Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/JPLNEWS1/0987.PR

Text File | 1993-04-23 | 8KB | 296 lines

PUTECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 9ll09. TELEPHONE [phone]


FOR IMMEDIATE RELEASE

          An l8-month decrease in the Sun's energy output,
recently

detected by a NASA satellite, may have been a factor in this

year's unusually harsh winter, according to a 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

a cause-and-effect relationship between the Sun's energy
output

and changes in Earth's weather and climate.

          A persistent decrease of a tenth of a 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 a 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, î
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 a
one
percent decrease would lower Earth's mean global temperature
by

more than l degree Kelvin (2 degrees Fahrenheit).  According
to

some models, a 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 î
atmosphere.  A 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 a 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 a longer-term trend in the Sun's irradiance.  Years of
careful

measurements would be required to identify such a 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. î
          The monitor also detected evidence of solar
oscillatory

phenomena -- global pulsations whose effects extend deep into
the

Sun.  The oscillations have a 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 C (77 F).  The current average global temperature is
l5

C (59 F).

          A gradual trend toward a 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
  îsatellite's attitude control system lost its capability to
point

precisely at the Sun.  A 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 a 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.
  î          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 a wide range of solar
phenomena

in a 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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