Solar Irradiance Variability Since 1978

Abstract

Since November 1978 a set of total solar irradiance (TSI) measurements from space is available, yielding a time series of more than 25 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 results from the detailed analysis of the VIRGO radiometry allow a good understanding of the effects influencing the long-term behaviour of classical radiometers in space. Thus, a re-analysis of the behaviour of HF/NIMBUS-7 and ACRIM-I/SMM was indicated. For the former the situation is complicated by the fact that there are no in-flight means to determine changes due to exposure to solar radiation by comparison with a less exposed radiometer on the same spacecraft. The geometry and optical property of the cavity of HF is, however, very similar to the PMO6-type radiometers, so the behaviour of the PMO6V radiometers on VIRGO can be used as a model. ACRIM-I had to be revised mainly due to a henceforth undetected early increase and a more detailed analysis of its degradation. The results are not only important for solar radiometry from space, but they also provide a more reliable TSI during cycle 21. The differences between the revised PMOD composite and the ACRIM and IRMB are discussed by comparison with a TSI reconstruction from Kitt-Peak magnetograms. As the PMOD composite is the only one which has reliable data for cycle 21, the behaviour of the three solar cycles can now be compared and the similarities and differences discussed.

Solar change and climate: an update in the light of the current exceptional solar minimum

“…Composites of measurements, related to a standard source, have given mean values I TS at Earth near 1365.5 W m −2 (e.g. Fröhlich 2006). However, data from the latest instrument, the total irradiance monitor (TIM) on the SORCE satellite launched in 2003, are yielding very similar variations, but values are persistently lower (see §5 and figure 2) with I TS = 1361 W m −2 (Kopp et al 2005).…”

“…In addition, the instruments degrade and some have been self-calibrating, whereas earlier instruments require retrospective re-calibration. The three available TSI data composites are termed: PMOD (Fröhlich 2006), ACRIM (Willson & Mordvinov 2003) and IRMB (Dewitte et al 2004). Figure 2 shows how the various observations of TSI over the recent decline from solar maximum to solar minimum compare.…”

Total solar irradiance since 1996: is there a long-term variation unrelated to solar surface magnetic phenomena?

“…IRMB composite (Institut Royal Météorologique Belgique) (Dewitte et al 2004), 3. PMOD composite (PhysikalischMeteorologisches Observatorium Davos) (Fröhlich 2006) As is discussed in Fröhlich (2006Fröhlich ( , 2009a, the ACRIM and the IRMB composites show unreasonable behavior in time, whereas the PMOD composite does not. Therefore we restrict ourselves in this study on the PMOD composite.…”

The DynaMICCS perspective

“…8 Composite TSI time series after bringing all instruments to the same space absolute radiometric reference (SARR) scale. From [49] agree on the shape of the solar irradiance variation within ±0.2 W/m 2 but SORCE produces an absolute value of the irradiance, 5 W lower than the other instruments. The instruments aboard DynaMICCS must reduce the uncertainty on the long-term solar irradiance variation by use of independent and simultaneous radiometers over long time periods.…”