tions indicated that it was not even likely that the temperature would be constant in the E region. Experiments with the anomalous propagation of sound mentioned earlier showed that atmospheric temperatures rose markedly between 30 and 55 km to between 336 K and 350 K at the latter altitude. Noctilucent clouds, on the other hand, strongly suggested very low temperatures at 80 km.21 The conclusion was forced, then, that the atmosphere was not isothermal, having temperatures which rose sharply above the stratosphere to somewhere at or above 55 km, fell again to very low values around 80 km, and then rose once more between 80 and 100 km.

Disagreements also arose over how the meteors became incandescent. One investigator objected to the idea of a gas cap, preferring to assume that the meteor was heated by direct impact with the air molecules. 22 In the early 1940s Fred Whipple obtained very accurate photographic records of meteor trails from which he could deduce decelerations. He developed an elaborate theory of how the properties of the upper atmospheric gases, the deceleration of the observed meteor and its heating to vaporization and incandescence, and its physical properties were all interrelated. Then, making some suppositions about properties of the incoming meteors and measuring deceleration and luminosity from the photographs, Whipple finally deduced the densities of the atmosphere along the trail.23 Again there were assumptions and corresponding uncertainties in the results.

For the ionosphericist the theoretical maze was even more complicated. The prober's principal tool was the radio wave. A signal sent into the ionosphere would be bent by the ionized medium, and if the charge density were great enough would be reflected downward again. For a simple layer in which the strata of equal ionization were horizontal, the condition for total reflection of a signal propagated vertically was:

where

(4π No e2)/mp2 = 1

(7)

No = value of the electron density at the point of reflection

Thus, a radio signal of low enough frequency sent into the ionosphere would continue upward until it reached a level at which the electron density was great enough to satisfy equation (7). At that point the wave would be reflected, returning to the ground after a delay corresponding to its flight along the upward and downward paths. As the wave frequency was increased, the wave would penetrate farther into the layer before being reflected, and the delay in the ionosphere would be increased. If the layer had

a maximum electron density, when the signal frequency exceeded the value (called the critical frequency) for which that maximum charge density would produce total reflection, then the wave passed through the layer and no return was observed at the ground.

By sweeping the signal frequency from low values to higher ones, one could generate a record of signal returns which could be displayed as shown in figure 5, curve E. The critical frequency could be read from the figure, from which the charge density at the point of reflection could then be calculated, using equation (7). With a little additional calculation, the height of the point of reflection could also be estimated, showing where the reflecting layer existed.

If, in the charge density, other maxima lay above and exceeded the initial maximum, then as the wave frequencies were increased new reflections would be observed, corresponding to the higher-altitude, more intensely ionized layers, as shown in curve F of figure 5. From the critical frequencies for these higher layers, estimates could be derived for the charge densities and heights of the upper layers.

By using an appropriate theory like that of Chapman concerning the formation of ionized layers by solar radiations (fig. 3), one could then estimate charge densities above and below the maxima obtained from the radio propagation measurements, and thus construct a continuous curve of charge densities versus altitude.

The concept was simple, but enormous complications entered when all the pertinent factors were considered. First, the ionosphere was by no means as simple as assumed in the foregoing example, and at times the propagation measurements indicated gross inhomogeneities. Moreover, one had to take into account the earth's magnetic field, collision frequencies among the particles in the ionosphere, and the fact that the ionization consisted not only of electrons but also of both positive and negative ions. The earth's magnetic field produced double refraction of the radio signals used to probe the ionosphere, splitting the signal into what were called ordinary and extraordinary rays, which followed different paths, had different points of reflection and different delay times, and were differently polarized—that is, the electric vectors of the two rays vibrated in different planes. When there were several ionospheric layers to deal with, and particularly under disturbed conditions, the problem of identifying properly the various return signals could become next to impossible. In addition, when the signal had to traverse a region in which the collision frequencies were high, as in a strong D region during times of high solar activity, the signal could be greatly attenuated or even blanked out. Not knowing the ions in the ionosphere simply added to the complication.

The mathematical expression of how all these factors affected the propagation of signals through the ionosphere was far more complicated than the simple expression of equation (7), and applying it to the determination

of charge densities in the ionosphere put great demands on ingenuity and insight.25

These two examples of how investigators restricted to working with observations obtained at or near the ground had to wrest the information they sought from long chains of supposition and theoretical reasoning illustrate the sort of opportunity that befell the rocket researchers, who expected to make direct measurements in situ. Since much, even most, of what went on in the upper atmosphere was caused directly or indirectly by energy from the sun, a most important contribution the rocket sounder could make was to measure the solar spectrum both outside the appreciable atmosphere and as affected by altitude within the atmosphere. Knowing the former would let the theorist know what wavelengths and intensities were generating ionization, various photochemical reactions, and ultimately heating in the atmosphere. Knowing the latter would immediately tell where the different wavelengths were having their effect. The importance Mitra put on this vital information is seen in his assertion that "the greatest obstacle in the study of the upper atmosphere, is undoubtedly the lack of our direct and precise knowledge of the energy distribution in the near and extreme ultraviolet radiation of the sun. For, conditions in the high atmosphere are almost entirely controlled by the sun."26

Many data the sounding rocket could obtain apparently could be obtained in no other way. In addition, many quantities that could be estimated from ground-based studies contained serious uncertainties which could be removed or lessened by rocket measurements. These circumstances made it possible for a number of young rocket experimenters in short order to compete respectably in upper-atmosphere research against much more knowledgeable scientists of many years' experience. The ways in which newcomers could contribute may be illustrated by listing some of the problems that in the mid-1940s still awaited solution.27

Diurnal, seasonal, and other temporal variations in atmospheric pressure, temperature, and density were needed.

