Identifying the Beginnings and Ends of Solar Cycles

Simulations of the conditions below the sun's surface yielded this two?dimensional map of a cycle-period for the solar dynamo, the physical process that generates the sun's magnetic field. The thick, continuous white line denotes the 11-year-cycle period. Thin white lines are other periods in half-year intervals and the horizontal broken line marks the maximum magnetic flux. Solar cycles 22 and 23 in this map are semi-transparent gray patches. CREDIT: Mausumi Dikpati and UCAR View full size image

This Behind the Scenes article was provided to LiveScience in partnership with the National Science Foundation.

A limited number of instruments can tell us about our planet's star: Our knowledge of solar cycles comesfrom Earth-based satellites and telescopes and theoretical models of solar phenomena based on the laws of physics.

One region of the sun is benefitting from such a combination of observational and theoretical techniques: The solar convection zone, where sunspots are born. They are then expressed at the solar surface throughout solar cycles.

The convection zone occupies the outer 30 percent of the solar interior, and the activity and characteristics of its various sunspots help scientists identify the beginnings and ends of solar cycles, as well as gain insights into the solar 'dynamo' — the physical process that generates the sun's magnetic field.

Convection zone

The sun's convection zone has some parallels to the convection of heat on the surface of the Earth. The sun's magnetized plasma — hot, ionized gas — circulates throughout the zone, with plasma flowing from the equator toward the Sun's poles. Like the Earth's oceans and atmosphere, which transport heat toward the Earth's poles, solar plasma actsas a conveyor belt, transporting heat poleward and changing the sun's magnetic field.

When the plasma nears the poles it sinks, then flows back toward the equator. The latitude at which that sinking occurs turns out to be very important, according to findings from a recent study by Mausumi Dikpati, Peter Gilman and Giuliana de Toma — all from the National Center for Atmospheric Research, supported by the National Science Foundation — and Roger Ulrich from the University of California, Los Angeles.

Modulations in plasma flux and the flow of plasma speed might explain why the latest solar cycle, number 23, was longer than previous cycles, the researchers thought.

"Cycles 19, 20, 21 and 22 each lasted about 10.5 years," said Dikpati. "Cycle 23 lasted notably longer — 12.6 years."

Observing the sun

Dikpati and her colleagues analyzed solar observations performed by Ulrich at Mount Wilson Observatory to see if the plasma flow from north to south within the Sun's convective zone changed between cycles 22 and 23. They also looked to see how close the flow came to the poles.

These observations, combined with data from the National Science Foundation-supported Global Oscillation Network Group, a six-station network of solar-velocity imagers, and the Solar Heliospheric Observatory, a joint European Space Agency-NASA satellite, provide observations that reach to 80 degrees latitude on the sun.

Historically, solar data were not considered reliable poleward of 50 to 60 degrees latitude. For the Earth, limiting observations to 50 to 60 degrees latitude would be equivalent to studying only Canada's most southern points, the northern-most regions of Mongolia or France, or the southern borders of Argentina. In other words, before this data researchers were unable to sufficiently study important areas of the sun.

These expanded observations were relatively easy to obtain with the new observatories and satellite. For the Mount Wilson Observatory, the improved instrumentation, resolution and calibration — and the higher-latitude reach of data collected and developed after 1985 — greatly aided comparisons between cycle 22 and 23, including the north and south circulation of the plasma. According to Dikpati and her colleagues, the new data indicate that the peak rate of plasma flow toward the pole was unchanged from cycle 22 to cycle 23.

Plasma flow

This model of magnetism below the sun's surface shows the extended reach of flux transport during the solar cycle that ended in 2008 (right), compared to the previous cycle (left). The larger loop may be related to the extended cycle.
CREDIT: Mausumi Dikpati and UCAR

View full size image

What they did see is that incycles 20 to 22 the poleward flow reached about 60 degrees while in cycle 23, plasma flow apparently reached all the way to the pole. This could be the difference responsible for the differences between the cycles.

To find an answer, the scientists dug deeper into the mystery by considering the lower two-thirds of the convection zone, which cannot be seen using observations alone. To enhance their understanding of unseen circulation, the scientists turned to a model — called the predictive flux-transport dynamo model — to approximate the dynamics of the system as a whole.

The model simulates the evolution of magnetic fields in the outer third of the Sun's interior, which correlates to how heat moves around the Sun. The model provides a basis for projecting the nature of upcoming solar cycles from the properties of previous cycles, including changes in period from one cycle to the next.

With this model, Dikpati and her collaborators showed that with constant maximum flow speed at the surface, but with greater extent of the plasma flow toward the poles, the amount of plasma traveling at the bottom of the convection zone toward the equator would diminish. By incorporating that insight into the dynamo model, the researchers learned that such factors influenced cycle 23, leading it to be about two years longer than the previous cycle, in agreement with the observations.

Having discovered a likely link between solar conveyor belt length and solar cycle length, Dikpati and colleagues are now trying to understand the frequency of such occurrences. It is notable that in the early phases of cycle 24, the present cycle, the flow is again stopping around 60 degrees latitude, suggesting cycle 24 may return to a shorter period. But we won't find out if that's true until the cycle ends.

Editor's Note: The researchers depicted in Behind the Scenes articles have been supported by the National Science Foundation (NSF), the federal agency charged with funding basic research and education across all fields of science and engineering. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. See the Behind the Scenes Archive.