Stretchy “conveyor belt” slows the Sun

Full disc image of the Sun

The Sun goes through an 11-year cycle of high and low activity. The quiet phase of solar cycle 23, just passed, was unusually long.

  • Last solar cycle had longer “quiet phase” than usual
  • Due to changes in the Sun’s equator-to-pole plasma flow
  • The flow reached the poles instead of turning back earlier

A new study of the unusually long solar cycle that ended in 2008 suggests that one reason for its length could be a stretching of the Sun’s “conveyor belt”…a current of plasma that circulates between the Sun’s equator and its poles.

The Sun goes through cycles lasting approximately 11 years that include phases with increased magnetic activity, more sunspots, and more solar flares, and then phases with less activity.

The level of solar activity can affect navigation and communications systems on Earth.

Diagram showing equator-to-pole plasma flow on the Sun

Looking into the Sun's surface layer. Normally, the plasma flow from the equator turns back before reaching the poles (left), but in solar cycle 23 it reached practically all the way to the pole.

Puzzlingly, solar cycle 23, which ended in 2008, lasted longer than previous cycles, with a prolonged phase of low activity that had scientists baffled.

The study was conducted by Mausumi Dikpati, Peter Gilman, and Giuliana de Toma, all scientists with the High Altitude Observatory of the National Centre for Atmospheric Research (NCAR), and by Roger Ulrich at the University of California, Los Angeles.

The NCAR analysis suggests that one reason for the long cycle could have been changes in the Sun’s conveyor belt.

Just as Earth’s global ocean circulation transports water and heat around the planet, the Sun has a conveyor belt in which plasma flows along the surface from the equator toward the poles, sinks, and returns toward the equator, transporting magnetic energy along the way.

Recent measurements gathered and analysed by Ulrich and colleagues show that in solar cycle 23, the poleward flow extended all the way to the poles, while in previous cycles the flow turned back toward the equator at about 60 degrees latitude.

Furthermore, the return flow was slower in cycle 23 than in previous cycles.

In 2004, the NCAR team’s computer model successfully predicted that cycle 23 would last longer than usual.

Adapted from information issued by NCAR / UCAR / Big Bear Solar Observatory.

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