Solar Spectrum

Summary

The impact of solar variability on the Earth’s climate has a long history as a research field, and has also become a hotly debated topic. Although there is strong evidence for this natural forcing to be weak in comparison to that of man-made greenhouse gases, large uncertainties remain regarding the magnitude of the variation of the spectral solar irradiance dataset and thus also on the magnitude of its impacts.

Solar Reference Spectrum

This reference spectrum is recommended by the CEOS Working Group on Calibration and Validation (CEOS WGCV), particularly the Infrared and Visible Optical Sensors Subgroup. Details can be found under the following links:

http://calvalportal.ceos.org/ceos-wgcv/ivos
http://ceos.org/ourwork/workinggroups/wgcv/subgroups/ivos/

The reference spectrum is based on the following key elements:

1. The absolute scale is determined by the latest observational SSI composite (Haberreiter et al., 2017) which takes into account the ATLAS1 observations by Thuillier et al., (2003). In particular, we use the solar minimum level during the year 2008 as the reference.
2. This dataset is also in agreement with the latest TSI value recommended by the IAU 2015 Resolution B3 (Prša et al., 2016).
3. The high-resolution component comes from the synthetic spectrum calculated with the radiative transfer code COSI developed at PMOD/WRC (Haberreiter et al., 2008; Criscuoli et al., 2020).

These elements guarantee the correct absolute scale as well as the high-resolution information of the spectrum.

Specifics:
Spectral Resolution: 0.005 nm
Spectral Range: 300 nm to 15 µm

The PMOD/WRC Solar Reference Spectrum can be found here on our public FTP server:
ftp://ftp.pmodwrc.ch/pub/data/SolarReferenceSpectrum/

The SOLID published composite can be found here on our public FTP server:
ftp://ftp.pmodwrc.ch/pub/projects/SOLID/database/composite_published/

Solar Spectrum Modelling

Variations of the Total Solar Irradiance (TSI) and Spectral Solar Irradiance (SSI) are known to affect the Earth’s atmosphere and climate (e.g. Matthes et al., 2017). For this reason irradiance variations have been monitored in the last four decades with various space-borne instruments.

Understanding the variations of solar irradiance requires a good understanding of spectrum emitted from the solar atmosphere.  Various spectral synthesis codes are available to model the emergent spectrum. As part of an ISSI International team, PMOD/WRC participated in a spectral synthesis model comparision, in which the COSI, RH, and the radiative scheme of the MURaM codes were compared.

The performance of the codes is currently being compared using 3D MHD simulations  (Haberreiter et al., under review).