calibration status of the solar irradiance monitor (sim) : the present and the future

ASIC3 Workshop Landsdowne, VA May 16-18, 2006 J. Harder

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

[email protected], (303) 492-1891


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


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.


SIM Measures the Broadband Solar Spectrum

20

2400

200

Total Solar Irradiance (TSI)

Watts 1362 m

Spectral Solar Irradiance (SSI)

96% of TSI

TIM

SIM

TSI E d

TSI E d


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)

E (W/m2)

E/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


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.


SIM Time Series at Fixed Wavelengths


Cross Sectional Views of SIM

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)

See:Harder et al., Solar Physics, 230, no. 1, pp. 141-167, 2005


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)


Measurement Equations

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


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)


Prism Degradation

C(t) Function of time alone Derived from prism

transmission experiments

Function of wavelength alone Derived from comparisons of

ESR and UV diode spectra


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.


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

calibration status of the solar irradiance monitor (sim) : the present and the future

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