The SORCE (Solar Radiation and Climate Experiment) folks have just published a very interesting paper with some surprising results about the solar irradiance over the end of Cycle 23.

As is well known, the TSI (Total Solar Irradiance) varies directly with the Sun's activity level, with an amplitude of about 0.1%. The Sun is about 0.1% brighter at activity maximum than at minimum. SORCE carries an instrument called TIM (Total Irradiance Monitor) that measures just this, but it also includes another intrument called SIM, the Spectral Irradiance Monitor. This instrument measures solar variability in six different wavelength bands, and SIM has turned up something very interesting.

© Harder, et al.

At left is part of Figure 1 from the Harder et al. paper. This shows two of the six wavelength regions the SIM observes, from 2004 to 2008. During this time, solar activity has declined to its current minimum between cycles 23 and 24. The key observational point is that while the UV irradiance has decreased (purple line, 201-300 nm), there has been an increase in the visible (green line, 400-691 nm). Other increases in the infrared are offset by declines in the red and near IR (691-972 nm) and near UV (300-400 nm).

Harder et al. make two basic points.

First, all the various SSI trends, added together, must reproduce the observed TSI. Since the trends in the visible and IR are out of phase with the solar cycle, the trends that are in phase, in particular the UV, are larger than has been previously assumed in models of solar variability. In fact, they find that UV variability between 200 and 400 nm is almost a factor of 10 larger than was estimated from earlier satellite data.

Second, they note that most general circulation models and climate models assume that the Sun's spectral variability tracks its total variability in all wavelengths. But as the figure above clearly shows, this is not the case. The interaction and effect of varying UV radiation in the stratosphere is well documented, even if the coupling of these effects to the troposphere is poorly understood. As I've noted in some other posts, although the globally averaged contribution of solar variations to climate change appears to be small, regional changes due to solar variability can be much larger (e.g., Europe during the Maunder Minimum). A nice 2008 paper by Judith Lean and David Rind makes the same point. Harder et al. conclude: "It is critical that these new solar spectral variability results from SIM be included in climate models to understand the potential direct and indirect consequences to climate change."

I wonder if Sun-like stars vary similarly in the UV and IR as their cycles wax and wane? I just might have to try to find out...