center of the earth. Hence the expressions for pressure and density were only approximate. More significantly, atmospheric temperature varied markedly with altitude, the scale height given by (3) varying proportionately. Thus, in regions of high temperature the pressure and density declined more slowly with height than where the temperature was low. In regions where the atmospheric temperature was constant or nearly so, each separate atmospheric gas individually followed laws like (1) and (2) with the average molecular mass M in (3) replaced by the molecular mass of the individual gas. Thus, the corresponding scale height H varied inversely as the molecular mass of the gas, and a heavy gas like carbon dioxide fell off in density much more rapidly than nitrogen, oxygen slightly faster than nitrogen, and light gases like helium much more slowly. As a result the lighter gases appeared to diffuse upward, while the heavier gases settled out. Such considerations led one to suppose that at the highest altitudes, hundreds or thousands of kilometers above the ground, the lighter gases predominated in the atmosphere. The outermost regions were expected to consist of hydrogen or helium primarily, although no experimental evidence confirmed the supposition. Starting at the ground, atmospheric temperature fell at a rate of about 6 K per km throughout the troposphere (or "weathersphere") to a value of around 220 K at the tropopause, or top of the troposphere, which was found at 10 to 14 km, the lower height corresponding to higher latitudes, the greater height to the tropics. Above the tropopause the temperature remained fairly constant to about 35 km. A slight increase in the proportion of helium in the air above 20 km suggested some tendency toward diffusive separation, which at one time led researchers to expect that the region above the tropopause would exhibit a layered structure-hence the name stratosphere for this quasi-isothermal region. Above the stratosphere, temperature rose again, as shown by the fact that the sound from cannon fire and large explosions was reflected from these levels of the upper atmosphere. Observations on this anomalous propagation of sound waves permitted one to estimate that the air temperature was about 370 K at 55 km height. Noctilucent clouds between 70 and 90 km suggested a low temperature in the vicinity of 80 km. These extremely tenuous clouds were seen only in high latitudes and only when illuminated by the slanting rays from the sun below the horizon. With the assumption that the clouds were composed of ice crystals, the temperature around 80 km was estimated to be about 160 K. The study of meteors, investigation of the electrical properties of the high atmosphere by radio techniques, and observations of the auroras showed that temperatures rose again above 80 km to 300 K at 100 km, and to 1000 K or possibly 1500 K at 300 km, with much higher temperatures beyond. Calculations from auroral observations were, however, not always consistent with this picture, often indicating considerably lower temperatures than those deduced from radio observations. Atmospheric composition near the ground was known to be: Meteorological processes kept the atmosphere mixed, maintaining this composition at least up to 20 km. Between 20 and 25 km, helium increased about 3 percent above the normal value, but winds and turbulence kept the atmosphere well mixed far above stratospheric heights, up to at least 80 km.7 In the absence of other agents, this stirring should have kept the composition fairly uniform throughout the mixing regions. But solar ultraviolet radiation in the region from 1925 Å to 1760 Å, absorbed in atmospheric oxygen above the stratosphere, gave rise to a chain of reactions leading to the formation of ozone. Simultaneously solar ultraviolet in the neighborhood of 2550 Å decomposed atmospheric ozone. An equilibrium between the formation and destruction of ozone, combined with atmospheric motions, distributed the gas so that in temperate latitudes it showed a maximum absolute concentration at about 25 km height, and a maximum percentage concentration at about 35 km. Although never more than the equivalent of a few millimeters at normal temperature and pressure, the ozone layer shielded the ground from lethal ultraviolet rays from the sun. Ozone concentrations were observed to be higher in the polar regions than in the tropics, and tended to correlate with cyclonic weather patterns. Above 80 km, solar ultraviolet dissociated molecular oxygen, the dissociation becoming fairly complete by about 130 km. Thus the region from 80 to 130 km appeared as one of transition from an atmosphere consisting of mostly molecular nitrogen and molecular oxygen, to one of molecular nitrogen and atomic oxygen. It was assumed that above 100 km diffusive separation of the atmospheric gases became increasingly effective, and that the dissociation