Scientists at NASA's Marshall Space Flight Center appear to have done a good job of predicting Cycle 23 of the sunspot cycle.
"Except for the last few days, it's followed our curve very closely," said Dr. David Hathaway, head of the solar physics branch at NASA/Marshall. "Last week there was one day when the sunspot number dropped to 20 as compared to an average of 90 for the past few weeks. But the sun normally does that sort of thing during any cycle."
Indeed, a plot of actual vs. predicted sunspot numbers for Cycle 23 shows a few peaks and valleys, but the sun is staying close to what was predicted by Hathaway, Robert M. Wilson, and Edwin J. Reichmann, also at NASA/Marshall, in September 1996.
"We can say with a little more confidence, now, that this prediction is right," Hathaway said. "When we made the prediction in September 1996, we were stepping out in front of things."
The explanation of the prediction was published in the May 1998 issue of the Journal of Geophysical Research (Space Physics).
"That it seems to be following our line as if it knows it was there is heartening," Hathaway said. But, he cautioned that the sun often defies predictions.
In their May article, Hathaway and his colleagues said that Cycle 23 will be above average but no record setter. The sun now is on the upswing of its 23rd activity cycle, a numbering scheme that dates from the mid-19th century, although the sunspot cycle may have lasted as long as the sun has burned. The "relative sunspot number" was introduced by Rudolf Wolf of the Zurich Observatory in 1848.
What is now called the International sunspot number or the Zurich number is a blend of actual numbers of individual spots and numbers of groups of spots on the sun. On average, the sunspot 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. 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.
The NASA/Marshall team's predictions are based variations in geomagnetic indices, the occurrences of high-latitude spots, the inferred strengths of the sun's polar fields, and the number of geomagnetically disturbed days over the course of the preceding cycle.
The get the latest sunspot numbers, check the sunspot predictions page. For the latest sunspot images, check the Current Solar Images page of the Solar Data Analysis Center at NASA's Goddard Space Flight Center.
Based on various precursor techniques, Hathaway, Wilson, and Reichmann predicted 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.