1505.01040
Dimming of the mid-20th century sun
Foukal
Advances in understanding of the white line faculae [bright regions on the sun, linked to subsequent appearance of sunspots in the same area] measured at the Royal Greenwich Observatory from 1874 to 1976 suggest that they offer a more direct measure of solar brightness by small diameter photospheric magnetic flux tubes than do chromospheric proxies. Proxies such as the area of Ca K plages [an unusually bright region on the sun], the Mg index or the microwave flux include many dark photospheric structures as well as pores and sunspots. The reconstruction of variation in total solar irradiance, TSI, based on the faculae indicates that the sun dimmed by almost 0.1% in the mid-20th century rather than brightening as represented in previous reconstructions. This dimming at the sun's highest activity level since the 17th century is consistent with the photometric behavior observed in somewhat younger sun like stars. The prolonged TSI decrease may have contributed more to the cooling of climate between about 1940 and 1970 than present models indicate.
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