A Solar irradiance spectrum data set provides the Sun’s spectral irradiance spectrum at a Sun-Earth distance of 1 astronomical unit. Solar irradiance spectrum data may come from observations—e.g. using a spectrometer onboard a satellite—or models such as the blackbody model.

## Data access#

All Solar irradiance spectrum data sets required by Eradiate are managed the data store (see Introduction for details).

### Identifier format#

Identifiers for Solar irradiance spectrum data sets (except blackbody_sun) are constructed based on the format {author}_{year}[_{extra}] where:

• author denotes the author of the data set,

• year stands for the year in which the data set was published,

• extra (optional) includes additional information, such as a data set post-processing operation.

## Structure#

Solar irradiance spectrum data sets include one data variable:

• the solar spectral irradiance (ssi)

and two dimension coordinates:

• the wavelength (w),

• the time (t).

Solar spectral irradiance data is tabulated against both wavelength and time.

Some of the following additional data set attributes are provided for data sets to which they apply:

• obs_start: observation start time,

• obs_end: observation end time,

• url: original data URL,

• comment: a comment indicating how the original data was processed.

## Description of available data sets#

Here is a quick description of each of the available solar irradiance spectrum datasets.

### blackbody_sun#

A theoretical irradiance spectrum, based on Planck’s law for the blackbody spectral radiance:

$L_{\lambda}(T) = \frac{2hc^2}{\lambda^5 (e^{hc/k\lambda T} - 1)}$

where $$h$$ and $$k$$ are the Planck and Boltzmann constants respectively, $$c$$ is the speed of light in a vacuum, $$\lambda$$ is the wavelength, with a blackbody temperature $$T$$ of 5800 K—which is roughly the temperature of the Sun’s photosphere. The envelope of the Sun’s irradiance spectrum approximates that of a blackbody radiator. While converting from spectral radiance to spectral irradiance, using the equation:

$\phi_{\lambda}(T) = \frac{\pi R^2}{D^2} L_{\lambda} (T)$

the radius of the blackbody ($$R$$) is set to the solar radius constant ($$695.7 \cdot 10^6$$ km) and the distance of the blackbody to the Earth ($$D$$) is set to 1 astronomical unit ($$149.5978707 \cdot 10^6$$ km) which is the average Sun-Earth distance. The wavelength range extends from 280 nm to 2400 nm to cover Eradiate’s wavelength range. Reference: [Lio02].

### meftah_2017#

A reference solar irradiance spectrum based on observations from the SOLSPEC instrument of the SOLAR payload onboard the internationial space station. The spectrum was built using observation data from 2008 for the [165, 656] nm wavelength range and from 2010–2016 for the [656, 3000] nm wavelength range. The spectrum is said to be representative of the 2008 solar minimum which corresponds to the end of the solar cycle 23 and the beginning of the solar cycle 24. Wavelength range: [165.0, 3000.1] nm. Resolution: better than 1 nm below 1000 nm, and 1 nm in the [1000, 3000] nm wavelength range. Absolute uncertainty: 1.26 % (1 standard deviation). Total solar irradiance: 1372.3 ± 16.9 W/m² (1 standard deviation). Reference: [MDB+17].

### solid_2017#

An observational solar irradiance spectrum composite based on data from 20 different instruments. The dataset provides daily solar irradiance spectra from 1978-11-7 to 2014-12-31. Wavelength range: [0.5, 1991.5] nm. Resolution: variable, between 1 and 16 nm. Reference: [HSDdW+17]. See also the Cal/Val Portal of the Committee on Earth Observation Satellites.

### solid_2017_mean#

A time-average of the solid_2017 dataset over all days from 1978-11-7 to 2014-12-31.

### thuillier_2003#

A reference solar irradiance spectrum based on observations from the SOLSPEC instrument during the ATLAS-1 mission (from 1992-03-24 to 1992-04-02) and the SOSP instrument onboard the EURECA satellite (from 1992-8-7 to 1993-7-1), and on the Kurucz and Bell (1995) synthetic spectrum. Wavelength range: [200, 2397] nm. Resolution: 1 nm. The mean absolute uncertainty is of 2 to 3 %. The spectrum is representative of moderately high solar activity. When contributions from the wavelength region $$[0, 200[ \, \cup \, ]2397, +\infty[$$ nm are added, the total solar irradiance evaluates to 1367.7 W/m². In [200, 2397] nm, the integrated solar irradiance spectrum evaluates to 1315.7 W/m².

Reference: [THL+03].

### thuillier_2003_extrapolated#

A version of the thuillier_2003 spectrum extrapolated to 2500 nm so that it covers Eradiate’s wavelength range. The figure below illustrates the original and extrapolated versions and highlights the extrapolation region.

Note

For the reference, we provide below the values of the integrated original and extrapolated solar irradiance spectra, evaluated by integrating the irradiance spectrum along wavelength using the trapezoidal rule.

\begin{align}\begin{aligned}\begin{split}\int_{200 \, \mathrm{nm}}^{2397 \, \mathrm{nm}} I_{\mathrm{original}} (\lambda) \, \mathrm{d} \lambda = 1315.68 \, \mathrm{W / m^2} \\\end{split}\\\int_{200 \, \mathrm{nm}}^{2500 \, \mathrm{nm}} I_{\mathrm{extrapolated}} (\lambda) \, \mathrm{d} \lambda = 1321.72 \, \mathrm{W / m^2}\end{aligned}\end{align}

Since the wavelength range is larger for the extrapolated irradiance spectrum, the corresponding integrated solar irradiance is also larger (by 0.46 %).

### whi_2008_*#

A combination of simultaneous satellite observations from the SEE and SORCE instruments (from 2008-03-25 to 2008-04-16) onboard the TIMED satellite and a prototype EVE instrument onboard a sounding rocket launched on 14 April 2008. Wavelength range: [116.5, 2399.95] nm (the wavelengths [0.5, 116.5] nm are cut off). Resolution: 0.1 nm. Representative of solar cycle minimum conditions. The WHI campaign produced three spectra, corresponding to three time periods (numbered 1, 2, 3 here):

• whi_2008_1: from 2008-03-25 to 2008-03-29, “sunspot active” spectrum. Total solar irradiance: 1360.70 W/m².

• whi_2008_2: from 2008-03-29 to 2008-04-4, “faculae active” spectrum. Total solar irradiance: 1360.94 W/m².

• whi_2008_3: from 2008-04-10 to 2008-04-16, “quiet sun” spectrum. Total solar irradiance: 1360.84 W/m².

whi_2008 is an alias to the quiet sun spectrum whi_2008_3. Reference: [WCH+08].