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

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 NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

2. 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) NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

3. 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-ments of the Sun during ca. 20 minutes per orbit. • The range of the other axis allows observa-tions for about half of all the orbits during an year. NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

4. 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. NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

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 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. NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

6. Science Objectives: TSI (1 of 3) • 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 improved. • SOVIM will provide one of those simultaneous time series. • Changes due to the exposure to the space environment and the solar radiation need to be corrected. NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

7. Science Objectives: TSI (2 of 3) • Results of almost 12 years of VIRGO radiometery on SoHO show what kind of changes can happen. • The PMO6V-A radiometer (purple line) and the left channel of DIARAD (blue line) is used operationally and the other is exposed rarely (red asterisk for PMO6V-B, blue diamonds for the right channel of DIARAD). • With these time series we have learned, how to correct the data for the observed changes. We hope that we can apply the same methods for the SOVIM observations although the environment is quite different on ISS. NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

8. Science Objectives: TSI (3 of 3) The SOVIM radiometers will contribute to the existing TSI datasets, which will improve the reliability of the composite. This composite is available from http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

9. Scientific Objectives: SSI (1 of 2) • 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 irradi-ance in the blue, green and red bands over the now almost 12 years of oper-ation. • As the measuring pace on ISS will not allow to do helioseismology, we will concentrate on monitoring SSI This will complement the very detailed measure-ment – albeit not very often – of SOLSPEC. • We have learned how to make out of the raw mea-surements decent time series covering a full solar cycle. NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

10. Scientific Objectives: SSI (2 of 2) With the help of the TSI solar cycle varia-tion and arguments for the possible changes of the back-up filter radiometers we were able to re-construct the time series of SSI in the three wavelength ranges: 402, 500 and 862 nm. Such records can then be compared to results of SOLSPEC These data are available from ftp://ftp.pmodwrc.ch/pub/data/irradiance/virgo/SSI NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

11. Data Products and Release Policy • Level0 are raw data, which will only be used by the SOVIM team to produce level1. • Level1 are in physical units and corrected for all a-priori known effects, as e.g. temperature, distance from and radial velocity to the Sun, …. • Level1 data will be available soon after they are received and treated in the standard pipeline. • Level2 data are corrected for changes in space and they can only be released after some time, e.g. 8-12 months after reception. These corrections need a thorough investigation of the medium to long-term behaviour of the instruments. • The data will be publicly available at a tbd URL. NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007

12. Conclusions • SOVIM will provide reliable TSI time series for the whole period of operation (1.5 years as planned, but maybe more). • SOVIM will provide reliable SSI time series at 402, 500 and 862 nm. • In contrast to VIRGO/SoHO the time series will not be continuous, but interrupted by periods of about one month with no data from time to time. Moreover, the duty cycle during an observing orbit is also only around 20%. • Nevertheless, these data will complement existing time series and help to improve the reliability of any composite TSI and SSI. NASA-JAXA-CSA-ESA ‚Incr16‘ Science Symposium, 11-12 Sep 2007