A high resolution extra-terrestrial solar spectrum determined from ground-based solar irradiance measurements

Abstract. A high resolution extraterrestrial solar spectrum has been determined from ground-based measurements of direct solar spectral irradiance over the range 300 nm to 500 nm using the Langley-plot technique. The measurements were obtained at the Iza\\`na Atmospheric Research Center from AEMET, Tenerife, Spain during the period 12 to 24 September 2016. This solar spectrum (QASUMEFTS) was combined from medium resolution (bandpass of 0.86 nm) measurements of the QASUME spectroradiometer in the range 300 nm to 500 nm and high resolution measurements (0.025 nm) from a fourier transform spectroradiometer over the range 305 nm to 380 nm. The KittPeak solar atlas was used to extend this high resolution solar spectrum to 500 nm. The expanded uncertainties of this solar spectrum are 2 % between 310 nm and 500 nm and 4 % at 300 nm. The comparison of this solar spectrum with solar spectra measured in space (top of the atmosphere) gives very good agreements in some cases, while in some other cases discrepancies of up to 5 % could be observed. The QASUMEFTS solar spectrum represents a benchmark dataset with uncertainties lower than anything previously published. The metrological traceability of the measurements to the International System of Units (SI) is assured by an unbroken chain of calibrations leading to the primary spectral irradiance standard of the Physikalisch-Technische Bundesanstalt in Germany.