Re-Analysis of the Long-Term Changes of the NIMBUS-7 Radiometer and Behavior of Total Solar Irradiance During Solar Cycle 21
Claus Fröhlich [cfrohlich@pmodwrc.ch], Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center, Davos Dorf, Switzerland

Only one radiometer - called Hickey-Frieden (HF) - is within the ERB package on NIMBUS-7 for the measurement of total solar irradiance (TSI) and thus changes due to exposure to solar radiation cannot be directly determined 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, it is essentially a copy of it with increased size. For the PMO6V on VIRGO/SOHO two main effects have been identified — a rapid early increase and a slow decrease, normally termed degradation, which can be modeled with a hyperbolic function taking the actual dose into account (Fröhlich, 2003). The corrections used by Fröhlich and Lean (1998) for the composite were based on early results from VIRGO and used simple exponential functions. With the recent results from VIRGO a re-analysis of the long-term behavior of HF is necessary. The results are not only important for solar radiometry from space, but also to improve the reliability of TSI before the start of ACRIM-I in early 1980. The latter will allow to better quantify the behavior of solar cycle 21 and to compare it with the two recent ones which differ in several aspects.