Reconstruction of solar UV irradiance in cycle 23

Abstract

Solar irradiance variations show a strong wavelength dependence. Whereas the total solar irradiance varies by about 0.1% during the course of the solar cycle, variations at the wavelengths around the Ly-α emission line near 121.6 nm range up to 50−100%. These variations may have a significant impact on the Earth's climate system. Being almost completely absorbed in the upper atmosphere, solar UV radiation below 300 nm affects stratospheric chemistry and controls production and destruction of ozone. Models of the solar UV irradiance remain far from perfect, even though considerable progress has been made in modelling the irradiance variations longwards of about 200−300 nm. We show that after correcting for the exposure dependent degradation of the SUSIM channels sampling irradiance at λ > 240 nm (making use of the Mg II core-to-wing ratio) the agreement between model and measurement is significantly improved. At shorter wavelengths the LTE approximation usually made in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths. The model reproduces observed solar cycle variations of the irradiance at wavelengths down to 115 nm and indicates an important role of UV irradiance variability: up to 60% of the total irradiance variations over the solar cycle might be produced at wavelengths below 400 nm.

Solar change and climate: an update in the light of the current exceptional solar minimum

“…It is not yet clear to what extent this unexpectedly large change reflects different instrumentation or how much it may be due to the long-term trends that have combined with the solar cycle to generate the current unusual solar minimum. The SATIRE modelling of the variability of the spectrum, extended into the UV by Krivova et al (2006), shows some similarities and some difference to the SIM results on both solar cycle and solar rotation time scales (Unruh et al 2008), but does not give as large a change in the UV as reported from SIM data.…”

The role of the Fraunhofer lines in solar brightness variability

“…2, also including the UV. We construe this as an independent support to both the Krivova et al (2006) method and the reliability of the "SATIRE-NESSY" time series.…”

Solar irradiance variability: a six-year comparison between SORCE observations and the SATIRE model

“…The error bars establish that the disagreement between SIM and SATIRE-S exceeds the estimated long-term uncertainty of the instrument. It should be noted that given that the decline in TSI over this period is approximately a third of the maximum-to-minimum variation, this sets the cycle variation a significant factor above that observed by UARS/SUSIM and UARS/SOLSTICE (Floyd et al 2003;Krivova et al 2006). Over a similar period in the previous cycle, August 1993 to May 1996 (solar minimum), the 200−300 nm region declined by 0.06 W m −2 from 14.51 to 14.45 W m −2 according to level 3BS V22 SUSIM data on UARS.…”

“…In the past it was assumed that the radiative forcing on Earth was directly proportional to this change in the total solar output, and although there is evidence that changes in TSI do affect the climate (Labitzke & van Loon 1995;van Loon & Shea 1999), it is becoming clear that spectral solar irradiance (SSI) is also an important factor when considering the Sun's impact on climate (Haigh 1994). It has previously been estimated that 60% of TSI variability is found in ultra-violet (UV) wavelengths below 400 nm (Krivova et al 2006). The UV domain is known to affect stratospheric temperatures and chemistry; most notable and well-established is its effect on ozone (Labitzke et al 2002;Haigh 2007).…”