upper atmospheric environment using these standard emissions within a ground radius of 300 to 1000 km, depending on height and project. WIDESPREAD EFFECTS A more pervasive consideration is the permanent and general illumination by a belt of sunlit satellites, whose finite reflectivity produced light estimated as brighter than perhaps 0.1 of a full moon. This light is seen not only close to the sources via small angle scattering but also is distributed more or less over the sky seen from a ground station, depending on the prevalence of fine haze. We note, but set aside here, the interactions of this background light with astronomy. In optical studies of the upper atmosphere itself, much research rests on ground-based spectrophotometric measurements of the faint light emissions, which give information on the energy delivered within the sun's outer environment of particles, waves, and radiation. How the solar wind energizes the earth's magnetosphere is traced by the pattern of auroras formed by incident electrons, and the chemistry of upper atmospheric processes is shown by the airglow. This highly developed science depends on measurement of faint-sky light, accurately discriminated as to wavelength, intensity, and variations with time and space. The airglow, with all its spectral components, is roughly equal to astronomical sources in providing the brightness of the dark sky. Intensities of auroras, which cover a wide range in various circumstances, often approach those of the airglow while yet maintaining much information content. Especially at midlatitudes, the subvisual stable auroral red arcs and the diffuse auroras require very sensitive spectral measurement and are unique indicators of the plasmapause region of the magnetosphere. Much future study will center on relatively weak emissions extending from the near ultraviolet to the near infrared. Many measurements press detection sensitivity to the limits of background light. The SAR arcs are reliably mapped and measured over the sky at levels of 50 Rayleighs. High-resolution studies of Doppler shifts in thermospheric winds are at the limit of photon statistics. The airglow continuum is already compromised by natural backgrounds, and the artificial light would directly overwhelm it in any but the best conditions.
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