Man's near-space environment is dominated by energy from the sun, which arrives in the vicinity of the earth in two forms: as electromagnetic radiation and as the plasma flow of the solar wind. This energy interacts with the earth's magnetosphere and atmosphere in a variety of ways. The study of these interactions has been a major goal of NASA's Explorer programs.
Most of the solar electromagnetic energy lies in the visible region of the spectrum and passes directly through the upper atmosphere. However, part of the solar energy in the ultraviolet interacts strongly with the upper atmosphere and produces the ionospheric and ozone layers. Understanding how this radiation controls the ionosphere and thermosphere has been a primary objective of the Atmosphere Explorer program.