Calibration Status of the Solar Irradiance Monitor (SIM) : The Present and the Future

1. Calibration Status of the Solar Irradiance Monitor (SIM) : The Present and the Future Jerald Harder, Peter Pilewskie, Juan Fontenla, and Erik Richard Laboratory for Atmospheric and Space Physics, University of Colorado jerald.harder@lasp.colorado.edu, (303) 492-1891

2. Presentation Outline • The importance of an absolute solar spectrum and solar variability to the Earth climate problem. • The SIM instrument • The SIM measurement equation • Important instrument characteristics • Instrument precision • Resolution • Critical in-flight recalibrations for long term stability of the instrument • Prism transmission and degradation • Photodiode radiant sensitivity correction • Conclusions, activities, and outlook

3. Spectral Irradiance Measurements Contribute to Key Climate Issues • Response of climate to solar variability is highly wavelength dependent: • Direct surface heating at near-ultraviolet wavelengths and longer. • Indirect processes through absorption of UV in the stratosphere and radiative and dynamical coupling with the troposphere. • Greatest relative variability occurs in the ultraviolet (indirect); greatest absolute variability occurs in mid visible (direct). • Relative uncertainty in direct solar forcing is very large and must be reduced in order to separate natural from anthropogenic radiative forcing. • Knowledge of TOA spectral distribution of solar radiation is crucial in interpreting the highly spectrally dependent radiative processes in the troposphere and at the surface. • The combination of TSI measurements, SSI measurements, solar imaging and sophisticated solar atmospheric modeling are needed to address the true nature of solar variability and its impact on climate. At the present none of these can stand alone.

4. SIM Measures the Broadband Solar Spectrum

5. SIM Partitions the TSI Into Discrete Bands as a Function of Time • The character of the variability in integrated bands is a strong function of wavelength.

6. Short Time Scale Solar Variability • Solar time variability is a function of wavelength. • TSI constrains the magnitude of the variability, but not its spectral distribution. • Solar surface features modulate spectral irradiance distribution. • The Earth’s response to solar variability is wavelength dependent.

7. SIM Time Series at Fixed Wavelengths

8. Cross Sectional Views of SIM See: Harder et al., Solar Physics, 230, no. 1, pp. 141-167, 2005 • Design Highlights • Dual instrument configuration for duty cycling and redundancy • Instrument coupled with periscope for direct prism calibration • Electrical Substitution Radiometer (ESR) for primary detector • Uses phase sensitive detection: noise floor ~2 nW Hz-½ • Spectrum acquired with only one optical element (Fery Prism)

9. Simplified SIM Measurement Equation • In-flight calibrations • SIM A / SIM B comparisons • ESR gain • Field of view • Wavelength Scale (Sun) • Prism degradation • Photodiode degradation • Preflight calibrations • Instrument metrology • Prism transmission • ESR sensitivity • Wavelength scale (lab sources)

10. Measurement Equations See: Harder et al., Solar Physics, 230, no. 1, pp. 169-204, 2005

13. Preflight Prism Transmission Measurement Calibration Requirements: • Light source must illuminate the prism the same way as the sun. • Must measure incoming and outgoing light beams with same detector. • Use phase sensitive detection. Results: • Regardless of prism rotation angle (59°±2.5°), incidence angle is near normal at the back surface. Effective reflectivity very weak function of angle • Effective reflectivity combines prism bulk losses with reflectivity of aluminized 2nd surface of the prism. • Angular dependence of transmission is due to Fresnel reflection losses on front face of prism (vacuumglass & glass vacuum)

14. Prism Degradation • k(l) • Function of wavelength alone • Derived from comparisons of ESR and UV diode spectra • C(t) • Function of time alone • Derived from prism transmission experiments

15. Photodiode Degradation • ESR table scans sample 60 discrete wavelengths from 250-2700 nm. • The ESR detector does not experience degradation. • From SIM A / SIM B comparisons. • Rate of change is found by matching the slope of the photodiode data to the ESR. • The correction is made to the radiant sensitivity, not to the time series.

16. Conclusions, Activities, and Outlook • Solar spectral irradiance is a key parameter in understanding solar variability and its impact on Earth Climate. • Climatological records of solar variability require: • High absolute irradiance accuracy • High measurement precision • The ability to self-correct long term drifts and sensitivity changes • Comparisons of side-by-side instruments • Direct measurement of optical components • Detector-to-detector comparisons. • Solar images, TSI, and solar modeling in conjunction with SIM measurements provide an effective suite research tools to investigate solar variability. • NIST calibration facilities such as SIRCUS and SURF will greatly improve the pre-flight calibration spectral instruments for future missions