Solar Irradiance Spectrum
CEOS Reference Solar Irradiance Spectrum
CEOS endorsed reference solar irradiance spectrum
For many Earth Observation based applications there is a need to utilise a solar irradiance spectrum. This may be as part of a bio-geo-physical ‘retrieval’ of some form, use of a Radiative transfer code in some way or a conversion in units e.g. radiance to reflectance. In most cases for any self-contained application the most important requirement is to be consistent in the choice of any exo-atmospheric solar irradiance spectrum and to be clear, in any subsequent documentation describing any result of its use, which spectrum and version was used. In this way, it allows others, in principle to be able to convert and compare similar results which may use different solar Irradiance spectrum. However, following requests from both the user community and satellite operators and developers, CEOS in the early 2000’s decided to establish a CEOS endorsed reference solar Irradiance spectrum, based on a community consensus of the ‘best’ observational spectrum available. At the time this was a composite spectrum, heavily based on a spectrum from the SOLSPEC mission, Thuiilier et al 2003 https://doi.org/10.1023/A:1024048429145. In recommending a spectrum it was noted that this represents a particular ‘quiet’ state in the solar cycle which is dynamic and follows a nominal 11 yr periodicity. However, the variability in solar spectral irradiance at wavelengths longer than the near-UV (>~350 nm) of the spectrum, other than during a particularly major short-term event are on average relatively small < than tenths of a percent and thus not a major impact to most Earth Observation applications at this time. In 2022, following results of new satellite missions, particularly the NASA TSIS-1 mission, which provided significantly reduced uncertainty, it was decided to revise the CEOS reference solar irradiance spectrum to that of Coddington et al (2021) https://doi.org/10.1029/2020GL091709 (please visit the dedicated page) and later to a version 2 which extends the spectral range to thermal infrared in Coddington et al (2023) https://doi.org/10.1029/2022EA002637
CEOS Recommendation on use of a solar Irradiance spectrum for EO applications
CEOS recommends that wherever a reference solar irradiance spectrum is used in an Earth Observation application, the choice of that spectrum and a link to an accessible, processable version of that spectrum should be included with any documentation associated with its use, ideally integrated in the metadata. The methodology used to convolve the chosen solar irradiance spectrum with that of other EO data and the associated uncertainty should also be documented and made available. For the purposes of harmonisation and interoperability, CEOS further recommends the use of its reference spectrum and where this is not done that a calculation indicating the difference of the chosen spectrum to that of the CEOS reference spectrum be made available.