Total solar irradiance since 1996: is there a long-term variation unrelated to solar surface magnetic phenomena?

Context. Total solar irradiance (TSI) has been measured with space-based instruments since 1978. The TSI during the recent solar minimum in 2009 has been lower than the two former minima around the years 1986 and 1996, which points to a long-term decrease. Aims. In this study, we address the question of whether the observed decrease in the TSI is the result of evolving solar surface magnetism (sunspots and faculae). Methods. We use a TSI model that is solely based on solar surface magnetic phenomena (sunspots and faculae including network). The information needed for this model is derived from Carrington rotation magnetogram and photogram synoptic charts measured with the Michelson Doppler Imager (MDI) instrument on-board the Solar and Heliospheric Observatory (SOHO). By combining these data with solar atmosphere calculations, TSI is reconstructed. Results. The TSI is reconstructed from June 1996 to May 2010. From the solar minimum of 1996 to the solar maximum of 2004 the model reproduces the observations well, but it fails to explain the observed decrease in TSI in the solar minimum of 2009 and the very recent data of 2010. Conclusions. The difference between modeled and observed TSI might be the result of underrepresented weak magnetic fields in the Carrington rotation synoptic charts, an uncertainty in the TSI measurement, or a decline of the global temperature of the photosphere. If latter were true, this would have important implications for reconstructions of TSI in the past. In order to study if an underrepresentation of weak magnetic fields in the Carrington rotation synoptic charts is the explanation for the difference between our model and the observation, full-disk images with higher spatial and temporal resolution should be analyzed in future