3.4 Total irradiance

The Total Solar Irradiance (TSI) is the radiant energy emitted by the Sun at all wavelengths crossing a square meter each second outside the Earth’s atmosphere. Although ground-based measurements of this “solar constant” and its variability were made decades ago (Abbot et al., 1913), accurate measurements of the Sun’s total irradiance have only become available with access to space. Several satellites have carried instruments designed to make these measurements: Nimbus-7 from November, 1978 to December, 1993; the Solar Maximum Mission (SMM) ACRIM-I from February, 1980 to June, 1989; the Earth Radiation Budget Satellite (ERBS) from October, 1984 to December, 1995; NOAA-9 from January, 1985 to December, 1989; NOAA-10 from October, 1986 to April, 1987; Upper Atmosphere Research Satellite (UARS) ACRIM-II from October, 1991 to November, 2001; ACRIMSAT ACRIM-III from December, 1999 to the present; SOHO/VIRGO from January, 1996 to the present; and SORCE/TIM from January, 2003 to the present.

While each of these instruments is extremely precise in its measurements, their absolute accuracies vary in ways that make some important aspects of the TSI subjects of controversy. Figure 10View Image shows daily measurements of TSI from some of these instruments. Each instrument measures the drops in TSI due to the formation and disk passages of large sunspot groups as well as the general rise and fall of TSI with the sunspot cycle (Willson and Hudson, 1988). However, there are significant offsets between the absolute measured values. Intercomparisons of the data have lead to different conclusions. Willson (1997) combined the SMM/ACRIM-I data with the later UARS/ACRIM-II data by using intercomparisons with the Nimbus-7 and ERBS and concluded that the Sun was brighter by about 0.04% during the cycle 22 minimum than is was during the cycle 21 minimum. Fröhlich and Lean (1998) constructed a composite (the PMOD composite) that includes Nimbus-7, ERBS, SMM, UARS, and SOHO/VIRGO which does not show this increase.

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Figure 10: Daily measurements of the Total Solar Irradiance (TSI) from instruments on different satellites. The systematic offsets between measurements taken with different instruments complicate determinations of the long-term behavior.

Comparing the PMOD composite to sunspot number (Figure 11View Image) shows a strong correlation between the two quantities but with different behavior during cycle 23. At its peak, cycle 23 had sunspot numbers about 20% smaller than cycle 21 or 22. However, the cycle 23 peak PMOD composite TSI was similar to that of cycles 21 and 22. This behavior is similar to that seen in 10.7 cm flux in Figure 9View Image but is complicated by the fact that the cycle 23 PMOD composite falls well below that for cycle 21 and 22 during the decline of cycle 23 toward minimum while the 10.7 cm flux remained above the corresponding levels for cycles 21 and 22.

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Figure 11: The PMOD composite TSI vs. International Sunspot Number. The filled circles represent smoothed monthly averages for cycles 21 and 22. The open circles represent the data for cycle 23. While the TSI at the minima preceding cycles 21 and 22 were similar in this composite, the TSI as cycle 23 approaches minimum is significantly lower. The TSI at cycle 23 maximum was similar to that in cycles 21 and 22 in spite of the fact that the sunspot number was significantly lower for cycle 23.

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