Total solar irradiance variability: What have we learned about its variability from the record of the last three solar cycles?

Since November 1978 a set of total solar irradiance (TSI) measurements from space is available, yielding a time series of almost 30 years. Presently, there are three TSI composites available, called PMOD, ACRIM and IRMB, which are all constructed from the same original data, but use different procedures to correct for sensitivity changes. The PMOD composite is the only one which also corrects the early HF data for degradation. The origin of the differences between the three composite are discussed by comparison with the record of ERBE. For the discussion of the similarities and differences of the three cycles the PMOD composite is used. The most interesting feature is the low value of TSI at the present minimum which cannot be seen in proxies such as F10.7, CaK and MgII indices. Thus, we see for the first time an effect on TSI which is not due to the direct effect of the superficial magnetic structures of solar activity.