VIRGO SSI time series now

Transcription

1 VIRGO SSI time series now Christoph Wehrli & VIRGO Team PMOD/WRC Davos An attempt to assess instrument degradation in a self consistent way by referring operational measurements to occasional backup operations correcting the backup channel by initial ageing of operational channel

2 VIRGO Sun Photometers Interference filter radiometer with 3 channels centered at 862nm, 500nm and 402nm (R,G,B); FWHM bandwidths 5nm; silicon PD detectors; rad-hard windows. Active (SPM-A) and Backup (SPM-B) instruments SPM-A: exposed continuously for helioseismology application SPM-B: exposed rarely for solar spectral irradiance measurements Calibrated by EG&G FEL lamps, NBS 1973 traceable

3 VIRGO SPM: Level1 data 2 VIRGO SPM-A 2 VIRGO SPM-B spectral irradiance [Wm -2 nm -1 ] % 20% 0.2 5% Sensitivity after 5825 days spectral irradiance [Wm -2 nm -1 ] Number of Backups 178 Total exposure time 2.6 days

4 First Light Irradiance VIRGO SPM-A 862 SPM-A 500 SPM-A 402 SPM-B 862 SPM-B 500 SPM-B st Light 1996/1 N&L rel.diff. -2.8% -3.0% -2.6% -3.1% -3.4% -2.3% Agreement to 0.5% between A & B Fröhlich et al., Sol.Phys. 175: , 1997

5 Variations of SPM-B: Solar or Instrumental? VIRGO SPM-B and TSI 5*(TSI/<TSI>) - 4 rel. Variation (TSI*5) SSI variations are about 5 times larger than TSI variation. Solar cycle 24 is not obvious, hidden in instrumental ageing.

6 Initial ageing of SPM-A VIRGO SPM-A First Light VIRGO SPM-A Operational TSI (*100) rel. Variation rel. Variation [TSI*100] Jan 18.Jan 19.Jan 20.Jan Date Jan 04.Feb 11.Feb 18.Feb 25.Feb 03.Mar Date 1996 Steep degradation during first hours! Linear degradation during first month {Commissioning activities until }

7 Ageing of SPM-A and SPM-B versus exposure time VIRGO SPM A & B rel. Variation polynomial fit SPM-A exposure time [d]

8 SPM-B corrected by 1 st Light degradation of SPM-A VIRGO SPM-B corrected for First-Light degradation rel. Variation % 1% Unrealistic increase in R and G over SC23

9 SPM-B corrected by operational degradation of SPM-A VIRGO SPM-B corrected for SPM-A operational degradation rel. Variation % Instrumental effects dominating over solar cycle

10 Comparison with SORCE-SIM Trend VIRGO SPM-B (First-Light degradation) lasp.colorado.edu/sorce 862nm Slope (ppm/day) nm VIRGO SIM nm same as TSI 500 < 2006: 2.72 >=2006: VIRGO SPM-B: dots; SORCE SIM: solid lines

11 Empirical Approach SSI timeseries represent a mixture of Solar Cycle and instrumental effects VIRGO SSI Active & Backup SPM degrade differently in time or exposure time Linear correction accounts for unrealistic decline of SSI, first order estimation of instrumental effect. Exponential correction eliminates most of solar cycle variation as well SPM-B500 [Wm -2 nm -1 ] %

12 Linear Detrending slope 1.20 [ ] [1/µm] TSI MissionDay SSI500 (res) 5 0 SSI500 (residuals) MissionDay TSI

13 Linear vs. Exponential Detrending TSI TSI MissionDay +2.0 TSI (residuals) MissionDay TSI (residuals) Lslope 0.54 [ ] Eslope 0.32 [ ] TSI

14 Long-Term Correlations Preminger et al., 2011, ApJLetters VIRGO Linear Exponential R [ ] [ ] G [ ] [ ] B [ ] [ ] TSI [ ] [ ]

15 Summary Normalization of SPM-A by SPM-B: Larger than expected variations of Backup channel Rapid initial degradation in Active channel Early increase of Backup is not observed in Active channel Recovery of SPM-B between expositions? Accumulation of contamination during dark periods, activation during exposure, Schwarzschild effect? Empirical correction of SPM-B: fitting degradation in time Linear or exponential detrending yield positive correlation with solar cycle in 3 visible channels.

16 Alternative analysis including proxies (C. Fröhlich, EGU 2011) SORCE Empiric correction versus Time (double exponential), Temperature (linear + Boltzmann), TSI and Mg-II Index.

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18 Additional material

19 Dark ageing: SOHO vacations 1.5 VIRGO/SPM Phoenix Ratio Irradiance [W/m 2 /nm] GAP Day of Mission Dark ageing of SPM-A during 104 days: -0.1% to -0.8%

20 Degradation Rates IPHIR on Phobos2 (1988) & SOVA on EURECA (1992) 1 1 VIRGO on SOHO (1996) Ratio to 1 st Light /500/862 nm /500/862 nm SOVIM on SSI (2008) PREMOS FR-A on Picard (2010) 1 1 Ratio to 1 st Light /500/862 nm /535/782 nm Exposure time [d] Exposure time [d] kkk

21 SOVA2/EURECA Wehrli et al., 1995, Metrologia