Relationship between solar activity and direct solar irradiance in Madrid (1910–1929)
Abstract Some works have reported variations in direct solar irradiance at surface with solar activity one order of magnitude higher than changes in total solar irradiance at the top of the atmosphere (TSI) over the solar cycle. In the present study, daily pyrheliometer records from the Astronomical Observatory of Madrid for the period 1910–1929 have been recovered. Metadata have been used to remove measurements affected by clouds, local pollution or any other factor dimming solar radiation noticed by means of visual observations of the state of the sky near the solar disk. Furthermore, corrections for volcanic eruptions, outliers and seasonal variations have been applied. When long-term changes in atmospheric transparency are taken into account in the analysis, non-significant variations in direct solar irradiance at surface with Sunspot Number (of the same order of magnitude as the variations in TSI) are found. Otherwise, a spurious high anticorrelation is obtained. This indicates, firstly, the influence of solar activity on solar irradiance at surface is so weak that atmospheric factors mask it, and secondly, careful analyses in order to infer this relationship have to be carried out not to reach results leading to wrong interpretations.