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Solving the mystery of the long solar minimum
by Jim Allen
Posted: 16 August 2010

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A team of researchers led by Mausumi Dikpati from the National Centre for Atmospheric Research (NCAR) and Roger Ulrich from the University of California, Los Angeles have suggested a cause for the unusually long lull in solar activity in the last decade.

Our Sun is constantly changing, differing in the number of sunspots and solar flares over an 11 year cycle. The changes in the level of the activity of the Sun can have wide ranging effects here on Earth, such as communication satellite blackouts and power grid failures.

The conveyor belt plasma flow in the Sun. Image: NASA.

The last solar cycle (cycle 23) ending in 2008 was very different to its predecessors, with a minimum in solar activity which was both less active and longer in duration than previous cycles. Ulrich and his colleagues suggest, in a paper featuring in the journal Geophysical Research Letters on 30 July, that the extended minimum in solar activity may be caused by a change in the flow of the Sun’s plasma. The surface of the Sun is constantly changing as layers of hot plasma move along the surface towards the poles and then back down towards the equator, in what scientists call the “conveyor belt”. Ulrich found that the currents carrying this plasma were flowing much closer to the poles than has been observed in previous solar cycles. The increase in the size of the current, and the slower return of the matter to the equator could cause the observed longer period of minimum activity.

To explain this Dikpati and her colleagues modelled how the conveyor belt may affect the length of the solar cycle. Modelling magnetic fields in the Sun’s interior, the authors have found that the extended plasma flow towards the poles could cause the lengthening of the solar cycle. Dikpati says “the key for explaining the long duration of [the previous cycle] with our dynamo model is the observation of an unusually long conveyor belt”. The team were able to use their model to predict that the last solar cycle would be longer than expected back in 2004.

With changes in the Sun's activity affecting navigation and communication technology on Earth, it is important that we are able to make predictions about the solar cycles. According to Ulrich “This study highlights the importance of monitoring and improving measurement of the Sun’s circulation,” adding “in order to improve predictions of the solar cycle, we need a strong effort to understand large-scale patterns of solar plasma motion.”