REU Training Solar Irradiance/Radiometry Jerry Harder

Things to remember about the Sun Radius 695,510 km (109  radii) Mass x kg (332,946  ’s) Volume x m 3 (1.3 million  ‘s) Density 151,300 kg/m 3 (center) 1,409 kg/m 3 (mean) Temperature 15,557,000° K (center) 5,780° K (photosphere) × 10 6 ° K (corona) 1 AU ×10 8 km TSI AU) 1,361 W/m 2 Composition 92.1% hydrogen 7.8% helium 0.1% argon

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

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

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):

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 cm 2, the surface subtends x 2 /S 2 steradians. –The irradiance (H) on this surface is the incident radiant power per unit area:

Point source illuminating a plane

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   = J o cos  Surface that obeys Lambert’s is known as a Lambertian surface

Brightness independent of angle for a Lambertian surface

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

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

Absolute Radiometry

Basic process for electrical substitution radiometry Remember: Joule Heating: P = I 2 R = V 2 /R

Implementation for SORCE (SIM)

Total Irradiance Monitor (TIM) Major Advances Phase sensitive detection at the shutter fundamental frequency eliminates DC calibrations Nickel-Phosphide (NiP) black absorber provides high absorptivity and radiation stability Goals Measure TSI for >5 yrs Report 4 TSI measurements per day Absolute accuracy<100 ppm (1 s) Relative accuracy10 ppm/yr (1 s) Sensitivity1 ppm (1 s)

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

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

TSI Record

Planck’s equation

Properties of the Planck distribution

Spectral Irradiance Monitor SIM Measure 2 absolute solar irradiance spectra per day Broad spectral coverage – nm High measurement accuracy –Goal of 0.1% (  1  ) High measurement precision –SNR  300 nm –SNR  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

SORCE SIM: ESR-based spectral radiometry

SIM Measures the Full Solar Spectrum

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.

The optical configuration matches illumination areas on the detector Interchanging entrance slits and exit slits provides ~ 2x10 5 dynamic range Different stellar/solar integration times provide ~ 10 3 dynamic range A optical attenuator (neutral density filter), which can be measured in flight, provides additional ~ 10 2 dynamic range in the MUV wavelength range for >220 nm SOLSTICE: Experiment Concept

SORCE SOLSTICE FUV & MUV Spectra

The Sun as a blackbody

Brightness Temperature

Sources of opacity in the solar atmosphere

Solar Emissions (VAL, 1992)

SIM Time Series at Fixed Wavelengths

27 Day Variability Depends on the Formation Region

Wavelength Dependence of Sun Images #2

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.

Recent quiet and active solar scenes 11 Feb Oct Jan 2005

Instantaneous Heating Rates

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