Introduction to Here Comes the Sun
The year 2000 may bring solar fireworks to Earth, astronomers predicted in 1997. More and more sunspots–dark areas with lower temperatures than the surface around them—began to appear across the face of the sun in 1996. These sunspots signaled the beginning of a new cycle of solar activity. Scientists were betting that the cycle would peak at the turn of the millennium, causing a similar peak in climatic and electrical disturbances all around the Earth.
Sunspots are the most obvious signs of disturbances on the sun. They occur along with mass ejections, eruptions that hurl large amounts of particles, energy, and radiation from the sun's surface. Solar physicists believe that sunspots and other disturbances occur when the magnetic field deep inside the sun is disrupted by the sun's rotation. Because the sun is composed primarily of gases, its equatorial region rotates faster than its poles do. This uneven rotation causes the sun's magnetic field to strengthen deep inside the sun. Portions of this magnetic field then buoy to the surface, in some cases preventing heat from reaching the surface, creating the cool sunspots.
Sunspots tend to occur in cycles that last about 11 years. During each cycle, the magnetic field twists, releasing energy in bursts—such as mass ejections—and then relaxes. The cycles progress from no or few sunspots to a large number of spots and back again to a minimum. The sunspots of a new cycle appear in areas midway between the poles and the equator first, and then in areas closer to the equator as a cycle progresses. The energy and radiation from eruptions become part of the solar wind—a stream of charged particles, primarily protons and electrons—that blows from the sun, through the solar system, and beyond.
Effects On Earth of Sunspots
When large bursts of solar energy reach Earth, they create geomagnetic storms, severe disturbances of Earth's magnetic field. Geomagnetic storms can produce a variety of effects on Earth, most of them disruptive. Among the most striking and best understood effects of increased sunspots and solar activity is an upswing in the occurrence of auroras (the northern and southern lights). Strong gusts of solar wind send clouds of charged particles from the sun toward Earth. The particles follow Earth's magnetic field lines into the Earth's atmosphere. There, the particles collide with atmospheric atoms and molecules producing the auroras—vivid, diffuse arcs and rays of multicolored light that dance across polar skies.
Another effect of the barrage of space particles is a shut-down of electric power transmission grids, the systems of electric power lines that carry electric current from producers to users. In 1989, the last time that the number of sunspots peaked, the flow of charged particles from the sun so disrupted the Earth's magnetic field that a hydroelectric power grid in Quebec, Canada, was knocked out of operation. The collapse shut down electric power throughout the province.
Abrupt power blackouts and more frequent auroras are the more obvious effects of the increased magnetic activity that accompanies a rise in the number of sunspots. But this increase in magnetic activity can also confuse the delicate electronics aboard spacecraft, causing potentially dangerous false or “phantom” commands to be given to the spacecraft. On a larger scale, the Earth's atmosphere often responds to a geomagnetic storm by heating and swelling, pushing the edge of the atmosphere outward. A higher, expanded atmosphere increases drag on low-orbiting satellites and can shorten their useful life by hastening their reentry into the atmosphere.
People, too, may be at risk from solar outbursts. Astronauts on a spacewalk must seek shelter in their spacecraft because high-energy radiation from a geomagnetic storm can destroy cells in the human body.
Predicting Sunspots
Thus, predicting how strong the solar cycle will be and when it will be at its maximum—the time when the sun is covered with the most sunspots and is producing the greatest number of eruptions—is important to our technological society and to human health. Such prediction is largely a process of looking back to find a pattern of activity that can then be successfully projected forward.
The prediction of solar activity is a relatively recent development in astronomy. Although ancient Chinese astronomers recorded sunspots, the first regular observations were not made until 1749, when astronomers at the Zurich Observatory in Switzerland began recording the cycle that modern astronomers call solar cycle 1. The 22 solar cycles since that date have been extremely irregular, making prediction difficult. Moreover, scientists now believe that truly reliable observations began in 1848, covering only 13 of the 22 cycles of the short historical record.
In many cases, according to Karen Harvey, a solar physicist at the Solar Physics Research Corporation and a Visiting Astronomer at Kitt Peak National Solar Observatory, both in Tucson, Arizona, the length of a particular cycle of sunspots is not clear until the new cycle begins. The reason for the uncertainty is that both old- and new-cycle sunspots are visible during the minimum period between cycles. One way that scientists can distinguish new sunspots from those of the preceding cycle is by plotting the points at which the spots appear on the sun. Spots from the old cycle are near the equator while spots from the new cycle appear at solar latitudes greater than 25 degrees.
To help astronomers today predict the solar cycle and monitor solar activity, the National Oceanic and Atmospheric Administration and the U.S. Air Force jointly operate a service known as Space Weather Operations. This facility in Boulder, Colorado, issues forecasts and warnings of impending solar disturbances. Such disturbances were forecast with about 30 percent accuracy in the mid-1990's, roughly the same as for weather forecasts before continuous observations of the Earth were possible with weather satellites. Solar scientists hoped that a new satellite, scheduled for launch in August 1997, would improve the accuracy.
Using A Satellite to Find Sunspots
Known as the Advanced Composition Explorer (ACE), the new satellite was to be placed about 1.5 million kilometers (930,000 miles) sunward from the Earth, at a point where the gravitational fields of the sun and Earth are balanced. From that location, ACE will sample the composition and magnetic characteristics of the solar wind flowing to Earth and provide warnings of hazardous solar gusts that will reach Earth about an hour after being detected by ACE.
Further improvement in long-term prediction, however, will have to await the results of studies being conducted by a number of ground-based instruments as well as by spacecraft from around the world. Ulysses, a joint mission of the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA), was launched in 1990 to orbit and observe the sun. The Solar and Heliospheric Observatory (SOHO), also a joint effort of NASA and ESA, has been surveying the sun with a battery of instruments since 1995. A Japanese satellite, Yohkoh, was launched in 1991 to study the sun's corona (outer atmosphere) by observing the X rays it emits.
Scientists spent much of early 1996 watching a nearly spotless sun and waiting for the first sign of the next round of sunspots to appear. In May, they noted a new sunspot region at about 29° N latitude, indicating the start of solar cycle 23.
After determining that a new solar cycle had begun, scientists turned to predicting the timing of the coming maximum and its effects on Earth. In September 1996, NASA assembled a scientific task force to make a “best guess” about the timing and size of the next solar maximum. The panel reported a “reasonable consensus” that solar cycle 23 will be comparable in magnitude to cycles 21 (1976 to 1986) and 22 (1986 to 1995), but will probably not exceed cycle 19 (1954 to 1964), the most active on record. The task force said the probability for severe geomagnetic storms would be greatest from 1999 through 2005. With those expected storms, the celestial fireworks of the new century would begin.