Changes in the flow of plasma deep below the Sun's surface could be behind the recent period of unusually low sunspot activity.
The number of spots on the Sun's surface varies periodically, going through successive maxima and minima in roughly 11 year solar cycles.
The most recent solar minimum had an unusually long number of spotless period of 780 days, compared to a typical solar minimum of about 300 days, making it the longest since 1913.
Now a team of scientists led by Assistant Professor Dibyendu Nandy from the Indian Institute of Science Education and Research in Kolkata, has developed a model based on magnetic dynamo simulations which may explain why some solar cycles are worse than others.
Nandy says sunspots are caused by magnetic field lines driven by plasma flows under the Sun's surface.
"The Sun's differentiated rotation causes these field lines to become twisted and eventually pop out as sunspots," he says.
Eventually they snap, flinging energy and plasma into space and occasionally towards the Earth where they can do damage.
While the cause of this solar cycle has long been known, it's been more difficult to predict its severity.
Nandy and colleagues examined four centuries of continuous sunspot observations from the time of Galileo Galilei to the present day.
Reporting in the journal Nature, Nandy and colleagues say the only exception was the Maunder minimum between 1645 and 1715, when hardly any sunspots were observed. This period is often referred to as the 'little ice age', when temperatures plummeted and the Thames River froze over.
"We then looked at the last solar cycle which peaked in 2001 and realised this also had an unusually deep solar minimum with few sunspots."
Variations in speed
Using their model they simulated 210 sunspot cycles, varying the speed of plasma circulation between the poles and the equator.
They found variations in the speed changed the number of sunspots on the surface. A fast flow in the first half of a solar cycle, followed by a slower flow in the second half, caused fewer sunspots and a deeper solar minimum.
Nandy says the Sun's activity entered a prolonged minimum towards the end of the last solar cycle.
"The was characterized by a very weak polar magnetic field and an unusually large number of days without sunspots."
"Our model was able to reproduce those characteristics."
According to Nandy the next step is to move the model forward and see what the upcoming solar cycle will do.
"We've not yet predicted what this solar max will bring, that's yet to be tried."
Nandy says NASA's Solar Dynamics Observatory spacecraft will provide solar seismology data on plasma flows below the Sun's surface.
"That will allow us to simulate forward and make some predictions."