Technology & Science

Solar study sheds light on sunspot doldrums

Unusually low sunspot activity in the past solar cycle may be due to changes in the flow of plasma deep below the sun's surface.
An image from November 2010 shows an active sunspot 1123, which unleashed a solar flare toward the Earth. Solar activity such as sunspots and flares are more common during the peak of the solar cycle. ((NASA))

Unusually low sunspot activity in the past solar cycle may be due to changes in the flow of plasma deep below the sun's surface.

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 spotless period of 780 days, compared to a typical solar minimum of about 300 days, making it the longest since 1913.

According to NASA, that was a "big event" linked to the cooling and collapse of the Earth's upper atmosphere and the weakening of the sun’s magnetic field, allowing cosmic rays to penetrate the solar system in record numbers.

Solar cycle

Solar activity waxes and wanes in 11-year cycles. During the maximum, the sun is stormy, generating sunspots and solar flares that can fling plasma toward the Earth. Solar storms can disable satellites used for weather forecasts and GPS navigation and affect the radio, navigation and computer systems of airplanes flying over the poles. On the upside, they can also produce beautiful shows of the northern lights when solar particles interact with the Earth's atmosphere and geomagnetic field.

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 computer model that may explain why some solar cycles are more extreme than others.

Nandy said sunspots are caused by magnetic field lines driven by plasma flows under the sun's surface. The movement of the plasma is simulated in the model.

"The sun's differentiated rotation causes these field lines to become twisted and eventually pop out as sunspots," he said.

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.

The sun had no sunspots for 780 days during the recent solar minumum. A more typical number is 300. ((NASA))
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 break from the cycle 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."

Using their model they simulated 210 sunspot cycles, varying the speed of plasma circulation between the poles and the equator.

Flow speed changes key

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 said 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 said 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."