by Bob Henson
These images from NCAR’s Mauna Loa Solar Observatory show helium emissions from the upper chromosphere on 30 October 2003 (left), near the peak of the last solar cycle, and 30 November 2008 (right), when no sunspots were being observed. (Images courtesy NCAR High Altitude Observatory.)
A quiet Sun has its virtues. For one thing, it doesn’t throw satellites and power grids into disarray. But solar experts are intrigued by the lackadaisical pace of the solar cycle now struggling to get started. Early last year, the Solar Cycle 24 Prediction Panel, coordinated by NOAA’s Space Weather Prediction Center (SWPC), called for the next 11-year cycle of activity to kick off between September 2007 and September 2008. But some stations went through August 2008 without observing a single sunspot, making it one of the quietest months in solar annals since 1913.
This minimum isn’t the longest on record (there were 568 spotless days in the early 1930s), and the sunspot pace is now beginning to pick up, but the year’s relative tranquility added spice to an already complex stew of predictions. The solar community is wrestling with two approaches: one that uses statistical patterns to glean clues about the Sun’s future, and another based on a dynamical modeling approach developed at NCAR. Most of the statistical techniques point toward a relatively calm cycle 24. In contrast, the flux-transport dynamo model pioneered by Mausumi Dikpati, Giuliana de Toma, and Peter Gilman (NCAR High Altitude Observatory) projects that cycle 24 will be stronger than its predecessor.
In its March 2007 outlook, the prediction panel acknowledged the competing approaches, noting that it was “unable to resolve a sufficient number of questions to reach a single, consensus prediction for the amplitude of the cycle.” Instead, the panel assigned two possible peak amplitudes for the smoothed International Sunspot Number (Ri): 140 +-20 (relatively strong) and 90 +- 10 (relatively weak). The panel says it won’t issue an update until 2009: “Important questions to be resolved in the year following solar minimum will lead to a consensus decision by the panel.”
Even though slow-starting cycles are often weak, NCAR’s Dikpati is sticking by her group’s prediction of a strong cycle 24. Its model focuses on large-scale processes and incorporates several types of data to simulate the solar dynamo over several decades. The group anticipated that cycle 24 would begin 12 to 18 months later than usual, based on an observed slowdown in the meridional flow at the Sun’s surface during much of cycle 23. The NCAR/HAO group is the only team to provide a physical rationale for the delay, says Dikpati.
Dikpati says the late start to cycle 24 doesn’t necessarily mean it will be weak. “The amplitude in our models is determined from the seed magnetic field produced at the bottom of the convection zone,” she says. Her group envisions the field as the result of remnant magnetic fields from past cycles mingling beneath the Sun’s surface.
Why is the new cycle taking even longer than expected to get going? One possibility is that the return flow of plasma has slowed down even more than the surface flow, which is the only portion that scientists can measure. If the return flow has been pushed deeper into the interior, says Dikpati, it could be encountering less diffusive transport. That would delay the cycle’s onset, she says, but there could be other consequences, such as an even stronger cycle than the NCAR team now expects.
“The long delay in the start of the next solar cycle, along with the low level of activity and weak polar magnetic fields, suggests that the next cycle will be small,” says David Hathaway, heliophysics team lead at NASA’s Marshall Space Flight Center. “This makes the Dikpati prediction all that more extraordinary. Given the conflict within the solar cycle predictions, we will undoubtedly learn important lessons about the solar cycle as the new cycle rises to its maximum over the next four years.”
With Earth’s average temperature in 2008 at its lowest point in seven years, some climate-change skeptics have invoked this year’s relatively quiet Sun as a factor. This doesn’t sit well with Douglas Biesecker (NOAA/SWPC), who leads the Solar Cycle 24 Prediction Panel. “Comparing the solar cycle to the temperature record over just seven years is irresponsible and questionable science,” says Biesecker. “We have records going back hundreds of years, and ignoring those records leads to faulty conclusions.”