1. SORCE - Spectral Irradiance Monitor Stéphane Béland 1, J. Harder 1, M. Snow 1, G. Thuillier 2 1 LASP – University of Colorado Boulder 2 LATMOS-CNRS SOSLPEC Workshop – March 2015

2. Spectral Irradiance Monitor (SIM) Outline: SIM Instrument overview Source of uncertainties Degradation model / Instrument measurement equation Estimated uncertainties Comparison of SIM and SOLSPEC Potential sources of scatter

3. Instrument type: 2 x Féry Prism Spectrometer Wavelength range: 200-2400 nm Wavelength resolution: 0.24-34 nm Detectors: ESR, n-p silicon photodiodes, InGaAs Field of view: 1.5 x 2.5° total Pointing accuracy/knowledge: 0.016°/0.008° Mass: 21.9 kg Dimensions: 88 x 40 x 19 cm Orbit average power: 17.5 Watts Orbit average data rate: 1.5 Kbits/s Relative accuracy: ~0.5 – 0.02% (210-2400nm) Long-term accuracy: 0.3-0.02%/year (210- 2400nm) Spectral Irradiance Monitor (SIM)

5. SIM: sources of uncertainties Short‐term, non‐accumulative effects: detector noise: ADC noise, stable over mission, ultimate limit of measurement uncertainties Detector/prism temperature: stable (±0.25°C) until power-cycling starts (±3°C within one orbit), correction applied. Small residual temperature effect still seen in diode data. spacecraft pointing: stable pointing throughout mission (≤1 arc-min), roll maneuvers affect 2.8% of data and are filtered out scattered light: out-of-band measurement stable (<100ppm) Long‐term accumulative effects Wavelength / Alignment shift: seen at safe-hold events, stretch-and-shift applied Detector degradation: measured diode degradation by comparing with stable ESR ESR servo gain degradation: small trend measured of ≈ 7 ppm/year Prism degradation: By far the dominant degradation factor Lambert’s law of absorption: measured Irradiance = I(λ) = E(λ)e -τ τ is function of wavelength and time Model based on comparison of two identical spectrometers used at different exposure rates, same physical, chemical and thermal environment Prism surface effect only

6. SIM Prism Degradation Model Based on comparing two identical spectrometers used at different rates of solar exposure Same physical, chemical and thermal environment Model validated when measurements of the sun at the same time with both spectrometers result in same calibrated irradiance Increased confidence by comparing integrated SSI with TIM TSI measurements On-Board-Computer events affected characteristics of degradation model

7. Instrument Measurement Equation Harder, J.W., et al., “The spectral irradiance monitor: measurement equations and calibration”, Solar Phys. 203, 169-204 (2005)

8. Instrument Measurement Equation Simplest degradation model: 1 / exp(-Kappa(λ)*C(t)) Separating the wavelength and the time dependencies (Kappa(λ), C(t)) Amount of prism degradation at a specific wavelength also depend on the geometry (ray path): beam enters and exits prism at different location of the “degradation spot”

9. Resulting degradation over SORCE mission Measured prism degradation over time for SimA and SimB

10. Wavelength dependent degradation: Kappa(λ) Comparing solar spectra from both spectrometers taken at the same time, using the solar exposure record as our time dependent factor, and adjusting the value of Kappa at each wavelength to minimize the differences Kappa is expected to be constant for the whole mission: depends on the material being deposited onto the prism Measured over the most stable time period of the mission

11. Wavelength dependent degradation: Kappa(λ) Combining the ESR and the UV photodiode data to cover full wavelength range Used to process data from every detectors

12. Remaining Time dependent degradation: C(t) Determine remaining time dependent effects NOT related to the Solar Exposure Lambert’s law of absorption: measured Irradiance = I(λ) = E(λ)e -τ Expand the degradation τ in 3 components: Kappa(λ), Solar Exposure (SE) and time dependent function F(t) Compare ratios of measured irradiance taken at time t 0 and t 1 for both instruments I A0 =E 0 e(-k∙F∙SE A0 ), I A1 =E 1 e(-k∙F∙SE A1 ) I B0 =E 0 e(-k∙F∙SE B0 ), I B1 =E 1 e(-k∙F∙SE B1 ) Minimize ratio of ratio by adjusting the value of F with time and at every wavelength (I B0 /I B1 ) / (I A0 /I A1 ) = exp {-k∙F∙(∆S A0 → 1 – ∆SE B0 → 1 ) }

