Smithsonian.com

A quiet Sun in 2008 (credit: NASA/SOHO)

Our Sun goes through cycles of activity on average every 11 years. At the height of a cycle, the Sun is a busy place, with flares, eruptions and sunspots. At its lowest point, the Sun is quiet. That quiet period usually lasts for about 300 days, but the last solar minimum stretched for 780 days from 2008 to 2010.

Scientists have proposed plenty of explanations for the lengthy solar minimum, but it’s remained somewhat of a solar mystery. Now scientists report in Nature that changes in the flow of plasma within the Sun were responsible for the lack of sunspots. “The Sun contains huge rivers of plasma similar to Earth’s ocean currents,” says Andrés Muñoz-Jaramillo, a visiting research fellow at the Harvard-Smithsonian Center for Astrophysics. “Those plasma rivers affect solar activity in ways we’re just beginning to understand.”

The astrophysicists created a computer simulation of the Sun and ran it through 210 solar cycles, varying the speed of the plasma as it cycled between the equator and the poles. They found that if the plasma was moving quickly in the first half of the solar cycle but more slowly in the latter half, the result was an extended minimum and a weak magnetic field, also a feature of the last solar minimum.

There might be one small problem with the model, though: it may match the last solar minimum, but it doesn’t match up with what’s going on with the Sun right now. “The Sun will ultimately tell us how to resolve this conflict because only it knows what the next cycle will bring,” NASA solar physicist Madhulika Guhathakurta told Wired Science.