A correct description of atmospheric composition at all altitudes would be invaluable. One could determine the distribution of ozone in the upper stratosphere and middle atmosphere and find the level at which most of the ozone was formed. Knowing the composition would also allow one to know definitely to what altitudes the atmosphere was completely or nearly completely mixed, and at what altitudes diffusive separation played an important role. In particular one would want to know where oxygen began to dissociate into atomic form and at what altitude the dissociation had become complete, and whether at some altitudes nitrogen also dissociated. At what level would lighter gases like helium become an appreciable or even dominant component of the air?

With respect to the ionosphere, radio sounding could not determine the ionization intensities in a region lying above one of higher charge density. One had to rely on theory to try to fill in the missing information. But in situ measurements might remove this lack. Moreover, if the precise nature and concentrations of both the positive and negative ions could be determined, a better understanding could be developed of how the balance between those agents creating the ionosphere and those tending to destroy it was established. One would then be in a better position to determine the specific causes of the temporal and geographic variations in the various ionospheric layers.

There was little doubt that excitation, dissociation, and ionization of atmospheric constituents, as well as various energy transfer and recombination processes, were responsible for the night sky radiations; but there were various possibilities among which to choose. Moreover, there were gross uncertainties in the altitudes from which many of the radiations were thought to arise. Again in situ measurements should help to resolve the difficulties, not only by pinning down altitudes, but also by providing additional insight into the recombination coefficients and other fundamental parameters involved.

As for magnetic field effects, a prime target would be to locate the electric currents that were responsible. One would hope, too, to be able to detect and identify the particles that caused the auroras.

With regard to cosmic rays, the precise composition of the primary radiation needed to be determined; for this purpose, measurements in outer space well above the atmosphere of the earth should be useful. Additional information on the effect of the earth's magnetic field upon the cosmic rays would be interesting, but more fundamental would be data on whether the radiation was isotropic or anisotropic in free space. An intriguing question was how many of the cosmic rays coming to the earth were from the sun and how many were from outside the solar system.

THE HARVEST

Such were the problems to which the rocket experimenters addressed themselves. Once started, the results of their research flowed in a steady stream into the literature, contributing to a growing understanding of upper atmospheric phenomena. A concise summary of some of the more important results from the first dozen years of high-altitude rocket sounding appears in the author's book Sounding Rockets.28 A deeper, more detailed insight into what had been achieved may be had from volume 12 of the Annals of the International Geophysical Year.29 The following brief review is derived from these and other sources.

It is not surprising that the first questions taken up by the rocket experimenters were those considered the most significant by the ground-based researchers. Naval Research Laboratory investigators built spectrographs and sent them aloft to photograph the solar spectrum at high altitude. On 10 October 1946 Richard Tousey and his colleagues obtained the first photographs of solar spectra from above the ozonosphere. 30 This event marked the beginning of many years of intensive research on the structure and energy content of the solar spectrum in both the near and far ultraviolet and eventually in the x-ray region, using a variety of techniques including spectrographs, photon counters, and photosensitive phosphors.31 Experimenters at the Applied Physics Laboratory of the Johns Hopkins University quickly followed up the NRL achievement with spectrographic experiments of their own, obtaining highly detailed spectrograms.32 In March 1947 the Naval Research Laboratory workers obtained additional spectra at various altitudes reaching to 75 km, and in June 1949 more spectrograms were recorded.33 In the years that followed, both University of Colorado and Navy workers developed pointing devices to keep rocket-borne spectrographs aimed at the sun, and with these obtained more detail and continually extended the spectra to shorter and shorter wavelengths. Using the pointing control, the group at the University of Colorado in 1952 flew a spectrograph to about 85 km. In addition to the by now familiar ultraviolet spectrum from 2800 Å to about 2000 Å, there was a strong emission line at 1216 Å. This was quickly identified with the Lyman alpha line of the neutral hydrogen atom.34 Between 1952 and 1955 both the Naval Research Laboratory and Air Force groups confirmed the presence of other emission lines between 1000 Å and 2000 Å. In 1958 the University of Colorado team used a specially designed spectrograph to photograph the solar spectrum from 3000 Å all the way to 84 Å in the extreme ultraviolet.35 About 130 emission lines were measured and their intensities roughly estimated. The resonance line of ionized helium at 304 Å was found to be very strong. In the years following, the Colorado workers, those at the Naval Research Laboratory, and a group at the Air Force Cambridge Research