Artificial satellites have been available for space research for slightly more than a decade, and it seems appropriate to take stock of the accomplishments of that decade from the viewpoint of the earth's environment in space. The results can only be described as revolutionary; few of the concepts of the early 1950's have survived without major revision and totally unexpected discoveries have provided fundamentally new theoretical challenges. The accomplishments are far too numerous to describe in detail, but a few prominent examples will serve to illustrate the great change in perspective that the space age has brought to our picture of the sun-earth system.
The early years of rocket astronomy were devoted to exploring the solar spectrum from the visible cut-off near 3000 A to its X-ray limit. Rockets provided the first means of directly examining these solar wavelengths which the earth's atmosphere blocks, and one of the earliest triumphs of rocket solar astronomy was the discovery of solar X rays originating in the million-degree coronal plasma. By 1950 the solar spectrum had been broadly mapped and the outlines of the mechanisms by which solar ionizing radiations produce and control the ionosphere became discernable. The past ten years of space research have presented us with a remarkably detailed knowledge of the solar spectrum, the nature of solar active centers, and the character of solar variability in the ionizing fluxes. The slowly varying behavior of the sun during its II-year cycle from maximum to minimum activity has been observed, and it is most pronounced in the higher energy radiations not detectable at ground level. While some of the principal ultraviolet emissions from hydrogen Lyman α(1216 A) to shorter wavelengths change by only 50 per cent from minimum to maximum, X-ray fluxes vary tenfold in the wavelengths absorbed in the E region of the ionosphere and a hundredfold for the short wavelengths (1 to 8 A) that affect the D region. Solar flares produce prodigious increases in the shortest wavelengths: the largest flare of recent years enhanced the 0- to 3-A flux by more than a hundred-thousand times. It is thus clearly established that sudden ionospheric disturbances are X-ray induced. The solar X-ray and ultraviolet energy input to all levels of the ionosphere is qualitatively understood; an improvement in flux values to a quantitative accuracy of only about 10 per cent is needed to satisfy the requirements of aeronomers for a detailed accounting of the atmospheric energy budget.