The frequency of spots visible on our Sun and the associated solar flares decreases to very low levels approximately every 11 years, and then begins to rise again. However, the strength of these solar cycles can vary greatly. In the past, during certain periods called grand minima, almost no sunspots were seen for decades. In contrast, in the second half of the last century, the Sun produced a series of very strong cycles – this period is referred to by solar physicists as the Modern Maximum. The Modern Maximum unexpectedly came to an end with the current solar cycle, which has proved to be significantly weaker than previous ones.
At the large meeting of European solar physicists held at Eötvös Loránd University in Budapest from September 4–8, 2017 (15th European Solar Physics Meeting, ESPM-15), the possibility of forecasting serious changes from cycle to cycle, which is akin to a Holy Grail in the field studying the origins of solar activity, the dynamo theory, also received considerable attention. In recent years, significant progress has been made in understanding the factors behind these changes. However, it has also become disappointingly clear that the unpredictability inherent in the appearance of individual sunspots, which can only be statistically averaged, may impose unavoidable limits on the predictability of the dynamo mechanism.
One of the conference speakers, Jie Jiang (Beijing Astronomical Observatory), conducted relevant research in collaboration with a research group at the Max Planck Institute for Solar System Research (MPS) in Göttingen. The group leader, Professor Manfred Schüssler, received the first-ever career achievement award in European solar physics (Senior ESPD Prize) at the conference. During their investigations, they found that certain large sunspots, appearing in unusual places, at unusual times, or showing unusual configurations, could "make history" by changing the entire future progression of solar activity. A few such spots might have abruptly ended the Modern Maximum.
Representing the team from the Department of Astronomy at ELTE TTK and the University of Montreal, Melinda Nagy (ELTE) reported that large sunspots deviating from the general statistical laws of solar cycles could have even more extreme effects. In one dynamo model developed and fine-tuned to reproduce the detailed observed behavior of the Sun, it was found that some of these "rogue" sunspots could even lead to the complete extinction of the solar cycle, causing a prolonged grand minimum. According to Kristóf Petrovay, head of the Solar Physics Group at ELTE, a simple explanation for this is that the enormous magnetic flux of these spots will eventually reach the Sun's polar regions, where it can extinguish the Sun's general magnetic dipole field of opposing polarity – thereby leaving no initial magnetic field for the dynamo to amplify sufficiently in subsequent cycles.
These results suggest that the possibilities for longer-term forecasting of solar activity are limited, particularly concerning sudden, large changes.
Predicting the onset of a grand minimum or an unusually strong ("giant") solar cycle similar to the one peaking in 1957 may not be possible more than a few years in advance – which is quite disappointing for solar and space physicists.
As if to confirm these speculations, during the conference, our Sun hosted a huge sunspot group with an unusual configuration that produced a significant solar flare on September 6 – the strongest in this cycle. Although detailed assessment is yet to follow, its characteristics very much fit the criteria for "rogue" sunspots. Its appearance may be slightly too late to significantly impact the next solar cycle expected to begin in about two years, but it could have a substantial effect on the one that follows.
The sunspot and magnetic map possibly responsible for the solar flare of the decade on September 6, 2017, captured in images from NASA's SDO observatory