SOLAR-ISS: A new reference spectrum based on SOLAR/SOLSPEC observations

Abstract

Context. Since April 5, 2008 and up to February 15, 2017, the SOLar SPECtrometer (SOLSPEC) instrument of the SOLAR payload on board the International Space Station (ISS) has performed accurate measurements of solar spectral irradiance (SSI) from the middle ultraviolet to the infrared (165 to 3088 nm). These measurements are of primary importance for a better understanding of solar physics and the impact of solar variability on climate. In particular, a new reference solar spectrum (SOLAR-ISS) is established in April 2008 during the solar minima of cycles 23-24 thanks to revised engineering corrections, improved calibrations, and advanced procedures to account for thermal and aging corrections of the SOLAR/SOLSPEC instrument. Aims. The main objective of this article is to present a new high-resolution solar spectrum with a mean absolute uncertainty of 1.26% at 1σ from 165 to 3000 nm. This solar spectrum is based on solar observations of the SOLAR/SOLSPEC space-based instrument. Methods. The SOLAR/SOLSPEC instrument consists of three separate double monochromators that use concave holographic gratings to cover the middle ultraviolet (UV), visible (VIS), and infrared (IR) domains. Our best ultraviolet, visible, and infrared spectra are merged into a single absolute solar spectrum covering the 165-3000 nm domain. The resulting solar spectrum has a spectral resolution varying between 0.6 and 9.5 nm in the 165-3000 nm wavelength range. We build a new solar reference spectrum (SOLAR-ISS) by constraining existing high-resolution spectra to SOLAR/SOLSPEC observed spectrum. For that purpose, we account for the difference of resolution between the two spectra using the SOLAR/SOLSPEC instrumental slit functions. Results. Using SOLAR/SOLSPEC data, a new solar spectrum covering the 165-3000 nm wavelength range is built and is representative of the 2008 solar minimum. It has a resolution better than 0.1 nm below 1000 nm and 1 nm in the 1000-3000 nm wavelength range. The new solar spectrum (SOLAR-ISS) highlights significant differences with previous solar reference spectra and with solar spectra based on models. The integral of the SOLAR-ISS solar spectrum yields a total solar irradiance of 1372.3 ± 16.9 Wm −2 at 1σ, that is yet 11 Wm −2 over the value recommended by the International Astronomical Union in 2015.

CME-induced Thermodynamic Changes in the Corona as Inferred from Fe xi and Fe xiv Emission Observations during the 2017 August 21 Total Solar Eclipse

“…Therefore, every emitted photon from the coronal ions will originate from a very recent photo-absorption, and the total number of photons emitted per unit time will be given by equation 3 for any coronal ion emission line that ends in the ground state. (Meftah et al, 2018). Bottom: Relative rate of photo excitation and spontaneous emission versus wavelength for any energy level transition based on the energy density from the top curve (scaled to 1 R ) as shown in the example in Section 3.2.2.…”

Solar survey at Pic du Midi: Calibrated data and improved images

“…The red curve is here for comparison. It is a calibrated solar spectrum taken at the ISS by Meftah et al (2018). The grey zone around the red curve represents the area covered by its error bars.…”

Sensitivity Analysis Method for Spectral Band Adjustment between Hyperspectral Sensors: A Case Study Using the CLARREO Pathfinder and HISUI

“…The 15 solar irradiance models listed in Table 2 were used in the numerical simulations. Descriptions of each irradiance model can be found elsewhere [39][40][41][42][43][44][45][46][47][48][49]. A previous analysis of actual data concluded that primary sources of uncertainties in the solar irradiance models would be the instrument calibration uncertainties, which would be 2-3% in the solar reflective range [43].…”