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 sa
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Contact: Dr. John M. Horack
john.horack@msfc.nasa.gov
256 544 1872
NASA/Marshall Space Flight Center--Space Sciences Laboratory
13-Apr-1998