Since scattering is inversely related to λ 4, the shortest wavelengths are preferentially scattered in the atmosphere, causing a cloudless sky to appear blue. Some of this light is back scattered into space while some is scattered forward and reaches the Earth's surface as skylight. Scattering of light by gas molecules in the atmosphere is proportional to 1/λ 4 (as described by Rayleigh's law). Scattering and absorption do not influence all wavelengths equally, thus significant changes in the spectral distribution of sunlight will occur as light penetrates the atmosphere. Reflectance, scattering, and absorption of light in the atmosphere can reduce this amount by 15–80% before reaching Earth's surface. Solar flux just outside Earth's atmosphere is referred to as the “solar constant” and has a value of approximately 1373 W m − 2.

Morris, in Reference Module in Earth Systems and Environmental Sciences, 2021 Role of the atmosphere Work of this type that seeks to define the significant atmospheric processes and that emphasizes the need for their accurate measurement in the laboratory continues to be valuable in shaping the direction of the field. In the latter work, the need to include ionization by solar photoelectrons as well as by direct absorption of solar EUV is stressed. In the earlier work, the F-region peak electron density is found to be sensitive to enhanced vibrational excitation of molecular nitrogen during solar maximum. Examples of this complexity are given by Richards and Torr, 1986, 1988, and references therein. It is a frequent comment in papers on ionospheric models that improved and continuing measurement of the solar EUV flux are needed, but unfortunately this type of routine measurement is probably a number of years into the future.Ĭomprehensive ionospheric and thermal models require knowledge of the reactions and collision processes occurring. The EUV flux varies greatly with solar activity compared to the FUV flux. These models also give the neutral composition and are therefore closely related to the atmospheric density applications of Section 15.5. A discussion of models of this type is given by Roble et al., 1987. Solar EUV and FUV flux measurements are inputs to large global circulation models that have as one of their outputs the electron and ion number densities. A useful catalog of measurements and models available through the National Space Science Data Center is by Bilitza, 1990. At the present state of development of remote sensing models, it is considered necessary to measure these lines individually, rather than to use some wide bandwidth approach. The EUV spectrum of interest has been divided into 37 lines and regions, as given most recently by Torr and Torr, 1985. The currently available flux values, primarily due to Hinteregger and associates, have been discussed in Chapter 7.

The key measurements for this approach are the solar EUV and FUV irradiance. It did, however, stimulate other remote sensing efforts in the ionosphere. This work indicated the promise of ionospheric sensing using solar flux data, but it was not suited for global use. Local electron densities have been calculated from solar flux measurements made by sensors on sounding rockets using a simplified ionospheric model, Heroux et al., 1974, 1975. Solar flux measurements have been considered as an input to global circulation models which will yield the electron density profile and other ionospheric characteristics, along with many other atmospheric properties. Huffman, in International Geophysics, 1992 17.3 UV solar flux approaches