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

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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_at_lasp.colorado.edu, [phone]

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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

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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.

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SIM Measures the Broadband Solar Spectrum
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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.

SIM Wavelength Range (nm) Irradiance (E) (W/m2) DE (W/m2) DE/E
200-300 14.8 0.15 1.0x10-2
300-400 93.5 0.58 6.2x10-3
400-680 504.2 1.28 2.5x10-3
680-1000 340.8 0.58 1.7x10-3
1000-1600 266.3 0.36 1.4x10-3
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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 Earths response to solar variability is
  wavelength dependent.

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SIM Time Series at Fixed Wavelengths
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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)

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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)

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Measurement Equations
See Harder et al., Solar Physics, 230, no. 1,
pp. 169-204, 2005
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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 (592.5),
  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)

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Prism Degradation

• C(t)
• Function of time alone
• Derived from prism transmission experiments

• k(l)
• Function of wavelength alone
• Derived from comparisons of ESR and UV diode
  spectra

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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.

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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