13. Fractional uncertainty per year Slope of A-B fractional difference over time Small negative slope through UV suggests too small of a UV correction Majority of the visible very flat except in 310-380 nm range with higher noise and in the 879-950 nm range where excess temperature noise is apparent

14. SIM Long-term Stability from A & B differences Wavelength uncertainty: 1.5x10 -3 x FWHM of resolution function at every wavelength UV Spectrum: 2σ < 2x10 -3, equivalently ≤ 2x10 -4 /yr Visible Spectrum: 1σ < 2*NEI + 2.6x10 -5, equivalently << 5x10 -4 /yr NIR: 1σ < 3x10 -3, pending further analysis with ESR detector Integrated SIM 240-1630 nm ± 0.22 Wm -2 (1σ) relative to TIM TSI Missing 54.17 Wm -2 of the TSI

15. SIM estimated uncertainties ±2σ error bands estimated from point‐to‐point difference between SIM A & B channels

16. Estimated uncertainties from A & B differences Small changes in character occur at the boundaries of the safe‐hold events UV differences influenced by the more‐structured spectrum Visible differences tend to follow noise curves – excess temperature noise at long wavelengths.

17. Integrated SIM vs TIM TSI

18. SOLSPEC vs SIM

19. SOSLPEC Reference Spectra vs SIM

21. SORCE SIM vs SOLSPEC 282.0 -> 284.0 nm

22. ±1σ

23. SORCE SIM vs SOLSPEC 282.0 -> 284.0 nm

24. SORCE SIM vs SOLSPEC 220.0 -> 222.0 nm

25. SORCE SIM vs SOLSPEC 240.0 -> 242.0 nm

26. SORCE SIM vs SOLSPEC 260.0 -> 262.0 nm

27. SORCE SIM vs SOLSPEC 276.0 -> 278.0 nm

28. SORCE SIM vs SOLSPEC 279.0 -> 281.0 nm

29. SORCE SIM vs SOLSPEC 282.0 -> 284.0 nm

30. SORCE SIM vs SOLSPEC 284.0 -> 286.0 nm

31. SORCE SIM vs SOLSPEC 220.0 -> 240.0 nm

32. SORCE SIM vs SOLSPEC 240.0 -> 260.0 nm

33. SORCE SIM vs SOLSPEC 270.0 -> 290.0 nm

34. SORCE SIM vs SOLSPEC 290.0 -> 305.0 nm

35. SORCE SIM vs SOLSPEC 320.0 -> 340.0 nm

37. SIM, SOLSTICE, SOLSPEC 279.0 -> 281.0 nm

39. SIM, SOLSTICE MgII Index

40. SIM, SOLSPEC MgII Index

41. SORCE SIM – Additional Corrections Residual Diode temperature effect (very noticeable after power cycling) Discontinuity / Misalignment of irradiance at OBC events (change in raypath?)

42. SORCE SIM – Additional Corrections Complete separation of wavelength and exposure time functions, Kappa(λ), C(t) in prism degradation is not possible: Kappa varies with both λ AND time. Residual, as of yet unknown systematic effects not corrected with the 2- instrument design. Two validation methods: Degradation model is calculated so that the calibrated irradiance measured from both instruments at the same time have the same values The time-series of the integrated SSI should closely follow the TSI measured with TIM Same amount of solar exposure on SimA in not equivalent to solar exposure on SimB since the Sun has changed between exposures: Is Lyman-αlpha sufficient to track differences/impact on degradation? Want to avoid the dependence on external Lyα measurements

43. SOLSPEC– Potential Sources of Scatter Wavelength misalignment / shift between scans: Time series of wavelength in flat area (~252.25 nm) of spectra shows same STDV Good wavelength alignment between two consecutive days with large variation

44. SOLSPEC– Potential Sources of Scatter Stability / Accuracy of pointing: Small change in wavelength with angle of incoming beam (shift and stretch?) Change in the location of the illuminated area on the optics – different degradation profile / system throughput with angle Others ?

45. Backup Slides