An l8-month decrease in the Sun's energy output, recently detected by NASA satellite, may have been factor in this year's unusually harsh winter, according to scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

This winter's severe weather conditions in the United States, coupled with the results from an experiment on the Solar Maximum Mission satellite, may be the first direct observation of cause-and-effect relationship between the Sun's energy output and changes in Earth's weather and climate.

A persistent decrease of tenth of percent in the total amount of solar energy reaching Earth (called solar irradiance) was detected over an l8-month period from February l980 to August l98l by the Active Cavity Radiometer Irradiance Monitor (ACRIM) experiment on the satellite.

"This is small change in the total energy output of the Sun, but has great potential significance for the Earth's fragile ecosystem," said JPL physicist Dr. Richard C. Willson, principal invesigator and designer of the experiment.

Climatologists are already studying the results of the experiment, which will be correlated with such global climate indicators as average temperatures, ice coverage and sea level, to evaluate the effects of the drop in solar irradiance.

A systematic increase or decrease in the Sun's release of energy -- as little as one-half percent per century -- can produce vast changes in Earth's climate. Scientists believe that one percent decrease would lower Earth's mean global temperature by more than degree Kelvin (2 degrees Fahrenheit). According to some models, decrease in solar energy of less than l0 percent could effectively freeze Earth's entire surface.

Nearly all life forms on Earth exist within the l0 kilometers (6.2 miles) above and below mean sea level. The temperatures within this thin environmental shell, called the biosphere, are determined by the amount of energy received by the Sun and delicate interactions between the atmosphere, ocean and land masses.

The climatic effects of short-term variations in solar irradiance are moderated by the heat capacity of the ocean and atmosphere. long-term increase or decrease, however, can eventually change the temperature of the ocean and atmosphere enough to change the weather and climate.

"These kinds of small but persistent trends in solar irradiance are believed to have been causes of climatic changes in the past," Willson said.

Solar magnetic activity reaches maximum approximately every ll years. The peak of the current solar cycle (called solar cycle 2l) occured in early l980, about the time the Solar Maximum Mission satellite was launched. The irradiance decrease detected by the monitor may be related to the general decline in solar activity since then. However, the decrease might be an indication of longer-term trend in the Sun's irradiance. Years of careful measurements would be required to identify such trend.

In its two years of operation, the irradiance monitor also observed short-term increases and decreases, lasting from days to weeks, in the amount of solar energy that reaches Earth. Analysis indicates the decreases are the effects of sunspots, dark, cooler patches on the Sun, while increases are caused by faculae, which are bright, extra-hot solar regions.

The monitor also detected evidence of solar oscillatory phenomena -- global pulsations whose effects extend deep into the Sun. The oscillations have five-minute periodicity. These five-minute oscillations match ground-based observations discovered in the late l970s. Study of this phenomenon, so-called "solar seismology," will provide new information on the inner workings of the Sun that cannot be obtained by other means.

During most of Earth's history, the climate appears to have been considerably warmer, with average global temperatures about 25 (77 F). The current average global temperature is l5 (59 F).

A gradual trend toward cooler climate began about l00 million years ago, resulting in the glacial climate of the last 20 million years. At least four major glacial epochs, each lasting nearly l00 million years, have occurred in the last billion years. The last epoch ended 250 million years ago. The present glacial period may yet prove to be another major epoch.

The Solar Maximum Mission was launched Feb. l4, l980, and in December l980, after l0 months of normal operation, the satellite's attitude control system lost its capability to point precisely at the Sun. less precise pointing technique was subsequently achieved by spinning the spacecraft so that it rotates every six minutes. In this configuration, three of the satellite's seven instruments continue to acquire useful data.

The Sun crosses ACRIM's field of view several times per orbit, providing an adequate quality and quantity of data for the experiment's primary mission objectives, though the data are deficient in some solar physics information (like the observation of solar oscillations).

While the spin-stablized pointing allows continued study of the Sun, the satellite's 550 kilometer- (340 mile-) altitude orbit is slowly decaying due to atmospheric drag.

At the present rate of decay, the Solar Maximum Mission satellite will reenter Earth's atmopshere in l984. Its demise will leave at least three-year gap in the precise record of solar irradiance observations made for the National Climate Program. The earliest successor experiment to the irradiance monitor is planned for deployment on NASA's Upper Atmospheric Research Satellite (UARS) in l987.

Solar Maximum Mission was the first NASA satellite designed to be retrieved by the Space Shuttle. An effort to retrieve the satellite, repair it on orbit and redeploy it in late l983 is being studied by NASA. The rejuvenated satellite would allow scientists to observe wide range of solar phenomena in different part of the solar activity cycle and sustain solar irradiance monitoring with the precision required for climate studies.

Authorization for the proposed Solar Maximum Mission repair mission is currently under consideration by Congress.

The irradiance monitoring experiment is conducted by JPL as part of the Environmental Observations program of NASA's Office of Space Science and Applications. Solar Maximum Mission is managed by NASA's Goddard Space Flight Center, Greenbelt, Md.

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