  Total and spectral solar irradiance are key parameters in the assessment of
solar influence on changes in the Earth's climate. We present a reconstruction
of daily solar irradiance obtained using the SATIRE-S model spanning 1974 to
2013 based on full-disc observations from the KPVT, SoHO/MDI and SDO/HMI.
SATIRE-S ascribes variation in solar irradiance on timescales greater than a
day to photospheric magnetism. The solar spectrum is reconstructed from the
apparent surface coverage of bright magnetic features and sunspots in the daily
data using the modelled intensity spectra of these magnetic structures. We
cross-calibrated the various data sets, harmonizing the model input so as to
yield a single consistent time series as the output. The model replicates 92%
of the variability in the PMOD TSI composite including the secular decline
between the 1996 and 2008 solar cycle minima. The model also reproduces most of
the variability in observed Lyman-alpha irradiance and the Mg II index. The UV
solar irradiance measurements from the UARS and SORCE missions are mutually
consistent up to about 180 nm before they start to exhibit discrepant
rotational and cyclical variability, indicative of unresolved instrumental
effects. As a result, the agreement between model and measurement starts to
deteriorate above this wavelength. As with earlier similar investigations, the
reconstruction cannot reproduce the overall trends in SORCE/SIM SSI. We argue,
from the lack of clear solar cycle modulation in the SIM record and the
inconsistency between the total flux recorded by the instrument and TSI, that
unaccounted instrumental trends are present. The reconstruction is consistent
with observations from multiple sources, demonstrating its validity. It also
provides further evidence that photospheric magnetism is the prime driver of
variation in solar irradiance on timescales greater than a day.
