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Uses of Total Solar Irradiance Data
The first long term solar monitoring was done by the Earth Radiation Budget (ERB) experiment on the NOAA Nimbus 7 spacecraft beginning in late 1978. JPL's ACRIM I experiment flew on the Solar Maximum Mission Spacecraft in 1980. ACRIM I compiled a nine and one half year database of TSI measurements and was the first instrument to clearly detected the intrinsic variability of TSI. ACRIM I was also the first instrument to detect a clear relationship between sunspot area and total irradiance. When there are many sunspots on the observed side of the sun, the measured TSI decreases.
Total solar irradiance (TSI) measurements serve two primary purposes. The first is to provide a long term data set for climate modelers to use in creating global climate models. ACRIM III data is also used in solar physics studies. Solar global oscillations have been detected in the ACRIM I data in both pressure modes (time scales of minutes) and possibly in gravity modes (time scales of hours to days).
The use of ACRIM data in climate modeling has been supported by the U.S. Global Change Research Program of the National Science Foundation. The sun is the key source of energy for the earth's climate and this energy is primarily transmitted in the optical wavelengths (ultraviolet and visible light). Climate modelers want to know how much of the sun's energy reaches the earth and to do this it is necessary to make two primary measurements. The first measurement is the total irradiance delivered by the sun to the earth. This is the measurement ACRIM makes. The second measurement is to determine how much of the incident light is reflected back into space so that it does not become part of the earth's energy system. Subtracting the amount of reflected light from the total delivered results in the amount of energy that becomes part of the earth system. It is this energy that creates the winds, heats the land, and helps to drive the oceans currents. Climate scientists take the total input energy along with complementary measurements of ocean currents, winds, and surface temperatures and make models that they hope will predict the climate behavior of the earth.
Solar physicists use the solar global oscillation information provided by ACRIM. Imagine that the sun is a ball of fluid (like JELLO) hanging in space. Now imagine the ball of JELLO wiggling. Those wiggles are oscillations. Physicists describe these oscillations as two different types - P-mode (pressure mode) and g-mode (gravity mode). These oscillations or waves that travel through the sun can be used to tell us things about what is going on in the interior of the sun. Using the periods (repetitive cycles) of these oscillations, solar physicists make theories about what the inside of the sun is like. Interpretation of the 5 minute oscillation results from the ACRIM I experiment has placed an upper limit on the differential rotation of the outer solar atmosphere as a function of the solar radius, and therefore on solar oblateness, providing support for the interpretation of the perihelion of Mercury observations.
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