REU Training Solar Irradiance/Radiometry Jerry Harder jerry.harder@lasp.colorado 303 492 1891

1. REU TrainingSolar Irradiance/RadiometryJerry Harderjerry.harder@lasp.colorado.edu303 492 1891

2. Things to remember about the Sun Radius 695,510 km (109  radii) Mass 1.989 x 1030 kg (332,946 ’s) Volume 1.412 x 1027 m3 (1.3 million  ‘s) Density 151,300 kg/m3 (center) 1,409 kg/m3 (mean) Temperature 15,557,000° K (center) 5,780° K (photosphere) 2 - 3×106° K (corona) 1 AU 1.49495×108 km TSI (@1 AU) 1,361 W/m2 Composition 92.1% hydrogen 7.8% helium 0.1% argon

3. Wavelength Dependence of Sun Images Extreme Ultraviolet Imaging Telescope (EIT) Fe XII 195 Å Yohkoh Soft X-ray Telescope (SXT) Ca II K spectroheliograms NSO Sacramento Peak He I 10830 Å spectroheliograms NSO Kitt Peak

4. Radiometric Terminology Advice: PAY ATTENTION TO YOUR UNITS!!!

5. Definition of Solid Angle (  ) • Solid angle subtended by sphere (from an ‘interior’point): =4 • For an area seen from a point of observation: • Approximation for a distant point ( small):

6. The inverse square law: Intensity • Consider a point source of energy radiating isotropically • If the emission rate is P watts, it will have a radiant intensity (J) of: • If a surface is S cm from the source and of area x cm2, the surface subtends x2/S2 steradians. • The irradiance (H) on this surface is the incident radiant power per unit area:

7. Point source illuminating a plane

8. Extended sources must be treated differently than point sources • Radiance (N): power per unit solid angle per unit area • Has units of W m-2 ster-1 • Lambert’s Law: J = Jo cos  • Surface that obeys Lambert’s is known as a Lambertian surface

9. Brightness independent of angle for a Lambertian surface

10. Lambertian source radiating into a hemisphere {P/A is ½ of what you would expect from a point source}

11. History of Absolute Radiometry • Ferdinand Kurlbaum (1857-1927) • Radiometric developments for the measurement and verification of the Stefan-Boltzmann radiation law. • Knut Ångström (1857-1910) • Observations of the ‘Solar Constant’ and atmospheric absorption

12. Absolute Radiometry

13. Basic process for electrical substitution radiometry Remember: Joule Heating: P = I2R = V2/R

14. Implementation for SORCE (SIM)

15. Total Irradiance Monitor (TIM) Goals • Measure TSI for >5 yrs • Report 4 TSI measurements per day • Absolute accuracy <100 ppm (1 s) • Relative accuracy 10 ppm/yr (1 s) • Sensitivity 1 ppm (1 s) Major Advances • Phase sensitive detection at the shutter fundamental frequency eliminates DC calibrations • Nickel-Phosphide (NiP) black absorber provides high absorptivity and radiation stability

16. Radiometer Cones Glory Prototype Cone Post-Soldered Cone Glory Prototype Cone Interior

17. TIM Baffle Design Glint FOV 46.6 degrees Vacuum Door Base Plate Shutter Cone Precision Aperture Shutter Housing Cone Housing Rear Housing Baffle 1,2,3 FOV Baffle

18. TSI Record

19. Planck’s equation

20. Properties of the Planck distribution

21. Spectral Irradiance Monitor SIM • Measure 2 absolute solar irradiance spectra per day • Broad spectral coverage • 200-2400 nm • High measurement accuracy • Goal of 0.1% (1) • High measurement precision • SNR 500 @ 300 nm • SNR 20000 @ 800 nm • High wavelength precision • 1.3 m knowledge in the focal plane • (or  < 150 ppm) • In-flight re-calibration • Prism transmission calibration • Duty cycling 2 independent spectrometers

22. SORCE SIM: ESR-based spectral radiometry

23. SIM Measures the Full Solar Spectrum

24. Solar Stellar Irradiance Comparison Experiment (SOLSTICE) Science Objectives: • Measure solar irradiance from 115 to 320 nm with 0.1 nm spectral resolution and 5% or better accuracy. • Monitor solar irradiance variation with 0.5% per year accuracy during the SORCE mission. • Establish the ratio of solar irradiance to the average flux from an ensemble of bright early-type stars with 0.5% accuracy for future studies of long-term solar variability.

25. SOLSTICE: Experiment Concept • The optical configuration matches illumination areas on the detector • Interchanging entrance slits and exit slits provides ~ 2x105 dynamic range • Different stellar/solar integration times provide ~ 103 dynamic range • A optical attenuator (neutral density filter), which can be measured in flight, provides additional ~ 102 dynamic range in the MUV wavelength range for l>220 nm

26. SORCE SOLSTICE FUV & MUV Spectra

27. The Sun as a blackbody

28. Brightness Temperature

29. Sources of opacity in the solar atmosphere

30. Solar Emissions (VAL, 1992)

31. SIM Time Series at Fixed Wavelengths

32. 27 Day Variability Depends on the Formation Region

33. Wavelength Dependence of Sun Images #2

34. Identification of solar active regions Solar Radiation Physical Model (SRPM) employs solar images from HAO's PSPT (left panel) to identify and locate 7 solar activity features (R=sunspot penumbra; S=sunspot umbra; P,H=facula and plage; F=active network; E,C=quiet sun) to produce a mask image of the solar features (center panel). The SRPM combines solar feature information with physics-based solar atmospheric spectral models at high spectral resolution to compute the emergent intensity spectrum.

35. Recent quiet and active solar scenes 11 Feb 2006 15 Jan 2005 27 Oct 2004

36. Instantaneous Heating Rates

37. References • “Modern Optical Engineering”, Warren J. Smith, McGraw Hill, 1990. • ‘Quantitative Molecular Spectroscopy and Gas Emissivities”, S. S. Penner, Addison-Wesley, 1959. • “Statistical Mechanics”, J. E. Mayer and M. G. Mayer, Wiley & Sons, 1940. • “Absolute Radiometry”, F. Hengstberger, Academic Press, 1989.