Spectro-radiometry of the low Sun - a photo on Flickriver

            In this and the following post, I present spectro-radiometry of the low Sun from 380nm to 2500nm (with a small gap from 780–900nm). These exhibit strong telluric (arising in the Earth’s atmosphere) absorption bands from molecules and short-lived molecular collisional pairs.

The radiometric calibration is provided in the visible spectrum from observations with a Sekonic C-7000 SpectroMaster (the red line spectrum from 360–780nm). This is fitted using a spectral extinction model of the atmosphere that includes Rayleigh and aerosol scattering as well as ozone absorption (thin blue line). The extinction model is applied to the known solar spectral power distribution outside the Earth’s atmosphere (grey line spectrum).

An Ocean Optics NIRQuest 256 channel infrared spectrometer is then used with a relative flux calibration derived from a quartz-halogen lamp operating at 3000K and assumed to be a Planck function at that temperature (red spectrum from 900–2500nm). The vertical scale of the infrared data is determined from the extrapolated visible extinction model. The dashed black line is a Planck function at 3000K and is used to normalise the infrared spectrum to have a continuum close to unity (green line spectrum).

The main infrared absorption bands are marked. These include oxygen, water, carbon dioxide and methane as well as O2-O2 and O2-N2 Collisionally Induced Absorption (CIA) bands. The departure of the solar spectrum above the Planck function centred on 1600 nm is the result of the minimum of the H-minus (the hydrogen negative ion, H-) opacity in the solar atmosphere in this region. This results in the emitted power arising at these wavelengths from deeper in the solar atmosphere where the temperature is higher that that emitting visible light.

The deviation between the extinction model and the spectral data in the blue below about 450nm (shown as the pale blue ‘wedge’) is the result of skylight being included in the field of view of the visible radiometer. The following post explains how this can be corrected. When the Sun is low in the sky and much reduced in intensity by atmospheric scattering and absorption, the light from the sky becomes significant by comparison with direct sunlight at short wavelengths.

While the y-axis of the plot is labelled 'spectral irradiance' (which has the correct units) the radiometer is being restricted to a field with a diameter of about 20 degrees and so excludes much of the scattered skylight.

Taken on: April 12, 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 • Telluric spectra more » • Water • Ozone • Natural Phenomena • Sky « less

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