Solar irradiance variations

Since November 1978 a set of total solar irradiance (TSI) measurements from space is available, yielding a time series of nearly 25 years. From measurements made by different space radiometers (HF on NIMBUS 7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of TSI can be constructed. This leads to a reliable record of TSI with an overall precision of the order of +/-0.1 Wm(-2) over the 25 years. Within this uncertainty no significant secular trend of TSI can be detected. This contrasts the results of the ACRIM composite which does show a significant trend. This difference is analyzed in detail and it can be demonstrated that the assumptions for the ACRIM composite are incorrect. The variability of TSI over the last cycle is characterized by analyzing the statistics of the time series and the behaviour of the power spectrum. Moreover, the composite TSI is compared to a proxy model based on sunspot darkening and brightening due to faculae and network, both in terms of time series and in frequency space. Since early 1996 spectral measurements by filter-radiometers of VIRGO provide continuous time series of spectral solar irradiance (SSI) at 402, 500 and 862 nm. These time series are analyzed and compared to TSI providing detailed information about the redistribution of energy within the spectrum during changes of TSI.