Detection and parameterization of variations in solar mid‐ and near‐ultraviolet radiation (200–400 nm)

Abstract

Nimbus 7 and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) spacecraft measurements of solar irradiance both exhibit variability at mid (200–300 nm) and near (300–400 nm) ultraviolet (UV) wavelengths that are attributable to the Sun's 27‐day solar rotation, even though instrument sensitivity drifts obscure longer‐term, 11‐year cycle variations. Competing influences of dark sunspots and bright faculae are the dominant causes of this rotational modulation. Parameterizations of these influences using a newly developed UV sunspot darkening index and the Mg index facular proxy replicate the rotational modulation detected in both the broadband Nimbus 7 filter data (275–360 nm and 300–410 nm) and in SOLSTICE 1‐nm spectra from 200 to 400 nm. Assuming that these rotational modulation influences scale linearly over the solar cycle, long–term databases of sunspot and global facular proxies permit estimation of 11‐year cycle amplitudes of the mid– and near–UV solar spectrum, unmeasured at wavelengths longward of 300 nm because of insufficient long‐term repeatability (relative accuracy) of state‐of‐the‐art solar radiometers at these wavelengths. Reconstructions of UV irradiances throughout the 11‐year solar cycle indicate variabilities of 0.173 W/m 2 (1.1%) in the integrated radiation from 200 to 300 nm and 0.24 W/m 2 (0.25%) in radiation from 300 to 400 nm. These two UV bands thus contribute about 13% and 18%, respectively, to the 1.34 W/m 2 (0.1%) total (spectrally integrated) radiative output solar cycle. The parameterizations allow customization of UV irradiance time series for specific wavelength bands required as inputs to general circulation model simulations of solar cycle forcing of global climate change, and have practical implications regarding the long‐term repeatability required for future solar monitoring.