Abstract
Direct measurements of the solar constant—the total irradiance at mean Sun-Earth distance—during the last ten years from satellites show variations over time scales from minutes to years and decades. Towards periods of several hours the power is steadily increasing and may be partly due to solar gravity modes. The most important variance is in the range from days to several months and is related to the photospheric features of solar activity: decreasing the irradiance during the appearance of sunspots, and increasing it by faculae and the magnetic network. Long-term modulation by the 11-year activity cycle is observed conclusively with the irradiance being higher during solar maximum. All these variations can be explained—at least qualitatively—by their manifestation on the photosphere. For the long-term changes the simultaneous changes of the frequencies of solar p-mode oscillations suggest a more global origin of the variations. Indeed, it seems that the observed irradiance modulation is true luminosity change with the magnetic cycle of the Sun.
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