UV solar irradiance in observations and the NRLSSI and SATIRE‐S models

Abstract

Total solar irradiance and UV spectral solar irradiance has been monitored since 1978 through a succession of space missions. This is accompanied by the development of models aimed at replicating solar irradiance by relating the variability to solar magnetic activity. The Naval Research Laboratory Solar Spectral Irradiance (NRLSSI) and Spectral And Total Irradiance REconstruction for the Satellite era (SATIRE‐S) models provide the most comprehensive reconstructions of total and spectral solar irradiance over the period of satellite observation currently available. There is persistent controversy between the various measurements and models in terms of the wavelength dependence of the variation over the solar cycle, with repercussions on our understanding of the influence of UV solar irradiance variability on the stratosphere. We review the measurement and modeling of UV solar irradiance variability over the period of satellite observation. The SATIRE‐S reconstruction is consistent with spectral solar irradiance observations where they are reliable. It is also supported by an independent, empirical reconstruction of UV spectral solar irradiance based on Upper Atmosphere Research Satellite/Solar Ultraviolet Spectral Irradiance Monitor measurements from an earlier study. The weaker solar cycle variability produced by NRLSSI between 300 and 400 nm is not evident in any available record. We show that although the method employed to construct NRLSSI is principally sound, reconstructed solar cycle variability is detrimentally affected by the uncertainty in the SSI observations it draws upon in the derivation. Based on our findings, we recommend, when choosing between the two models, the use of SATIRE‐S for climate studies.

Variation of the Mn I 539.4 nm line with the solar cycle

“…Reproducing the large solar cycle variation of the Mn I line with the SATIRE-S model without optimizing it in any way (the single free parameter of the model remains fixed to the value that was independently determined by Ball et al 2012) is a success for the model and strengthens the assumption underlying it, namely that variations in TSI and SSI (in the optical wavelength range) are caused by the evolution of the solar surface magnetic field. Furthermore, the fact that SATIRE-S reproduces the correct level of the variation in the Mn line provides strong support to the overall spectral profile of the irradiance variability computed with SATIRE-S. On the one hand, Yeo et al (2015) have recently demonstrated that the magnitude of the changes in the UV returned by SATIRE-S agrees well with the most stable satellite measurements. On the other hand, since spectral lines determine the amplitude and even the A33, page 7 of 8 phase of the visible irradiance variations (Shapiro et al 2015), our finding here indicates that the magnitude of the variability in the visible is also adequate.…”

A Remarkable Recent Transition in the Solar Dynamo

“…The difference is insignificantly small: it falls within the error bounds. Hence, this information cannot be used for the present study but it seems worth to keep it in mind for later investigations (Yeo et al 2015). Anyway, realizing that the TSI reflects the concentration of magnetic elements in the chromosphere and specifically the magnetic flux of the facular fields around the sunspots, it shows that during the Transition the magnetic activity in the sunspot belt was very low.…”

Solar forcing for CMIP6 (v3.2)

“…NRLSSI2 and SATIRE-SSI show significantly different spectral profiles of the variability between about 250 and 400 nm. This has fueled a debate (e.g., Yeo et al, 2015) that is unlikely to settle soon. The two models have been derived independently, and as of today there is no consensus regarding their relative performance.…”