1998/1999

The Eleven-Year Switch

The sun is in its most active state in a decade.
But you're in the dark.

1610: Galileo Galilei and four other astronomers independently discover sunspots through telescope observations. Galileo uses the spots' motion to discern the sun's rotation period of 27 days. 1807-17: William Wollaston and Joseph Fraunhofer independently discover "dark lines" in the spectrum of solar light, giving birth to the practice of solar spectroscopy. 1843: After 17 years of meticulous observations, amateur German astronomer Samuel Schwabe discerns a periodic rise and fall in sunspot activity. He estimates the time between peaks at 10 years; the number is later pegged at closer to 11 years. 1908: The Mount Wilson Observatory, near Los Angeles, begins collecting the world's first daily photographic record of the sun's disk. 1940-50: Walter Orr Roberts, a student at Harvard College, founds the High Altitude Observatory near Climax, Colorado, setting up the first coronagraph in North America. From his coronal images, Roberts creates the first movie loop of a solar prominence. (In 1960, Roberts becomes the founding director of NCAR, which incorporates HAO as a scientific division.) 1980-89: The Solar Maximum Mission satellite collects the most detailed data yet on coronal mass ejections. 1990: NCAR's advanced Stokes polarimeter is installed; it later provides the first picture of the three-dimensional magnetic fields of sunspots. 1995-96: NCAR hosts two major workshops to discuss the Solar Magnetism Initiative, a proposal to unite NCAR and university-based physicists in search of the roots of magnetism driving the solar cycle.

On the night of March 13-14, 1989, curtains of red and green danced above the Arctic Circle. It was one of the most dramatic displays of the northern lights in years, visible from much of the United States. But after the show, some people found they couldn't turn on their kitchen lights. The solar explosion that injected high-energy particles into the earth's outer atmosphere to produce the aurora borealis also disturbed the earth's magnetic field so vigorously that a main power line feeding the city of Montreal, Quebec, failed. Six million people in Canada, Sweden, and the United States were without power for up to nine hours.

X-ray image of the sun from the Yohkoh mission, courtesy Japanese Space Agency/ Lockheed Martin Corp.

But in 1989, no instrument could provide advance warning by viewing this storm as it emerged from the sun, because it came from a part of the corona that faced the earth. At that time, only mass ejections on the edge of the sun could be observed--and those are unlikely to affect us, since they're thrust into space at right angles to the earth instead of toward us.

Without advance notice of solar storms, there's no hope of preparing society for possible disruption. But things have changed between the last solar maximum and the one now approaching. In the 1990s, NCAR's High Altitude Observatory (HAO) and several other labs have placed a new generation of instruments in space and on the ground. These electronic cameras photograph the entire solar disk, not just the sun's edge, for clues to solar storms in the making. "It's the [mass ejections] on the disk that are dangerous," says NCAR senior scientist Oran White, who is anticipating the fourth solar peak of his career. "That's the whole point of our chromospheric helium imaging photometer [CHIP]--to find the events that are 'geo-effective.' "

HAO's eclipse team scored a major success on 26 February 1998. Solar eclipses--spectacular in their own right--are one of the best ways for scientists to freeze and study a snapshot of the 11-year solar cycle. One NCAR group documented the 1998 eclipse from the Caribbean island of Curaçao, using their highest-resolution cameras to date to obtain the image on page 5. Nearby, another team led by Robert MacQueen (Rhodes College) and Jeffrey Kuhn (Michigan State University) observed the eclipse through a porthole in the NSF/NCAR C-130 aircraft, searching for infrared light beyond interference from the earth's lower atmosphere. Pleased at data from the flight that confirm theoretical predictions of strong emissions at the four-micron wavelength, HAO investigator Phil Judge said, "This could prove to be the most sensitive indicator of coronal magnetic field strengths available."

