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Available solar reference spectra Chance and Spurr: composite high resolution Kurucz: model spectra Thuillier et al: composite medium resolution SUSIM on ATLAS and UARS: low resolution
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High resolution solar reference
spectrumRobert Voors
KNMI
Introduction Solar Reference spectra used for
calibration Wavelength calibration Radiometric calibration
Current spectra do not meet requirements Spectral resolution and sampling Radiometric accuracy
Available solar reference spectraChance and Spurr:
composite high resolution Kurucz:
model spectra Thuillier et al:
composite medium resolutionSUSIM on ATLAS and UARS:
low resolution <410nm; long time periodASTM solar standard spectrum
Composite medium resolutionGOME, SCIAMACHY…Many more…
What’s the problem?
Bladibla
Combine best of both worlds High resolution spectrum:
assume relative line strengths are OK Low resolution spectrum:
assume absolute radiances are OK.
Recipe for a new reference spectrum1. Convolve a high resolution spectrum with
poor radiometric calibration with the best slit function that is available for the lower-resolution solar reference spectrum
2. Interpolate the thus obtained high sampling, low-resolution spectrum on the wavelength grid of the low-resolution reference spectrum
3. Divide that spectrum by the low-res spectrum, to obtain the fraction by which to multiply the original high-resolution spectrum, used in 1
4. Interpolate the fraction from 3 to the high resolution wavelength grid
loreshireshisamplores SFF
)(Regrid hisamplores
losamplores FF
measlores
losamplores
losamp FFQ
)(Regrid losamphisamp QQ
hires
hisampREFhires FQF
Recipe for a new reference spectrum The original high resolution spectrum How to find the best low(er)-resolution
reference spectrum How to determine the instrument transfer
function (slit function) of the low-resolution instrument.
Wavelength calibration of the low-resolution spectra.
The original high resolution spectraSpectrum Wavelength
Range [nm]
Spectral
Resolution [nm]
Spectral
Sampling [nm]
Reference
Hall &
Anderson
200-310 ~0.025 0.01 1
Kitt Peak 296-1300 <0.005 0.0003-0.001 2
(1) Balloon spectrum, corrected for atmospheric effects.(2) Ground-based measurements (Kurucz 1984), corrected for atm. effects
The original low resolution spectrum
Spectrum Wavelength
Range [nm]
Spectral
Resolution [nm]
Spectral
Sampling [nm]
Reference
SUSIM < 410 ~0.15 0.05 1
Heidelberg
balloon
400 - 652 ~1.5 0.25 2
(1) Average of ~1 solar cycle of SUSIM data(2) LPMA balloon measurement from SCIAMACHY validation
The instrument transfer function
SUSIM: • scanning• triangular
LPMA: • non-scanning• flat/gaussian
Wavelength scale of low resolution spectra
Final result
CaII H&K
Comparison at 1 nm resolution
Solar variability
Discussion Solar variability not a serious issue, except near 270 nm and
at strong Fraunhofer lines CaII H&K lines pose a problem New spectrum compares well with other solar spectra, e.g.
from SCIAMACHY and Thuillier. Differences mostly < 4 percent Differences possibly due to
Solar reference spectrum Absolute radiometric scale of Thuillier, OMI, SCIAMACHY etc. Assumed low-res slit function Wavelength calibration