The TSI record from PREMOS/PICARD

International audienceSince the launch of PICARD in 2010 the PMO6 absolute radiometers of the PREMOS experiment had been measuring Total Solar Irradiance. We will present the TSI data record, corrected for sensor degradation, and review the stability of the radiometers during the entire mission. Implications for the uncertainty of Composite Total Solar Irradiance time Series data sets will be discussed

Assessing the beginning to end-of-mission sensitivity change of the PREcision MOnitor Sensor total solar irradiance radiometer (PREMOS/PICARD)

The switching of the total solar irradiance (TSI) backup radiometer (PREMOS-B) to a primary role for 2 weeks at the end of the PICARD mission provides a unique opportunity to test the fundamental hypothesis of radiometer experiments in space, which is that the sensitivity change of instruments due to the space environment is identical for the same instrument type as a function of solar-exposure time of the instruments. We verify this hypothesis for the PREMOS TSI radiometers within the PREMOS experiment on the PICARD mission. We confirm that the sensitivity change of the backup instrument, PREMOS-B, is similar to that of the identically-constructed primary radiometer, PREMOS-A. The extended exposure of the backup instrument at the end of the mission allows for the assessment, with an uncertainty estimate, of the sensitivity change of the primary radiometer from the beginning of the PICARD mission compared to the end, and of the degradation of the backup over the mission. We correct six sets of PREMOS-B observations connecting October 2011 with February 2014, using six ratios from simultaneous PREMOS-A and PREMOS-B exposures during the first days of PREMOS-A operation in 2010. These ratios are then used, without indirect estimates or assumptions, to evaluate the stability of SORCE/TIM and SOHO/VIRGO TSI measurements, which have both operated for more than a decade and now show different trends over the time span of the PICARD mission, namely from 2010 to 2014. We find that by February 2014 relative to October 2011 PREMOS-B supports the SORCE/TIM TSI time evolution, which in May 2014 relative to October 2011 is ~0.11 W m−2, or ~84 ppm, higher than SOHO/VIRGO. Such a divergence between SORCE/TIM and SOHO/VIRGO over this period is a significant fraction of the estimated decline of 0.2 W m−2 between the solar minima of 1996 and 2008, and questions the reliability of that estimated trend. Extrapolating the uncertainty indicated by the disagreement of SORCE/TIM and PREMOS with respect to SOHO/VIRGO, we can conclude that it is currently not possible to assess centennial timescale changes in solar irradiance based on any of the presently existing TSI composites. It is imperative to accurately estimate solar irradiance changes from observations in order to extrapolate centennial scale trends important for understanding both long-term solar irradiance changes and the Sun’s influence on the Earth’s climate

Assessing the beginning to end-of-mission sensitivity change of the PREcision MOnitor Sensor total solar irradiance radiometer (PREMOS/PICARD)

The switching of the total solar irradiance (TSI) backup radiometer (PREMOS-B) to a primary role for 2 weeks at the end of the PICARD mission provides a unique opportunity to test the fundamental hypothesis of radiometer experiments in space, which is that the sensitivity change of instruments due to the space environment is identical for the same instrument type as a function of solar-exposure time of the instruments. We verify this hypothesis for the PREMOS TSI radiometers within the PREMOS experiment on the PICARD mission. We confirm that the sensitivity change of the backup instrument, PREMOS-B, is similar to that of the identically-constructed primary radiometer, PREMOS-A. The extended exposure of the backup instrument at the end of the mission allows for the assessment, with an uncertainty estimate, of the sensitivity change of the primary radiometer from the beginning of the PICARD mission compared to the end, and of the degradation of the backup over the mission. We correct six sets of PREMOS-B observations connecting October 2011 with February 2014, using six ratios from simultaneous PREMOS-A and PREMOS-B exposures during the first days of PREMOS-A operation in 2010. These ratios are then used, without indirect estimates or assumptions, to evaluate the stability of SORCE/TIM and SOHO/VIRGO TSI measurements, which have both operated for more than a decade and now show different trends over the time span of the PICARD mission, namely from 2010 to 2014. We find that by February 2014 relative to October 2011 PREMOS-B supports the SORCE/TIM TSI time evolution, which in May 2014 relative to October 2011 is ~0.11 W m−2, or ~84 ppm, higher than SOHO/VIRGO. Such a divergence between SORCE/TIM and SOHO/VIRGO over this period is a significant fraction of the estimated decline of 0.2 W m−2 between the solar minima of 1996 and 2008, and questions the reliability of that estimated trend. Extrapolating the uncertainty indicated by the disagreement of SORCE/TIM and PREMOS with respect to SOHO/VIRGO, we can conclude that it is currently not possible to assess centennial timescale changes in solar irradiance based on any of the presently existing TSI composites. It is imperative to accurately estimate solar irradiance changes from observations in order to extrapolate centennial scale trends important for understanding both long-term solar irradiance changes and the Sun’s influence on the Earth’s climate

The PREMOS/PICARD instrument calibration

International audiencePREMOS is a space experiment scheduled to fly on the French solar mission PICARD. The experiment comprises filter radiometers and absolute radiometers to measure the spectral and total solar irradiance. The aim of PREMOS is 1. to contribute to the long term monitoring of the total solar irradiance, 2. to use irradiance observations for 'nowcasting' the state of the terrestrial middle atmosphere and 3. to provide long term sensitivity calibration for the solar imaging instrument SODISM on PICARD. In this paper we describe the calibration of the instruments. The filter radiometer channels in the visible and near IR were characterized at PMOD/WRC and the UV channels were calibrated at PTB Berlin. The absolute radiometers were compared with the World Radiometric Reference at PMOD/WRC and a power calibration relative to a primary cryogenic radiometer standard was performed in vacuum and air at NPL