Scientists at the Indian Institute of Science Education and Research, Kolkata, have come up with a path-breaking prediction for the solar cycle — the repetitive 11-year period of activity after which the sun’s north and south poles flip.
Prantika Bhowmik, a PhD student at the Center of Excellence in Space Sciences India (CESSI), and Dibyendu Nandi, her professor and co-author of the new paper, have devised a mathematical technique that predicts the next solar cycle ten years in advance. Their study was published in the journal Nature Communications.
What is a solar cycle?
The solar disc or the surface of the sun as we see it is home to tremendous amounts of magnetic energy. This leads to entanglement in magnetic fields, in the form of dark sun spots, each of which are more than ten times the size of earth. These are formed when magnetic flux becomes super concentrated, preventing the convection of heat from sun’s core to the surface.
They can last anywhere from a few days to months, and are sometimes even visible to the naked eye.
These manifestations are accompanied by auxiliary activities such as solar flares — powerful increase in brightness near a sun spot, and coronal mass ejections — ejections of material that flow into the solar system. Such ejections travel in the ‘solar wind’, which carries high energy particles from the sun outward and far away. They can often impact or even kill satellites, scrape off parts of our atmosphere, and knock out power grids. When harmless, they create aurorae. But a powerful solar storm in 1859 once created aurorae over places as far away from the poles as Hawaii.
These actives don’t occur at random, though.
Sun spots have been observed and recorded for over four centuries, and it was found in the late 19th century that solar activity peaks and lulls approximately every 11 years. The cycles were reconstructed by combing through history, all the way back to 1745. The cycle that commenced that year and ended in 1766 is numbered ‘1’.
The cycles, however, are not exactly 11 years long; they vary between 9 years to 14 years, although 11 seems the average length. The next one, Cycle 25, is set to begin whenever Cycle 24 ends, possibly in early 2020, when all the spots start reflecting a reversed magnetic orientation.
A solar cycle functions in ‘maxima’ and ‘minima’. The start of a cycle is at minimal solar activity. This slowly picks up till it peaks mid-cycle in a ‘maximum’, where there can be up to 200 sun spots a day, travelling across the surface of the sun as it rotates. Then solar activity starts coming down again in a ‘minimum’ towards the end of the cycle. At the end of the cycle, the magnetic polarity of the sun flips. At present, the activity is at a minimum between Cycles 24 and 25.
The biggest topic of discussion surrounding solar cycles apart from general space weather seems to be climate change.
Solar activity has historically possibly caused climate change. The Maunder Minimum was a period between 1645 and 1715 where sun spots had become extremely rare, with a 30-year period recording fewer than 50 sun spots, in contrast with 50,000 sun spots we would see today in a 30-year period.
It was around this time that the earth saw the phenomenon called Little Ice Age, where Europe and North America experienced below average temperatures, resulting in global cooling. The link between low solar activity and global cooling is questionable, but increasingly, climate change sceptics seem to propagate this theory. Additionally, many believe that low solar activity in Cycles 24 and 25 would mitigate anthropogenic or human-triggered global warming.
But this isn’t the case.
“There is no clear link between low solar activity and global cooling,” explained Nandi. “Anthropogenic warming is very different from effects that result due to the sun alone. Low solar activity offsetting the warming we are causing is a myth.
“The sun is mostly likely responsible for less than 15 per cent of the global temperature increases we’ve seen over the last century,” experts say.
Since Cycle 24 has been the weakest cycle in over a century, speculation is rampant that a weak Cycle 25 could alleviate global warming and bring down earth’s temperatures. And the myth that a ‘mini ice age’ is due soon continues to propagate.
Crucial to daily life
Global warming aside, it is imperative for us to understand the solar cycle better since it is so crucial to daily life.
Solar activity has been known to affect the ozone levels in our atmosphere. Ozone causes warming and when it is damaged, causes cooling right below it. This, in turn, causes temperature variation between the tropics and the polar region, which leads to instabilities in global wind flows, jet streams, and flight patterns. Periods of maxima have also caused increases in storms and precipitation due to change in winds.
As we make technological leaps and bounds, and also on the side, venture farther into space, a complete understanding of the sun becomes more and more important.
Cycle 24 and 25
Cycle 24 was attempted to be predicted by several scientists, primarily two big teams. Both teams used the same data but arrived at the opposite conclusions: One claimed the cycle will be low while other said it would be harsher. Furthermore, all models of prediction can only predict the maximum of a cycle when the minimum has already begun.
“This gives everyone a prediction window of just 4 to 5 years,” said Bhowmik. “This was very inefficient. Both the big predictions got contradictory results despite have the same input data about the past. The process was too inaccurate to be reliable.”
So the Bhowmik and Nandi set about refining it. The advent of machine learning and complex algorithmic modelling meant they could do much more than what had been done for Cycle 24.
Their model is driven by observed data.
First, they observe the sun spots and note down parameters such as timings, frequency, and ‘tilt angle’.
They feed the information about these spots into the first part of the algorithm which maps the polar flux on the sun’s surface using magnetic field evolution models. Once that is mapped, it is used as input to the next part of the algorithm, which uses a dynamo model to predict what happens inside of the sun.
The algorithm then spits out the final prediction which combines the predictions for the upcoming minimum and the following maximum. Once the maximum is predicted, the decline is easy to predict.
Bhowmik and Nandi’s methods have been retro-tested for the data from the previous 100 years, and managed to accurately predict past cycles. It increases the prediction window to nearly 10 years, a historic high.
The model finds that there is not a lot of difference between the current and next solar cycles. In fact, Cycle 25 could even be slightly harsher than 24, with more sun spots. This becomes pertinent once again, especially since more and more nations are sending up satellites and other missions, including ISRO, which has a whole host of launches lined up.
“Cycle 25 is expected to peak around 2024, so if ISRO is planning any interplanetary or trans-lunar missions, it would be safe for them to launch at least a couple of years before or after the peak,” said Bhowmik.
The study was supported by India’s ministry of human resource development, the Indo-French Centre for the Promotion of Advanced Research, and NASA.