high resolution solar reference spectrum robert voors knmi

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