direction to the pole. The tilt in the pairs comes from the twisting of the magnetic field caused by the Sun’s rotation.
Over time, the sunspots are torn apart by surface flows that act in all directions. A latitudinal flow (moving from pole to pole on the Sun) transports the remaining fragments of the magnetic field from the trailing spot up towards the pole, where they neutralise and replace the existing polar fields with a magnetic field of opposite polarity. Eventually, around the peak of the cycle, this causes both of the Sun’s poles to flip. Activity then decreases until the Sun is back to solar minimum.
After 11 years the whole cycle starts again, only with polar fields aligned in the opposite direction. And just to add further complexity, the Sun also undergoes longer fluctuations over 90, 200 and 2,400 year-timescales that can weaken or strengthen activity.
Prudent predictions
Scientists still do not yet fully understand the behaviour of the magnetic field inside the Sun, so solar cycle prediction is extremely challenging. Since
1989 the NASA-NOAA Solar Cycle Prediction Panel, a panel of experts from across the world, has met every decade to discuss the various methods and release an official prediction for the upcoming cycle.
“Solar cycle predictions have a chequered history, particularly for the recently concluded Cycle 24,” says Professor Dibyendu Nandi from the Indian Institute of Science Education and Research-Kolkata, a solar physicist who studies the solar magnetic cycle. “Wildly diverging predictions, the inability of the panel to reach an early consensus and a revision of its forecast has led to a scrutiny of the science of solar cycle predictions.”
Given the great difficulty of the task and the fact that there is still much to learn to accurately forecast future cycles, the most recent prediction suggests that our current cycle, Cycle 25, will be weaker than average, on a par with the previous one, Cycle 24.
For Solar Cycle 24, the Prediciton Panel was split and could not come to a consensus, initially issuing a prediction that would either be moderately strong or weak. In April 2014, Cycle 24 peaked with a sunspot number of 113; it was the fourth weakest cycle since records began and weakest for a century.
Many different methods are used to provide the prediction. For Cycle 25 the panel considered 61 predictions that fell into seven different categories.
“After reviewing all of the available predictions, the panel agreed that physics-based methods had proven to be the most successful in the past,” says Dr Lisa Upton, solar scientist at Space Systems Research Corporation, Colorado and co-chair of the NASA-NOAA Prediction Panel. This meant that the panel came to a consensus prediction fairly easily for Solar Cycle 25.
These prediction methods build on what we think is happening inside the Sun and on the surface, along with observations to predict what will happen.
“We have made meaningful progress over the last decade,” says Dibyendu. “This is reflected in all the physically well-grounded solar cycle models indicating a weak Solar Cycle 25. I am eagerly waiting for observational verification of our theoretical prediction models, but the Sun could yet again surprise us and send us back to the blackboard!”
Solar minimum was predicted to occur between October 2019 and 2020 with the cycle reaching a maximum around July 2025, plus or minus six months, with a peak sunspot number of 115. It has since been determined that solar minimum occurred in December 2019, when the appearance of new sunspots at high latitudes, with an oppositely orientated polarity, gave the signal that the new cycle had officially begun.
“With solar minimum occurring near the start of our predicted range, we might expect maximum to also occur closer to the beginning,” says Upton. “We are still on track for Solar Cycle 25 to be on a par with Solar Cycle 24.”
Does this mean that we are heading for a prolonged period of low solar activity, though? Perhaps not, as some solar scientists are predicting that this Solar Cycle 25 will actually break the trend of decreasing activity, with future cycles becoming more active.
“While this means that we have another fairly weak cycle ahead of us, the downward trend from Solar Cycle 22 to 24 has levelled off,” suggests Upton.
This is good news for solar scientists and amateur astronomers, as we may expect to see a more active Sun in future cycles. “I’m excited to see what Solar Cycle 26 might have in store for us,” says Upton. “While it’s still far too early to say, we might see the solar cycles picking up in strength again.”
In the meantime, ESA’s Solar Orbiter, a 10 year-long mission to look closely at the Sun, should provide us with a missing part of the puzzle. During solar maximum the spacecraft will image the Sun’s polar fields from a high viewpoint for the first time. These views should provide an insight into the inner workings of our star, meaning that in the future we can predict what the solar cycle will be like better than before.