In October, the Upper Atmosphere Research Satellite (UARS) completed
the first measurement of the solar ultraviolet radiation spectrum
over the duration of an 11 year solar cycle, a period marked by
cyclical shifts in the Sun’s activity. This long measurement record
by two instruments aboard UARS will give researchers better insight
into how fluctuations in the Sun’s energy affect ozone and the
Earth’s climate. In turn, the dataset gives scientists tools to
document the influence of man-made chemicals on ozone loss.
Though mission success was initially declared only 18 months after
its launch in September 1991, UARS has continued to track ozone
levels and atmospheric gases that react with ozone. The satellite has
now also recorded the Sun’s influence on ozone and other gases over
an entire solar cycle.
During the 11-year solar cycle the Sun undergoes periodic changes in
activity from the “solar maximum,” to a period of quiet called the
“solar minimum.” During the solar maximum there are many sunspots,
solar flares, and coronal mass ejections, which can affect
communications and the atmosphere here on Earth.
“Having a complete solar cycle of data provides information necessary
to distinguish the natural variations in the Earth’s atmosphere from
man-made variations,” said Charles Jackman, UARS Project Scientist at
NASA’s Goddard Space Flight Center, Greenbelt, Md.
“UARS has lasted so long that we now have an 11 year mission with a
single set of observations spanning the entire solar cycle,” said
Gary Rottman, a senior scientist at the University of Colorado and
Principal Investigator for the SOLar Stellar InterComparison
Experiment (SOLSTICE) instrument on UARS.
This complete solar cycle UV radiation dataset provides key
measurements toward better determination of the roles of natural and
man-made influences on ozone.
Also, by observing a full solar cycle, scientists hope to use the
additional data to better understand the Sun’s behavior.
Observatories on the Earth have found fewer sunspots in this solar
cycle than the last one, but UARS measurements indicate the amount of
UV radiation that struck Earth’s atmosphere during each solar maximum
was about the same.
“The expected correlation between sunspot activity and UV irradiance
over the long term was not found,” said Linton Floyd, a researcher
working at the Naval Research Laboratory, Washington, and Project
Scientist for the Solar Ultraviolet Spectral Irradiance Monitor
(SUSIM) instrument on UARS. Floyd hopes that more long-term records
will help clear up such mysteries about the Sun.
UARS includes ten instruments designed to understand the radiation,
chemistry, and dynamics of the Earth’s upper atmosphere. Of those
ten, seven instruments still work.
The SUSIM and SOLSTICE instruments measure UV light from the Sun and
provide insights into the relationship between UV radiation and
atmospheric ozone. These two instruments were independently
calibrated, each providing a check on the other. Another set of
instruments measure gases like ozone, methane, water vapor, and
chlorofluorocarbons (CFCs) in Earth’s atmosphere. The third group
measures winds in the stratosphere, mesosphere, and the lower
thermosphere and help researchers understand the global movement of
gases.
In January 2003, NASA will launch the Solar Radiation and Climate
Experiment (SORCE) satellite, which will provide further measurements
of the Sun. By having an overlap with UARS, NASA will have two
satellites making essentially the same measurements simultaneously,
thereby providing a “truthing” for comparisons and an even longer
term data record, Floyd said.
For more Information, please see:
http://www.gsfc.nasa.gov/topstory/2002/1114uars.html