To get another take on the sun, scientists look to space, where a new generation of instruments is continuously recording solar changes. One, called the Solar Heliospheric Observatory (SOHO, deployed by the National Aeronautics [NASA] and Space Administration and the European Space Agency), sits a million kilometers (620,000 miles) above the earth with a dozen instruments from six nations. Another, Yohkoh (deployed by the Japanese Space Agency), orbits the earth at a much closer range, about 600 km (370 mi) up. It focuses on sampling the sun's x-ray output, which can vary wildly in both the short and long term.

Scientists at NCAR are finding both satellites a boon to long-term research, but satellite data may also serve as warning tools in the next solar cycle. If a storm as big as the March 1989 event occurs, says White, "between CHIP and SOHO and Yohkoh, we'll see it." With the help of these sentries, alerts can be sent by the National Oceanic and Atmospheric Administration's Space Environment Center (SEC) to storm-vulnerable utilities and satellite operators.

In late 1996, White joined an international panel of scientists chaired by the SEC's Jo Ann Joselyn for a NASA-sponsored study of the timing and structure of past solar peaks, with an eye toward predicting the strength of the next one. Their prognosis: an event equal to the past several maxima in strength, with a peak in late 1999 or early 2000. According to the panel, "Severe geomagnetic storms are likely to occur during the period from 1999 through 2005."

Photo courtesy NCAR High Altitude Observatory

One of NCAR's newest solar cameras delivered this image from the eclipse of February 26, 1998.

A fundamental need in forecasting "space weather" is to understand the basic physics that drive the solar cycle and its offshoots. Although solar physicists have a working theory of the sun's interior as a dynamo that generates magnetism, they have yet to figure out why the amount of magnetism on the sun's surface waxes and wanes every 11 years. Even more enigmatic are the radiation and mass ejections that affect the earth's atmosphere. "You could say space weather is at the same point where weather prediction was 50 years ago," says HAO director Michael Knölker. The U.S. Space Weather Program is now tackling the forecast problem. Meanwhile, Knölker and Robert Rosner (University of Chicago) are leading a solar magnetism initiative that would link U.S. laboratories and university centers of solar physics, including HAO, in a concerted effort to understand the solar magnetic machine.

If the next solar cycle pans out as forecast, it will be the latest in a string of increasingly powerful peaks. Sunspot activity at solar maximum is now about twice as extensive as it was in the early 1900s. Even so, the total radiative output from the sun only increases about 0.2% at solar peak. However, at short wavelengths, such as extreme ultraviolet and x-rays, the change--and the impact on the earth's atmosphere--are much larger.

NCAR's Harry van Loon, a climatologist, has worked with Karin Labitzke (Free University of Berlin) for over a decade on a painstaking search for connections between the sunspot cycle and variations in the earth's atmosphere and climate. Van Loon and Labitzke first found a correlation between solar activity and Northern Hemisphere stratospheric pressures and temperatures. After more satellite data were analyzed, the correlation appeared to extend to temperatures in the upper troposphere, and in 1998, the two researchers confirmed their findings for the Southern Hemisphere as well. "This has increased our confidence that the solar-stratospheric relationship is more than a statistical coincidence," says van Loon.

Could recent rises in global surface air temperature be related to changes in the sun's output? The Little Ice Age occurred during the so-called Maunder minimum in solar activity (1640-1710), when sunspots virtually ceased and the earth chilled. But the chill continued for another century, even after solar activity resumed. While the similarity between solar activity and the globe's surface temperature record since the 1600s points to a solar driver in the past, solar activity hasn't increased enough since the 1970s to comfortably account for recent global temperature increases.

"There's no complete theory that predicts the solar cycle and its effect on the total output from the sun," says Oran White. But solar physicists are accustomed to large-scale complexities: sunspots big enough to swallow the earth, a billion tons of particles ejected from the sun in the space of an hour, magnetism powerful enough to scramble the earth's power grids. NCAR scientists, studying their best pictures to date of these astronomical realities, remain undaunted by the uncertainties.

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