Reconstruction of solar irradiance variations in cycles 21-23 based on surface magnetic fields

Aims. We present a reconstruction of total solar irradiance (TSI) back to 1974, i.e. from the minimum of cycle 21 to the declining phase of cycle 23. We also present a cross-calibration between the magnetograms obtained by the 512 channel magnetograph and the spectromagnetograph at Kitt Peak. Methods. The TSI reconstruction is carried out using data from the 512-channel Diode Array Magnetograph and the newer spectromagnetograph on Kitt Peak. The model is based on the assumption that all irradiance changes on time-scales of a day and longer are entirely due to the variations of the surface distribution of the solar magnetic field. The reconstructed irradiance is compared with the composite of total solar irradiance measurements from PMOD/WRC (version 41). Results. A good correspondence is found with the PMOD TSI composite, with no bias between the three cycles on time-scales longer than the solar rotation period, although the accuracy of the TSI reconstruction is somewhat lower when 512 channel magnetograph data are used. This suggests that the same driver of the irradiance variations, namely the evolution of the magnetic flux at the solar surface, is acting in cycles 21-23. Different methods of comparing the magnetograms obtained by the two Kitt Peak magnetographs give somewhat different results, with factors by which 512 channel data must be divided in the range 1.38-1.63 being found. This is due to the non-linearity of the relationship between the magnetic field measured by the two instruments.