Apr. 15, 1998
April 13, 1998: The current sunspot cycle will be above average but no record setter, according to scientists at NASA's Marshall Space Flight Center.
"It's like saying we're going to have a mild or cold winter," said Dr. David Hathaway. "We're in a similar state in predicting what the sun's climate is going to do."
Hathaway is a co-author with Robert M. Wilson and Edwin J. Reichmann, also at NASA/Marshall, of "An Estimate for the Size of Cycle 23 Based on Near Minimum Conditions." It will appear in the May 1998 issue of the Journal of Geophysical Research (Space Physics).
The sun now is on the upswing of its 23rd activity cycle, a numbering scheme that dates from the mid-19th century, following introduction of the "relative sunspot number" by Rudolf Wolf of the Zurich Observatory in 1848. Wolf's sunspot number (now called the International sunspot number or the Zurich number) represents a blend of actual numbers of individual spots and numbers of groups of spots on the sun.
On average, this number varies from a minimum through a maximum to the next minimum in about 11 years. Because the solar magnetic fields reverse at the peak of each 11-year cycle, solar activity cycle actually spans a 22-year "Hale cycle." Cycle 23 is the last half of the current Hale cycle (composed of Cycles 22 and 23) that began in 1986.
"The consensus [among solar physicists] is that this cycle will be above average in size and probably a fast riser," Wilson said. "Sunspot maximum should not be perceived as the top of the cycle curve, but instead it should be thought of as an interval of peak activity which usually spans about 2 to 4 years and includes the actual maximum in sunspot number." For Cycle 23, the peak interval starts in 1999.
Predicting the solar cycle is more than a matter of scientific curiosity. An active sun can cause geomagnetic storms that endanger satellites and disrupt communications and power systems on Earth. It also heats the Earth's outer atmosphere so that spacecraft are exposed to more atmospheric drag and to greater erosion by atomic oxygen.
We even have tantalizing hints that the Earth's climate may be linked to sunspots. The "Little Ice Age" corresponded with a 70-year period, 1645-1715, when sunspots were sparse in number, the Maunder minimum. Also, there are strong statistical associations linking current trends in climate (surface temperatures) to trends in solar activity, as outlined in another paper by Wilson for the Journal of Geophysical Research (Atmospheres).
Still, with almost 250 years of observations - of which only the last 150 years are considered truly reliable- predictions are akin to the Farmer's Almanac, Hathaway said.
"There's no real physics involved," he explained. "It's all statistical inferences." The reason is that while scientists believe that sunspots are driven by the dynamo that lies hidden beneath the photosphere, they are unsure of what controls the dynamo.
So what they do is study measurements of several activities related to the sun and look for patterns that proceed the sunspot cycle. Hathaway and Wilson said these include variations in geomagnetic indices (like the aa and Ap indices, which respond to the changing conditions in the solar wind), the occurrences of high-latitude spots, the inferred strengths of the sun's polar fields, and the number of geomagnetically disturbed days (those days when the Ap index exceeds 25) over the course of the preceding cycle.
A part of the aa (or Ap) index is directly proportional to sunspot number. This component varies directly with the current solar cycle and is related to sporadic solar events like flares and disappearing filaments and prominences. On the other hand, "the remaining portion seems to be associated with the next solar cycle" Hathaway said, "although there's no conclusion on what produces this connection." This latter component seems to be related to recurrent features on the sun.
"It's probably related to the polar fields," Wilson said. "Polar coronal holes tend to reach maximum size near cycle minimum, and the maximum in the geomagnetic indices usually occurs within a couple of years or so before minimum."
In turn, this serves as a good indicator of where the sunspot cycle is headed, but it's not an absolute measure. Hathaway has several graphs showing different prediction methods applied to past sunspot cycles. That is, they used the data from early in a cycle to "predict" what the rest of the cycle would do. Most predictions match reality fairly close, except for Cycle 19, the current record cycle, which peaked at 190 in sunspot number in 1957, just as the first space physics satellites were being launched.
Based on the various precursor techniques, Hathaway, Wilson, and Reichmann predict that Cycle 23 will rise faster than normal to its peak, attaining maximum amplitude sometime during the latter half of 1999 to the first half of 2000, and that it will measure about 170 plus or minus 20 units (yearly sunspot number). They expect Cycle 23 to continue until sometime in 2006 when the next cycle, Cycle 24, should begin.
Editor's Note: This news release, complete with further links, can be found at http://science.msfc.nasa.gov/newhome/headlines/ast13apr98_1.htm
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