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Solar Variability and Irradiance Monitoring (SOVIM) within SOLAR on ISS

SOVIM project aims to measure total and spectral solar irradiance and calibrate the total solar irradiance records. The instruments, contributors, and technical details are discussed in this report.

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Solar Variability and Irradiance Monitoring (SOVIM) within SOLAR on ISS

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  1. Solar Variability and Irradiance Monitoring (SOVIM) within SOLAR on ISS Claus Fröhlich Physikalisch-Meteorologisches Observatorium Davos World Radiation Center CH 7260 Davos Dorf Switzerland and the SOVIM-Team Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  2. Das SOVIM Team • The experiment SOVIM was developed at PMOD/WRC in cooperation with IRMB. The main objective is the measurement of the total and spectral solar irradiance and calibration of the now 30-years record of TSI. • Principle Investigator: Claus Fröhlich, PMOD/WRC • Instrument CoI: Wolfgang Finsterle und Christoph Wehrli, PMOD/WRC, Steven Dewitte und Sabri Mekaoui, IRMB • Further CoI: Bernhard Fleck, ESA, Nigel Fox, NPL, Judith Lean, NRL, Gerhard Schmidtke, IPM/FHG, Werner Schmutz, PMOD/WRC, Sami Solanki, MPI, Gerard Thuillier, SA/CNRS • Technical Personell: Hansjörg Roth (TM), Dani Pfiffner, Marcel Specha, Jules Wyss, PMOD/WRC; André Chevalier, Christian Conscience, Pierre Malcorps, IRMB Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  3. Solar Variability and Irradiance Monitoring (SOVIM) • Originally, SOVIM was proposed as a re-flight of SOVA which flew on EURECA • The physical dimensions were not really adequate and a new package had to be designed which fitted in an overall envelope of about 150x380x510 mm3 • Moreover, the thermal control had to be completely changed from a collectively controlled instrument mounted on a cold plate to a independently, self-contained thermally controlled experiment. • This led to the present design of the instrument which still contains some of the original H/W, but looks completely different. The radiation shield in front controls the temperature of the instrument during sunlit and deep space looking periods. • SOVIM contains: 4 radiometers of different type (2 PMO6V, 1 PMO6R (from SOVA/EURECA), DIARAD with 2 channels, 2 filter radiometers (SPM) and a Two-axis Sun Sensor (TASS) Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  4. SOLSPEC SolACES Pointing Device SOVIM SOVIM on the SOLAR CPD • Course Pointing Device (CPD) of SOLAR with the 3 instruments mounted during EMC tests • The range of the CPD around the de-rotation axes allows measure-ment of the Sun during ca. 20 minutes per orbit. • The range of the other axis allows observa-tions for about one third or half of all the orbits during an year. 720 hours per year were guaranteed. Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  5. Instruments within SOVIM The instruments are contributed by IRMB and PMOD/WRC, with the latter being responsible for the overall scientific and technical management with PI: Claus Fröhlich and TM: Hansjörg Roth. The radiometers are based on electrically calibrated cavity receivers and measure the total solar irradiance (TSI). They are operated in active mode, hence they are often called ACR (active-cavity radiometers). There are three types included: 1 PMO6-R and 2 PMO6-V; both are developed by PMOD/WRC, the former has flown on EURECA, the latter is of the same type as those of SoHO/VIRGO; DIARAD is a dual channel radiometer developed by IRMB, it is similar to SOLCON, flown on the shuttle and to DIARAD within SoHO VIRGO. Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  6. Instruments within SOVIM The instruments are contributed by IRMB and PMOD/WRC, with the latter being responsible for the overall scientific and technical management with PI: Claus Fröhlich and TM: Hansjörg Roth. The filterradiometers (formerly called sunphotometers, SPM) are developed by PMOD/WRC and measure the spectral solar irradiance (SSI) at 402, 500 and 862 nm with a bandwidth of 5 nm, as on SOHO/VIRGO. The coarse pointing device does not provide very accurate pointing, thus a two-axis sun-sensor(TASS, already flown on EURECA) is included, which measures the pointing in two directions with a sensitivity of a few arcsec. Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  7. Science Objectives: TSI • The absolute uncertainty of the presently used radio-meters in space is of the order 0.2%. • As the precision is much better, simultaneous measurements with different instruments on different platforms allows to combine them into a composite. It spans now almost three solar cycles and allows detailed studies of the solar variability. The absolute uncertainty, however, is not really improved. • SOVIM can only provide calibration points, because of the often and for long time interrupted observa-tions. Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  8. Science Objectives: TSI The SOVIM radiometers provide calibration points, and this is very important as TSI during the present minimum is low. This composite is available from http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  9. Scientific Objectives: SSI • The filter radiometers have been mainly used for helioseismology due to their very low inherent noise (<0.02ppm). However, the ones on VIRGO have also demonstrated their ability to monitor the spectral irradiance in the blue, green and red bands over the now almost 13 years of operation. • The measuring pace on ISS will obviously not allow to do helioseismology, so we are concentrating – again – to get a calibration. • We have learned how to make out of the raw measurements decent time series covering now a full solar cycle. • Now we want to know where the SOVIM data are positioned…. These data are available from ftp://ftp.pmodwrc.ch/pub/data/irradiance/virgo/SSI Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  10. SOVIM Results: TSI What do we get now from the observations we have since launch. Unfortunately I can only report on TSI, but it is already not only interesting, but very important. Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  11. Conclusions • SOVIM provides reliable TSI values for the whole period of operation (we have 6 months only, but good results). • SOVIM will also provide reliable SSI calibration of the SoHO time series at 402, 500 and 862 nm. • These data do complement existing time series and help to improve the reliability of any composite TSI and SSI. Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  12. Mission Performance • You have got a mission time line which shows that the objective has not been achieved (only about 50% of what was promised!). • This is documented in SOVIM History080215.pdf which has been distributed to everybody – just read it carefully! You will be astounded what or better what not they (ESA, ALENIA etc) provided to us… • As to the pointing I would like to remind you about several issues, which had finally a solution from our initiative and not by ESA nor by ALENIA. • There are still some open issues of how failures were managed which need to be high-lighted and discussed in the open public. Please do not hide yourselves – even an independent investigation within ESA may be necessary… • I have updated SOVIM_TASS_003.pdf with some results from power spectral analysis which clearly shows how CPD is ‘working’ Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  13. Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  14. Mission Performance • You have got a mission time line which shows that the objective has not been achieved (only about 50% of what was promised!). • This is documented in SOVIM History080215.pdf which has been distributed to everybody – just read it carefully! You will be astounded what or better what not they (ESA, ALENIA etc) provided to us… • As to the pointing I would like to remind you about several issues, which had finally a solution from our initiative and not by ESA nor by ALENIA. • There are still some open issues of how failures were managed which need to be high-lighted and discussed in the open public. Please do not hide yourselves – even an independent investigation within ESA may be necessary… • I have updated SOVIM_TASS_003.pdf with some results from power spectral analysis which clearly shows how CPD is ‘working’ Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

  15. Pointing performance of CPD • We (the scientists, and not those responsible) have determined the offset needed to point the instruments to the sun. This is, nevertheless, the most important point in a solar mission. • The offset of the sun sensor is 67 arcmin in X and 16 arcmin in Y – out of spec anyway, but fly as is! • A final point is on the control loop…. Bad as everything. Face2Face Meeting, Bruxelles, 4 - 5 Dec. 2008

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