High-Resolution Image

The fact that the Sun is behaving differently at ultraviolet, visible, and near-infrared wavelengths is important for determining the response of Earth's climate, since the different wavelengths are forcing different components of the Earth system. The ultraviolet primarily forces the stratosphere and upper troposphere; the visible penetrates to the land surface and into the upper ocean layers, and the near-infrared is forcing the whole troposphere down to and including the surface. This plot shows the Sun’s equivalent blackbody temperature (blue curve) for wavelengths from 258 nm to 2700 nm as measured by the Spectral Irradiance Monitor flying on the NASA EOS Solar Radiation and Climate Experiment (SORCE).

This is the temperature that a blackbody would have if it produced the observed spectral solar irradiance (SSI) at the given wavelength. Note that the Sun’s temperature is approximately constant over visible wavelengths (380 -760 nm), and approximately equal to 5780 K, which is the blackbody temperature approximately corresponding to the canonical total solar irradiance (TSI) value of 1367 W m-2. The measured spectrum from SIM is in close agreement with that obtained from the SOLSPEC spectrometer onboard the Space Shuttle during the ATLAS and EURECA missions in the early 1990’s (Thuillier et al., 2003). The black curve shows the ‘Quiet Sun’ spectrum as calculated by the Solar Radiation Physical Modeling (SPRM) model (Fontenla et al., 2005). The agreement between the observations and the model are limited by uncertainties in measured irradiance, by uncertainties in the solar abundances of O, C, and N, the incompleteness of the atomic databases, and the need for non-local thermodynamic equilibrium radiative transfer modeling in the UV part of the spectrum. The SRPM model uses seven semi-empirical spectral models to represent sunspots, plage, network, and quiet atmosphere and these models are combined with an analysis of solar images to synthesize the irradiance of the sun at any given time. Time series comparisons of the modeled and measured SIM spectra provide a powerful tool for understanding solar irradiance variability.

We know that the TSI typically varies with a relative amplitude of about 0.1%, or 1000 parts per million (ppm). Does the whole spectrum vary in sync with the TSI, as many climate change simulations have assumed, or is the variability a function of wavelength? The two inset plots show that the latter is true, and that irradiance at different wavelengths display quite different characteristics. The inset at upper left shows the relative variations of SSI at wavelength 430 nm (red curve), during the time period from 5/1/2004 to 9/1/2005, compared to the variations of TSI (black curve) over the same time period. At this visible wavelength the presence of active network and slowly decaying active regions enhance the irradiance above the nominal quiet sun value; this enhancement is most apparent in December 2005. Note also in this plot that the ‘dips’ in irradiance are due to the passage of sunspots and the contrast is about twice as much as the TSI. The inset at lower right shows the relative variations of SSI at wavelength 1550 nm (magenta curve), during the same time period, again compared to the TSI (black curve). At this near-infrared wavelength the variations of SSI are generally not as large as for TSI, but during certain times there are irradiance enhancements that are not readily explained in terms of our current understanding of the solar atmosphere; these features are most apparent in March 2005. The large broad peak in the solar blackbody temperature near 1550 nm is due to the transitions of the weakly bound "extra" electron in H- ions in the sun’s photosphere, a phenomenon first highlighted in a 1939 paper by Rupert Wildt.

Understanding the physics of the different solar variations that occur at infrared, visible, and ultraviolet wavelengths, and improving our ability to correlate and eventually predict these, will be crucial to advancing our understanding of the solar forcing of Earth’s climate.

Image provided by Jerry Harder (UCO/LASP); text by Robert Cahalan and Jerry Harder.

References:
Fontenla et al., Astrophysical Journal, 2005, in press.
G. Thuillier, M. Hersé, D. Labs, T. Foujols, W. Peetermans, D. Gillotay, P.C. Simon, H. Mandel Solar Physics 214: 1–22, 2003.
Wildt, R., ”Electron affinity in Astrophysics,” Astrophysical Journal, 89: 295, 1939.