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Solar cycle ups and downs continues to mystify scientists

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Solar cycle ups and downs
continues to mystify scientists

No massive events are in the forecast

Dec. 16, 1999: A popular rumor making the rounds on the Internet is that on Jan. 1, 2000, the sun will let loose with a powerful coronal mass ejection (CME) that will zap important communications satellites and short-circuit terrestrial power grids.

Not! The sun indeed is becoming more active, but it's nothing new.

Right: Against an X-ray image of the sun taken on Dec. 9, a graph traces the actual sunspot average for Cycle 23 (jagged line) against the predictions (center curve) of the NASA/Marshall team. The anticipated maximum and minimum ranges are given by the dotted lines. Links to 1283x997-pixel, 563KB JPG. Credit: NASA/Marshall Space Flight Center and the Yohkoh Soft X-ray Telescope team.

"The sun's been doing this for a long, long time," said Dr. David Hathaway, solar physics group leader at NASA's Marshall Space Flight Center. And even though the sun is climbing towards the peak of yet another sunspot cycle maximum, "This cycle is a little different from what we've seen, but not out of the ordinary. For us on Earth it's going to be life as usual.

Hathaway is one of several scientists who will talk about the current solar maximum in a panel discussion at the American Geophysical Union's annual fall meeting in San Francisco.

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"The big difference for us is our increasing dependence on technology in general and on space-based technology in particular. Certainly it's not going to wipe out the planet although it may affect a satellite or two."

The story about a powerful CME is gaining wide circulation on the Internet, largely because there are elements of truth to it. The solar cycle is climbing towards maximum. CMEs can generate geomagnetic storms that disrupt or damage satellites. Geomagnetic storms have overloaded the power grid in the American northeast. And then there was the "planet buster" comment offered in jest but taken seriously by a few people.

Here are the basics. The sun has an 11-year-long cycle during which an increasing number of spots appear at high latitudes and drift towards the equator. The spots are actually regions of intense magnetic activity where the solar atmosphere is slightly cooler than the surrounding atmosphere. This makes the regions appear black when viewed through filters that reduce the light to bearable intensities for cameras and eyes. [Note: never look at the sun through anything but filters designed just for that purpose. Make sure the filters have no scratches or other damage that can pass direct sunlight.]

A comparison of the sun at mid-latitude last month and three years ago.
Links to 449x900-pixel, 96KB JPG showing two full solar disks. Credit: NASA/Marshall Space Flight Center.

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The length of the cycle isn't fixed at 11 years; that's just the average. (Further, solar physicists often think in terms of a 22-year cycle since the sun's magnetic poles reverse direction every 11 years.) We are now in the start of Cycle 23, the 23rd since reliable observations became available.

"It looks like this cycle, while bigger than usual, is certainly no record setter," Hathaway continued. "In fact, it keeps looking wimpier than expected."

That's because Hathaway and two other scientists at NASA/Marshall earlier this year published a means of predicting sunspot numbers ("A synthesis of solar cycle prediction techniques," Journal of Geophysical Research, Oct. 1, 1999). Their model does a credible job of predicting the sunspot numbers for the first 36 months of cycles 19 through 22. (Scientists often make what appear to be after-the-fact predictions in order to test new math models of how things should work.) Three of the predictions are "dead on," and the fourth is close.

But almost as soon as their report was published, the sunspot numbers took a dive, then rose, then dove again.

"I still have confidence in the techniques that say the cycle ought to peak in the middle of next year, although it may be in 2001," he affirmed. Some scientists believe the cycle may have peaked already, but that goes against the record that shows big cycles peak early and small cycles peak later.

"This would be extremely odd if we had a small cycle that peaked early," he noted.

Further, he points to "butterfly" diagrams, maps showing spots appearing first at high latitudes and drifting toward the equator, then forming later at low latitudes. So far, spots have not made it to the equator, another indicator that the cycle is peaking. Nor has the polar magnetic field reversed yet for this cycle.

Right: Butterfly diagrams, covering 1966 to the present, depict how sunspots form at the higher latitudes and drift towards the equator with time. Late in each solar cycle the birthplace of sunspots also moves towards the equator until the sun is relatively quiet and starts over. Links to a 1900x1280-pixel, 128KB GIF covering 1870 to the present. Credit: NASA/Marshall Space Flight Center.

