Solar spectrum shortly before sunset (sky subtracted)

This is a visible–IR spectrum of the Sun shortly before sunset with an altitude above the horizon of 5°. This is a time when its visible brightness is about fifty times less than the brightest it can be when high in a clear sky where it dominates the surrounding sky.

Before sunset, as the Sun becomes highly attenuated by scattering and absorption, the sky remains reasonably bright, especially in the blue, and so a radiometric measurement of the Sun alone needs to be corrected for the presence of skylight included by the instrument. The C-7000 device measures Spectral Irradiance from the entire hemisphere centred on the direction in which it is pointed. For the measurement made here, I used a tubular mask as described in a previous post: www.flickr.com/photos/bob_81667/49676872537/ to isolate the Sun and a modest region of sky between 10 and 20° in diameter. A second measurement of the sky is made with the tube pointed sufficiently far away from the Sun that no direct sunlight falls on the instrument aperture.

The visual spectrum measurement shown here has been 'sky-subtracted' in this way. It can be seen that the 'blue wedge' seen in the previous spectrum has gone away and the extinction model now fits the measured data well.

The infrared spectrum is collected with a fibre collimator which 'sees' a similar patch of sky to the shielded C-7000 device. In the infrared, however, the Sun is less attenuated and the sky is relatively much darker and so no sky subtraction is necessary at this level of precision.

The vertical axis in the plot should therefore not strictly be labelled Spectral Irradiance but rather as an energy flux. If the measurement were made from space, outside the atmosphere, it would be effectively an irradiance.

The small triangular markers labelled 'TFC' represent regions of the continuous solar spectrum which are relatively free from either Fraunhofer absorption in the solar atmosphere or telluric absorption from Earth.

Taken on: April 11, 2020

Tags:

sun

spectrum

spectral

visible

infrared

sunset

nirquest

c-7000

sekonic

extinction

model

molecular

identification

energy-distribution

cia

o2-o2

o2-n2

planck

h-minus

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Sets appears in: • Spectroscopy • Water more » • Ozone • Natural Phenomena • Sky « less

Groups appears in: • Earth Science Teaching Resource • Natural Phenomena more » • Astronomy Spectrography « less

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