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Solar Ups and Downs

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see captionMay 9, 2000 -- In early April 2000, the Boulder sunspot number soared above 300 -- the Sun was literally peppered with spots. It seemed like a fitting prelude to the solar maximum, which experts predict will arrive in mid-June.

Now, a little more than a month later, the Sun's visible disk is almost featureless, sporting just a few diminutive spots.

What's going on here? Is the sunspot maximum coming or not?

"These are just normal up and downs in the sunspot cycle," explains Dr. David Hathaway, a solar physicist at the Marshall Space Flight Center who specializes in tracking and predicting sunspot activity. "On a daily or weekly basis the sunspot number can fluctuate wildly, but when we average the counts over a month they agree fairly well with our predictions that Solar Max is very near."

Right: What a contrast. The Sun was peppered with spots on April 3, 2000, but little more than a month later the Sun appears almost featureless. These white light images of the Sun were captured by the Michelson Doppler Imager on the Solar and Heliospheric Observatory (SOHO). [more images from SOHO]

Solar astronomers keep track of sunspots in two ways: by counting them and by monitoring their total area. Although the two quantities are related, they are not perfectly correlated. It's possible, for instance, to have a large number of sunspots that simply don't cover a very large fraction of the solar disk. Apparently, that's what happened this week.

"The Boulder sunspot number on May 7 was 130," says Hathaway. "That's not extraordinarily low. What makes the Sun look so blank right now is the small total area that's covered by spots."

On any given day near the sunspot maximum, the areas of all the sunspots added together cover about 1200 millionths of the Sun's disk. On May 7, 2000, that number dropped all the way to 130 millionths. [Editors note: On May 7, 2000, the sunspot area and the Boulder sunspot number were coincidentally the same -- 130.]

"That's about ten times less than the average for the past two months," says Hathaway. "Meanwhile, the sunspot number is only about 25% less than the recent average. What we've got is a whole bunch of very small, hard-to-see sunspots."

see caption

Left: The Boulder sunspot number and the total sunspot area are shown for the first 130 days of 2000. The sunspot areas, expressed in millionths of the total area of the solar disk, have been divided by a factor of 10 to make the values fit on the same scale as the sunspot number. In early May (at the extreme right hand side of the plot) the sunspot area dropped to approximately 10 times less than it's average value over the 90 day period.

"These ups and downs [big sunspot counts followed by much lower ones] come about because [the Sun] is a tremendous ocean of hot turbulent fluid, and it's the nature of turbulence to be chaotic.

"That's why it's so difficult to predict [solar activity]. Turbulence is a very thorny problem ... some of the great minds of this century, when they looked at turbulence they decided to do something 'easy' like general relativity," grinned Hathaway.

A Two-sided Sun?

Hathaway speculates that what we are seeing now might be a "two-sided Sun."

"There are particular longitudes [on the Sun] where sunspots erupt through the surface," explains Hathaway. "They're not always the same spots -- different groups tend to pop up at these same longitudes time after time. These are called active longitudes or 'activity nests.' If one or more of these are on the same side of the Sun, we can get a "two-sided" Sun. One side with lots of spots, one side with fewer. Have we hit one of these gaps, now? Maybe so. We'll have to wait and see if this pattern keeps up for more than one 27-day solar rotation."

The current dearth of sunspots probably won't last. According to the NOAA Space Environment Center, active regions could soon appear over the Sun's east limb and raise solar activity back to normal levels. Also, two of the small spots (numbered 8989 and 8990) now visible on the solar disk are growing in size.

Despite the unusual absence of large sunspots this week, solar maximum is apparently still on the way. Stay tuned to Science.nasa.gov for more updates about solar and geomagnetic activity as the solar cycle nears it peak.

SOHO is a cooperative project between the European Space Agency (ESA) and NASA. The spacecraft was built in Europe for ESA and equipped with instruments by teams of scientists in Europe and the USA.

Web Links

SunspotCycle.com -daily Boulder sunspot numbers and more about sunspots

Sunspot Cycle Predictions -from the Marshall Space Flight Center

SOHO home page -real-time images, screen savers, and more

Thursday's Classroom -- lesson plans and educational activities about sunspots