The Sun's activity peaks and wanes over a period of 11 years in what's called the solar cycle. The intensity of each cycle also changes, with some being more powerful than others. Longer periodicities are believed to be the cause, but a bold new proposal goes against these assumptions about the Sun.
Solar Cycle 24, which concluded a year ago, was a short, weak cycle. Traditional predictions suggest that Solar Cycle 25 will be an equally chilled time for the Sun. The level of activity is correlated to the number of sunspots that appear during the cycle. The average is between 140 and 220 sunspots, but the official prediction for Solar Cycle 25 is between 95 and 130 when it peaks around 2025.
The study published in Solar Physics predicts that the opposite will be true. The team from the National Center for Atmospheric Research (NCAR) predicts Cycle 25 will see between 210 and 260 sunspots. If this is the case, it will be among the most active on record.
The team arrived at their controversial prediction by considering the magnetic cycle of the Sun, which takes place roughly every 22 years. Their evidence suggests magnetic field bands are moving across the Sun from the poles to the equator and are responsible for the powerful flickers of extreme ultraviolet light detected on the Sun. They also believe these bands are related to the formation of sunspots – crucial indicators of increased magnetic activity.
According to the team, the formation of these bands happens every 11 years, but their motion towards the equator is not regular, with some being slower or faster. When they meet at the equator, these bands disappear as they have opposite polarity. The researchers call this the “terminator”. When that happens, a new cycle begins.
The team believes the delay in reaching the terminator is a way to better predict the activity of a cycle. A slow terminator will eat up the time from the subsequent cycle and will lead to a weaker one. A fast terminator will lead to a more active Sun.
“Scientists have struggled to predict both the length and the strength of sunspot cycles because we lack a fundamental understanding of the mechanism that drives the cycle,” NCAR Deputy Director Scott McIntosh, who led the study, said in a statement. “If our forecast proves correct, we will have evidence that our framework for understanding the Sun’s internal magnetic machine is on the right path.”
The team have matched the sunspots and behavior of the terminators for the past 270 years and believes their findings are justified. Most excitingly, their prediction can be put to the test. The next decade, and in particular the next five years to the peak, will show which scenario is more likely to be correct.