CAOSP abstracts, Volume: 41, No.: 2, year: 2011

Abstract: The last solar activity minimum during 2008/09 was unusually long and with extended periods without sunspots. During this period the total solar irradiance (TSI) was much lower than during the previous minima and no solar activity proxies show similarly low values. Proxy models for TSI use a measure for the darkening of sunspots, the so-called photometric sunspot index (PSI) and for the brightening of faculae and network a chromospheric index. Because none of these can explain the low TSI, a further component is needed, which describes the trend between minima due to a still controversially discussed mechanism. A new algorithm for the calculation of PSI is described which uses individual factors for the different observing stations and a better representation of the size-dependent contrast of spots. The proxy model based on the new PSI, the long- and short-term Mg II index and a trend based on the minima values of the open field explains almost 85 % of the variance of TSI over the last three solar cycles. Moreover, it confirms the factor of ≈ 4 between the observed trend of TSI and those of the chromospheric and other solar activity indices.