Total Solar Irradiance, 1978 - 1999
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Caption: Measurements from six independent space-based radiometers since 1978 (top) have been combined to produce the composite total solar irradiance (TSI) over two decades (bottom). They show that the Sun's output fluctuates during each 11-year sunspot cycle, changing by about 0.1 percent between maxima (1980 and 1990) and minima (1987 and 1997) of solar activity. Temporary dips of up to 0.3 percent and a few days duration are the result of large sunspots passing over the visible hemisphere. The larger number of sunspots near the peak in the 11-year cycle is accompanied by a general rise in magnetic activity that creates an increase in the luminous output that exceeds the cooling effects of sunspots.
The data are from the Hickey-Frieden (HF) radiometer of the Earth Radiation Budget (ERB) experiment on the Nimbus-7 spacecraft (1978-1992), the two Active Cavity Radiometer Irradiance Monitors (ACRIM I and II) placed aboard the Solar Maximum Mission satellite (SMM, 1980-1989) and the Upper Atmosphere Research Satellite (UARS, 1991-), respectively, and the VIRGO radiometers flying on the SOlar and Heliospheric Observatory (SOHO, 1996-). Also shown are the data from the radiometer on the Earth Radiation Budget Satellite (ERBS, 1984-), and SOVA2 as part of the Solar Variability Experiment (SOVA) on the European Retrievable Carrier (EURECA, 1992-1993). Offsets among the various data sets are the direct result of uncertainties in the absolute radiometer scale of the radiometers (±0.3%). Despite these biases, each data set clearly shows varying radiation levels that track the overall 11-year solar activity cycle.
References
T.J. Quinn and C. Fröhlich. Accurate radiometers should measure
the output of the sun.
C. Fröhlich and J. Lean. The sun's total irradiance: Cycles and
trends in the past two decades and associated climate change
uncertainties.
Instruments: VIRGO + 5 other missions (see last paragraph of image caption); Taken: 1978-1999