"Three independent measurements say we're not there folks," he added.

So if the sun will continue to be active, aren't we likely to get a powerful CME thrown our way?

Hathaway said that he and other scientists wished that they could predict CMEs and other events even a few minutes ahead of time, let alone days or months.

"We can predict the likelihood that an active region is apt to erupt," he said. If large spots with opposite polarity are linked and close together, "then something's going to blow." But it's like taking odds on a football game. The other side can always pull a surprise play that leave you looking the fool. "Our capability just isn't there yet."

Web Links

October 14: Solar Cycle Update - Updated predictions from NASA scientists place the solar maximum in mid-2000.
July 22: Seasons of the Sun - Predicting what the Sun will do next.
April 13, 1998: New sunspot cycle to be bigger than average, but no record setter.

The Sunspot Cycle - Information about the Sunspot Cycle from the Marshall Solar Physics group. - daily updates on sunspot numbers and solar activity

The potential for a CME to cause a geomagnetic storm that disrupts life on Earth is very real, though, as demonstrated several times. The1989 power blackout in the American northeastand Canada was triggered by a geomagnetic storm that overloaded one part of the power grid and caused a blackout to cascade through the system. Several satellites have been disrupted, and at least one or two killed, by radiation enhanced by geomagnetic storms.

For these reasons, operators of satellites, power systems, pipelines, and other sensitive systems keep an eye on solar-terrestrial activities by way of the Space Environment Center in Boulder, Colorado. The center, in turn, collects data from ground and orbiting solar telescopes, magnetometers, and other instruments, and posts warnings on the web and through direct contact.

Systems operators can then take the appropriate measures. Satellite controllers may shut down all but the most basic housekeeping systems to prevent electrical charging that could short vital systems. Utility companies can isolate portions of a power grid so a blackout does not cascade across the grid.

But beyond making a brighter aurora, a powerful CME won't have any noticeable effect on the ground. On June 4 the Solar and Heliospheric Observatory (SOHO) saw a large CME that Dr. Richard Fisher of NASA's Goddard Space Flight Center called "a real planet-buster." The comment was meant in jest, but was picked up and circulated by traditional news media and the rumor mill. It was far less than a blast from Darth Vader's Death Star.

Right: This is the "planet buster" that wasn't. On June 1, 1999, the LASCO coronagraph on SOHO observed a massive cloud of plasma leaving the sun. Its apparent brightness is caused by the sun's brilliant disk being blocked by the telescope optics. The CME is actually quite faint. Credit: ESA/SOHO.

Where the sun could deal us a surprise is an event known as the Maunder Minimum. During 1645-1715, the Northern Hemisphere was locked in a "Little Ice Age." At the same time, no sunspots were observed.

Because astronomy was so primitive, that's about the limit of what we know from then. It was even forgotten until 19th century English astronomer E.W. Maunder investigated it in 1890. Other minima are believed to have occurred, but sunspot records don't exist before 1610 (The effects of the Sporer Minimum, 1460-1550, can be seen early in the movie "Orlando" with ice skating and even a full staged play on the frozen Thames River). These studies depend on records (often spotty) of auroral activities and of radioisotopes in ancient ice cores.

It also raises the question of whether a "Maunder Maximum" might be possible.

Left: As more than 300 years of observations show, the "constant" sun has highly variable. Links to 497x375-pixel, 66KB JPG. Credit: NASA/Marshall.

"If we look at other stars, you can tell that they have sunspot cycles, too," Hathaway replied. While features on the stars themselves cannot be seen (except in a few cases), starspots can be tracked as changes in emissions of ionized calcium.

Since 1966, Olin Wilson, Jr., at the Mt. Wilson Observatory near Los Angeles has studied spots on stars similar to our sun in age, size, and mass.

"You look at them and some aren't having cycles at all, others match our sun, and others are complete chaos," Hathaway said. "The sun presumably does go through hyperactive phases at well." But as with predicting CMEs, our level of understanding isn't yet up to the task of predicting major changes in the sunspot cycle, or even changes in the current cycle.

"I'm still sticking with what is our best prediction and expecting a sunspot number of about 140 at maximum around mid-2000," Hathaway said. "I'm a little worried about the prediction. But I see no reason to throw it out. The sun is a variable object and there are going to be some ups and downs."

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