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Marshall Space Flight Center
For release: 06/19/03
Release #: 03-097

Powerful 'conveyor belts' drive Sun's 11-year cycle, new evidence
suggests

NASA Marshall Center and university astronomers have found evidence
the 11-year sunspot cycle is driven in part by a giant conveyor
belt-like, circulating current within the Sun. Previously, scientists
believed this equator-ward drift was a wave-like process involving
magnetic forces. But new evidence suggests this drift is produced by
a giant circulation system 125,000 miles below the Sun's surface.

NASA and university astronomers have found evidence the 11-year
sunspot cycle is driven in part by a giant conveyor belt-like,
circulating current within the Sun.

The astronomers, Dr. David Hathaway, Robert Wilson and Ed Reichmann
of NASA's Marshall Space Flight Center in Huntsville, Ala., and Dr.
Dibyendu Nandy of Montana State University in Bozeman, reported their
findings the week of June 16 at the annual meeting of the Solar
Physics Division of the American Astronomical Society in Laurel, Md.
The results were also published in the May 20 issue of the
Astrophysical Journal. 

The astronomers made their discovery by reviewing the positions and
sizes of all sunspots seen on the Sun since 1874. "The sunspots
appear in two bands on either side of the Sun's equator," said
Hathaway. "Although the individual sunspots come and go from
week-to-week, the central positions of the bands in which they appear
drift slowly toward the solar equator over the course of each 11-year
sunspot cycle." 

Previously, scientists believed this equator-ward drift was a
wave-like process involving magnetic forces. However, this new
evidence suggests this drift is produced by a giant circulation
system in which the compressed gases, 125,000 miles below the Sun's
surface, move from the Sun's poles to its equator at about three
mph - a leisurely walking pace. The gases then rise near the equator
and turn back toward the poles, traveling in the surface layers where
the gas is less compressed - moving at a faster rate of approximately
20 to 40 mph. Recent progress in theoretical modeling of the sunspot
cycle has emphasized the important role of this circulation. 

The speed of this circulation system, called a meridional
circulation, changes slightly from one sunspot cycle to the next. The
circulation is faster in cycles shorter than the average 11-year
period and slower in cycles longer than the average period. This is a
strong indication that this circulation acts like an internal clock
that sets the period of the sunspot cycle.

The circulation also appears to influence the strength of future
cycles, as seen in the number and sizes of the sunspots produced, not
in the cycle immediately following, but rather in a two-cycle or
22-year time lag. When the flow is fast, it concentrates the magnetic
field at the Sun's poles. These stronger fields are then transported
downward into the solar interior where they are further compressed
and amplified to become the intense magnetic fields that form
sunspots years later. 

The Sun is now in the declining phase of the current sunspot cycle
that peaked in 2000 and 2001. Because the circulation flow was fast
during the previous cycle, the astronomers believe the next cycle
will be a strong one, peaking in the years 2010 and 2011.

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