Abstract
The solar spectral irradiance (SSI) dataset is a key record for studying and understanding the energetics and radiation balance in Earth’s environment. Understanding the long-term variations of the SSI over timescales of the 11-year solar activity cycle and longer is critical for many Sun–Earth research topics. Satellite measurements of the SSI have been made since the 1970s, most of them in the ultraviolet, but recently also in the visible and near-infrared. A limiting factor for the accuracy of previous solar variability results is the uncertainties for the instrument degradation corrections, which need fairly large corrections relative to the amount of solar cycle variability at some wavelengths. The primary objective of this investigation has been to separate out solar cycle variability and any residual uncorrected instrumental trends in the SSI measurements from the Solar Radiation and Climate Experiment (SORCE) mission and the Thermosphere, Mesosphere, Ionosphere, Energetic, and Dynamics (TIMED) mission. A new technique called the Multiple Same-Irradiance-Level (MuSIL) analysis has been developed, which examines an SSI time series at different levels of solar activity to provide long-term trends in an SSI record, and the most common result is a downward trend that most likely stems from uncorrected instrument degradation. This technique has been applied to each wavelength in the SSI records from SORCE (2003 – present) and TIMED (2002 – present) to provide new solar cycle variability results between 27 nm and 1600 nm with a resolution of about 1 nm at most wavelengths. This technique, which was validated with the highly accurate total solar irradiance (TSI) record, has an estimated relative uncertainty of about 5% of the measured solar cycle variability. The MuSIL results are further validated with the comparison of the new solar cycle variability results from different solar cycles.
Highlights
The solar spectral irradiance (SSI) dataset is a key record for studying and understanding the energetics and radiation balance in Earth’s environment
Meeting a stability requirement is reduced to understanding instrument degradation trends to better than this stability requirement. This can be challenging for this stability requirement, and understanding instrument degradation is at the heart of the controversy for solar cycle variations from the Solar Radiation and Climate Experiment (SORCE) Spectral Irradiance Monitor (SIM) (Harder et al, 2009)
The German Solar Auto-Calibrating EUV Spectrometers (SolACES) and French Solar Spectrum (SOLSPEC) instruments are integrated onto the European Space Agency (ESA) SOLAR platform that was deployed onto the International Space Station (ISS)
Summary
The solar spectral irradiance (SSI) dataset is a key record for studying and understanding the energetics and radiation balance in Earth’s environment. This can be challenging for this stability requirement, and understanding instrument degradation is at the heart of the controversy for solar cycle variations from the SORCE Spectral Irradiance Monitor (SIM) (Harder et al, 2009) These SORCE SIM measurements indicate that there is more near-ultraviolet (NUV: 300 – 400 nm) solar cycle variation than earlier observations reported (Harder et al, 2009; Unruh, Ball, and Krivova, 2012; DeLand and Cebula, 2012; Ermolli et al, 2013). These variations have been debated on the basis that they are inconsistent with some SSI model estimates The extension over the extreme ultraviolet (EUV: 10 – 115 nm) is made possible by the TIMED
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