Solar cycle
Solar irradiance

The total solar irradiance (TSI) is the amount of solar radiative energy incident on the Earth's upper atmosphere. TSI variations were undetectable until satellite observations began in late 1978. A series of radiometers were launched on satellites since the 1970s. TSI measurements varied from 1355 to 1375 W/m across more than ten satellites. One of the satellites, the ACRIMSAT was launched by the ACRIM group. The controversial 1989–1991 "ACRIM gap" between non-overlapping ACRIM satellites was interpolated by the ACRIM group into a composite showing +0.037%/decade rise. Another series based on the ACRIM data is produced by the PMOD group and shows a −0.008%/decade downward trend. This 0.045%/decade difference can impact climate models. However, reconstructed total solar irradiance with models favor the PMOD series, thus reconciling the ACRIM-gap issue.
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