NASA and the National Oceanic and Atmospheric Administration (NOAA) on Tuesday jointly released a consensus statement announcing the commencement of solar cycle 25. As the star governing our solar system, Sun’s activities impact Earth and the overall space weather.
What is a solar cycle?
Like seasons on Earth, the Sun follows a cycle of 11 years, during which solar activities fluctuate between solar minima and maxima. Depending on the number of sunspots detected on the Sun, scientists term it is as solar maxima (highest number of sunspots) or solar minima (lowest number of sunspots). Sunspots are small and dark, yet cooler areas formed on the solar surface, where there are strong magnetic forces. They start appearing at Sun’s higher latitudes and later shift towards the equator as a cycle progresses. In short, when the Sun is active, there are more sunspots in comparison to fewer sunspots during the lesser active phase. Maxima or minima is not a specific time in the 11-year cycle, but is a period that can last for a few years.
How are solar cycles determined?
One of the important elements researchers look out for on the Sun’s surface is the number of sunspots. A new cycle commences when the Sun has reached its lowest possible minima phase. Every time the cycle changes, the Sun’s magnetic poles reverse. Since the Sun is a highly variable star, data of sunspot formation and its progress need close monitoring. Data of six to eight months are required to confirm whether the star has undergone a minima phase. A Solar Cycle Prediction Panel comprising solar physicists, led by NASA and NOAA, along with an international community, release a consensus statement once a decade. The team predicts the number of sunspots, the year or period when the cycle would reach its peak, and maxima and minima after taking into consideration the forecasts from global scientific teams. But the panel does not undertake independent predictions.
Traditionally, telescopes were used to record sunspots and recorded data since 1755 is available. With the advance in technology in the recent decades, satellites are also used to make real-time sunspot observations. On this basis, scientists announced the completion of solar cycle 24, which lasted between December 2008 and December 2019. With the Sun’s activities having reached its lowest minima between the two cycles, the new solar cycle 25 has now commenced.
How has the transition between solar cycles 24 and 25 been?
The Sun’s activities were notably lesser during 2019 and early 2020. There were no sunspots for 281 days in 2019 and 181 days in 2020. Since December 2019, the solar activities have slowly picked up, corroborating the beginning of the new cycle. The panel termed solar cycle 25 to be a weak one, with the intensity similar to that of Solar cycle 24.
“Just because it is a below-average cycle, it does not mean that there is no risk of extreme space weather. Violent solar eruptions can occur at any time,” said Doug Biesecker, panel co-chair and solar physicist at NOAA’s Space Weather Prediction Centre, Colorado.
The transition between the two cycles has been an unusual one, said physicist Dibyendu Nandi from IISER, Kolkata, who said cycle 25 has begun with a ‘hesitant’ start. Generally, during the transition phase, it is common for sunspots from the two consecutive cycles to overlap with each other. At times, there are no sunspots for longer days at a stretch, suggesting a ‘quiet’ Sun, said Nandi.
So, between cycles 24 and 25, there reached a point when the number of sunspots dived to 1.8, which lasted for a considerably longer time, meaning a ‘deep minima’.
“Initially, at the transition, new sunspots started appearing but it later stopped. This trend has continued. It is now triggering some concern among scientists,” said Nandi, whose team noted the start of solar cycle 25 in February this year.
Being the weakest in a century, solar cycle 24 was the fourth smallest in intensity since 1755. At its peak in April 2014, the highest number of sunspots observed were 114, whereas the average number is 179. The forecast suggests that solar cycle 25 would peak in July 2025 and the number of sunspots would be around 115.