of oxygen enhanced the tendency of nitrogen to settle out and the oxygen to rise. Whether nitrogen also dissociated in the higher levels was not known. In the upper levels of the atmosphere was the ionosphere. The term was used in two different ways, either to mean the ionized constituents of the high atmosphere, or to mean the regions in which the ionization was found. An ionosphere was postulated by Balfour Stewart in 1878 to explain small daily variations observed in the earth's magnetic field. Later, in 1902, A. E. Kennelly in America and O. Heaviside in England suggested that a conducting layer in the upper atmosphere, which could reflect radio waves beyond the horizon, might explain how Marconi in 1901 had sent wireless signals from Cornwall to Newfoundland.9 The first real evidence of such an ionosphere was obtained in 1925 when E. V. Appleton and M. Barnett in England detected sky waves coming down to their receiver after being reflected by a high-altitude layer.10 Additional evidence of the KennellyHeaviside layer came from experiments by G. Breit and M. A. Tuve in America." These experimenters sent a radio pulse upward, and observed two or more delayed pulses in a receiver a few kilometers away from the transmitter. The initial received pulse was assumed to be from the direct ray along the ground, and the other pulses to be echoes from the ionosphere. The method of Breit and Tuve became the basis for probing the ionosphere, using the reflections to determine the heights of various layers. Sophisticated formulas were worked out to explain how the various reflections observed were generated by the ionosphere. From these formulas and the experimental data, theorists could estimate layer heights, electron densities, magnetic field intensities, collision frequencies of the electrons and atmospheric particles, and reflection and absorption coefficients for the ionized media. 12 The ionization was assumed to be caused by solar radiations, and ultraviolet was taken to be the most likely agent. Sydney Chapman showed how a monochromatic beam of ultraviolet light would generate a parabolic distribution of electron concentrations in an exponential atmosphere of molecules (like oxygen) susceptible to ionization by the radiation (fig. 2).13 Starting with this basic theory and considering the effect of the various solar wavelength regions likely to influence the upper atmosphere, it was possible to estimate the variation with height of electron intensities and to make some guesses as to what the heavier ions might be. From both radio observations and theory, scientists concluded that the ionosphere had two main regions of ionization, region E, centering on 110 km, and region F2 centering on 275 km. The ionosphere was found to vary with time of day, season of the year, and phase of the sunspot cycle. For regions E, and F2 halfway between the minimum and maximum of solar activity, the average ionization intensities corresponded to 105 and 106 electrons per cc, respectively.14 Mainly during the daytime, regions E and F, formed at heights of 140 km and 200 km. Region D, at some uncertain distance below the E region, was observed at times of high solar activity, and presumably because of the increased molecular collision frequency at those lower altitudes caused pronounced absorption of radio signals of medium wavelength. At great distances from the earth, the earth's magnetic field was taken to be essentially that of a uniformly magnetized sphere; i.e., a magnetic dipole (fig. 3). Closer in, the field was observed to depart somewhat from that of a dipole, consisting of the dipole, or regular, part, and an irregular part. Some 94 percent of the earth's field, including some of the irregular field, was found to have its origin inside the earth. Of the remaining 6 percent of external origin, about half appeared to be caused by a flow of electric current between the atmosphere and the earth. The remainder, about 3 percent of the total field, appeared to be due to overhead electric currents. 15 Such electric currents could be produced by atmospheric motions at high altitude caused by solar or lunar tides, or by nonuniform heating of the atmosphere by the sun as the earth turned. While these more or less regular daily variations could easily be accounted for by electric currents in the ionosphere, magnetic storms which occurred at times of solar activity were more likely associated with streams of charged particles from the sun. The initial increase in magnetic field observed during a storm could be explained by the arrival of charged particles from the sun, which compressed the earth's magnetic field slightly and thereby increased its value temporarily. The strong decline in intensity to below normal values which soon followed the initial phase might be caused by a huge ring current around the earth, fed by the particle stream from the sun, as suggested by |