5.2 Variability of solar spectral irradiance and heating rates

From the above discussion it is clear that the response of the middle atmosphere to solar variability will depend critically on the spectral composition of the changes in solar irradiance. The UARS satellites provided measurements of the solar UV spectrum during the 1990s. More recently the spectral range has been extended into the visible and near infrared through the instruments on ERS-2, 1994 – 2003 (External Linkhttp://www.iup.uni-bremen.de/gome/), EnviSat, 2002 – present (External Linkhttp://www.iup.uni-bremen.de/sciamachy/) and SORCE, 2003 – present (External Linkhttp://lasp.colorado.edu/sorce/). Otherwise estimates are based on semi-empirical models. Figure 26View Image presents variations in spectral irradiance deduced for an 11-year solar cycle corresponding to a peak-to-peak TSI variation of 1.2 Wm–2 (similar to solar cycles 21 and 22). This figure also shows the difference in spectral irradiance between a current cycle mean and the Maunder Minimum. Although, as discussed above, the amplitude of the TSI variation over this period is uncertain, the relative spectral variations are probably more reliable.

It is apparent that much larger fractional changes take place at shorter wavelengths than the ∼ 0.1% variation in TSI, shown in Figure 22View Image, which represents the changes in the visible portion of the spectrum. Thus the direct impact of solar irradiance variability is larger in the middle and upper atmosphere than it is at lower altitudes.

Figure 27View Image (upper panels) shows vertical profiles of the solar fluxes and heating rates for solar minimum conditions in the ultraviolet, visible and near infrared spectral regions. The magnitude of the UV flux is much smaller than in the other two regions but its absorption by ozone, coupled with low atmospheric density, causes the largest heating rates in the middle atmosphere. The weaker absorption of visible radiation in the lower stratosphere and of near infrared radiation in the troposphere give much smaller heating rates. The right-hand panel reproduces the data of the centre panel with a linear pressure scale for the ordinate (emphasising the troposphere) and expanded abscissa scale.

Also shown in Figure 27View Image (lower panels) are the differences in fluxes and heating rates between 11-year solar cycle minimum and maximum conditions, based on the spectral differences shown in Figure 26View Image. At the top of the atmosphere the increases in incoming radiation in the visible and UV regions are of similar magnitude but the stronger absorption of UV produces much greater heating. The spectral changes prescribed for these calculations were such that near-infrared radiation was actually weaker at solar maximum so decreases in heating rate are shown. This is contentious but the changes are anyway very small being about one part in ten thousand.

View Image

Figure 27: Solar fluxes (left) and heating rates (middle and right). Top: solar minimum values; bottom: difference between solar min and max. The spectrum is divided into UV (220 – 320 nm), visible (320 – 690 nm) and near infrared (690 – 1000 nm) bands. The right hand column has a linear pressure scale for the ordinate so emphasising the troposphere. From Larkin (2000Jump To The Next Citation Point).

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