Author: William T. Ball
Department of Physics, Imperial College London, UK
Energy received from the Sun is the main driver of processes in Earth’s climate system. Therefore, it is important to understand how solar irradiance, the power per unit area received at the top of the Earth’s atmosphere, varies in time. Space-based measurements of the wavelength-integrated Total Solar Irradiance (TSI) began in 1978 and since then we have discovered that the spectral solar irradiance variations are highly wavelength dependent. Spectral irradiance variations below 400 nm, which may contribute 60% of TSI variability , are particularly important to understand since this spectral region leads to the largest contributor of heating within the Earth’s stratosphere through the interaction between the solar UV radiation and ozone.
Total solar irradiance observations and models
Space-based radiometers measuring TSI now provide high-precision and accurate absolute measurements on rotational and yearly time-scales. It is therefore possible to test model accuracy at reproducing TSI. We can explain almost all variations in TSI, longer than a few hours, by considering changes only in features related to surface photospheric magnetic flux. Attaining good agreement with TSI observations using models is straightforward and can be achieved in several different ways (see ).
There is no single instrument that has observed TSI continuously from 1978 to present day. Therefore, data are combined to produce composites. There are currently three of these resulting from different approaches that attempt to account for instrument degradation, glitches and data gaps. While all three composites show similar rotational and cycle variability, the inter-cycle, secular change in irradiance between the solar minima is different in all three (see Fig. 1) .We used the SATIRE-S model [4, 5] to independently reconstruct irradiance over the entire observational record. SATIRE-S has one underlying assumption: that irradiance variations are caused by changes in the surface magnetic flux. It employs daily continuum intensity images and magnetograms, such as those taken by the SoHO/MDI instrument. Four components are identified in the images: sunspot penumbra and umbra, faculae and the quiet Sun. Each component is assigned intensities, as a function of wavelength and limb angle, and we integrate over these to produce daily spectra and TSI. The SATIRE-S TSI dataset, with an accompanying uncertainty estimate that tries to account for modelling uncertainies, spans 1974 to the end of 2009 and is plotted in blue in Fig. 1 .
We found that the model best approximated the PMOD composite (Physikalisch-Meteorologisches Observatorium Davos ), accounting for 92% of the variability over the entire 1978-2009 period and 96% during cycle 23 between 1996 and 2008; this is the highest agreement any model has achieved with TSI. The model also reproduced the same secular variation as PMOD. This indicates that the PMOD composite is probably the best composite of TSI datasets currently. There is an on-going project, bringing together instrument scientists, that aims to reassess TSI data and produce a single composite with daily, time-dependent error estimates in an attempt to provide the most reliable TSI data product to date .
The problem with solar-cycle spectral irradiance changes
Understanding and modelling the spectral solar irradiance (SSI) variations is more difficult. This is because: (i) models used to reconstruct irradiance are sensitive to assumptions about the solar atmospheric structure and (ii) the space-based SSI instruments, that make observations that can validate models, are subject to damage from the Sun. Instruments observing the solar spectrum have wavelength-dependent degradation that is not easy to account for.
By the time of the launch of the SOlar Radiation and Climate Experiment (SORCE) satellite in 2003, observations of the Spectral Solar Irradiance (SSI) had focused on the UV below 400 nm. Models were able to reproduce both the rotational and cycle variability of SSI observations, in particular those from the SUSIM and SOLSTICE instruments on-board the Upper Atmosphere Research Satellite (UARS) mission.
SORCE was the first mission to observe SSI variations at visible and IR wavelengths using the SIM instrument (200-2400 nm) and a new SOLSTICE (115-310 nm). While the rotational variability is in good agreement with the SATIRE-S model at almost all wavelengths, both SOLSTICE and SIM observe significantly larger UV cycle variability than both the model and the UARS instruments, up to a factor of five at some wavelengths . Figure 2 shows the 200-290 nm integrated flux lightcurves for UARS/SUSIM (red) and SORCE/SOLSTICE (black) along with SATIRE-S (blue); absolute fluxes have been shifted so they are in agreement in 2004. The upper two horizontal dashed lines show the approximate cycle amplitude for UARS/SUSIM and SATIRE-S whereas the lower dashed line is at the level of SORCE/SOLSTICE at the solar minimum of 2008. From the change between 2004 and 2008, the cycle amplitude implied by SORCE/SOLSTICE is at least twice as large as UARS/SUSIM and SATIRE-S for this integrated wavelength band.
Concluding remarks: the future of solar irradiance
While models are now successful at reproducing observations of total solar irradiance well, the differences between UARS and SORCE SSI observations currently leave the question open as to what the magnitude of solar cycle spectral irradiance variations of the Sun really are. SORCE data are undergoing a reanalysis with indications that their implied UV cycle variability will decrease . Nevertheless, both SORCE instruments still show larger variability than the SATIRE-S model and UARS observations. The SORCE mission is reaching an end so, for now, the potential range of solar cycle spectral variability is likely to remain quite large and therefore uncertainty will remain in the impact it has on the Earth’s climate system. A SATIRE-S SSI dataset is now being prepared, again with accompanying uncertainties, for the same time period and covering the wavelength range 115 – 160,000 nm.
Resolving the problem of solar cycle SSI variability will require new observations. The successor of SIM, to be launched on the Joint Polar Satellite System NOAA mission, may not begin operations until 2017 at the earliest. For the time being, models will continue to develop our understanding of irradiance variations. For example, although SATIRE-S performs extremely well, there is much that can be improved: by including non-local thermodynamic equilibrium calculations to improve variability estimates in the UV; and to account more accurately for the geometric effects from faculae, which are currently estimated based on a 1D atmospheric model.
With improvements in all solar irradiance models, and with additional observations, we will be able to better assess the role the Sun plays in changes to our planet.
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