Beyond the Atmosphere: Early Years of Space Science

 
 
CHAPTER 6
 
THE HARVEST
 
 
 
[75] 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 A 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 [76] Center in Massachusetts contributed much detail on the solar spectrum in the far ultraviolet.
 
As had been anticipated, the ultraviolet spectrum of the sun, which proved to be very complex, did not correspond to a simple black body radiating at a 6000 K temperature as in the visible part of the spectrum. This finding was dramatically shown in a comparison of actual intensities obtained by NRL on 7 March 1947 with the 6000 K blackbody curve, shown in figure 6.
 
On 5 August 1948 in an Aerobee rocket flight to 96 km, T. R. Burnight detected what appeared to be x-rays in the upper atmosphere. Burnight did not follow up on his discovery, however, and it was left to others to pursue the subject.36
 
These data on the solar spectrum below the atmospheric cutoff at about 2800A supplied theorists with much of the missing information to explain how and where the sun's radiation produced different atmospheric layers. The workers at the Naval Research Laboratory and the Applied Physics Laboratory used observations on the change in solar ultraviolet intensities with altitude to determine the distribution of ozone in the upper atmosphere.37 It was established that the level of maximum ozone production lay in the vicinity of 50 km, hence that the higher concentrations of ozone a lower altitudes had to be due to atmospheric circulations.
 
Solar ultraviolet could be tied with confidence to the E region of the ionosphere. The intense Lyman alpha radiation of the neutral hydrogen atom penetrated to 70 km and influenced the lower E region and upper D region of the ionosphere. But x-rays in the vicinity of 2 Å penetrated deep into the D region and were far more efficient in producing ionization in the D layer than was hydrogen Lyman alpha.
 
Atmospheric structure-that is, the variation of pressure, temperature, and density with altitude-also received the early attention of the rocket experimenters.38 Almost every flight carried gauges to measure these fundamental parameters. Signal Corps and University of Michigan groups adapted anomalous sound propagation techniques to the rocket by sending explosive grenades aloft to be set off at high altitude; the sound waves could be used to determine both air temperatures and winds up to 60 km or higher.39 Those measuring x-ray intensities used the observed absorption of x-rays in the ionosphere to estimate air densities there.40 As a result of many rocket observations, in the early 1950s the Rocket and Satellite Research Panel was able to issue an improved estimate of upper-atmospheric structure for use by geophysicists.41 By the time Sputnik went into orbit, the groundwork had been laid to describe the structure through the F region of the ionosphere and to give a considerable amount of information about both geographical and temporal variations of these quantities.42
 
The ionosphere was also receiving immediate attention in the sounding rocket program. Among the early experimenters, J. Carl Seddon undertook....
 

 
[77
 
Line graph of the Solar spectrum
 
Figure 6. Solar spectrum. Solar intensities above the ozonosphere at White Sands, New Mexico, Spectrum of 7 March 1947, replotted on a linear intensity scale relative to the intensity of a black body at 6000 K. Durand et al, Astrophysical Journal 109 (1949): 1-16. Illustration courtesy of the Astrophysical Journal, published by the University of Chicago Press, copyright 1949, The American Astronomical Society. All rights reserved.
 

 
....to adapt the propagation techniques of the ground-based probers to the rocket. He used the influence of the ionosphere on radio signals from the flying rocket to deduce charge densities existing in the atmosphere.
 
The phase speed c , wavelength X [Greek letter Lambda] , and frequency f of a steady-state radio signal satisfy the equation
 
c = Xf
 
while the relation between c and co, the phase velocity in free space, is
 
c = c0/n
 
where n is called the index of refraction of the medium in which c is the phase velocity. If the signal source is in motion relative to the observer, a shift in frequency, the well known doppler shift, results:
 
[Delta] f = - f(v/c)
 
= - fnv/c0
 
The original transmitted frequency could be carefully fixed in an experiment, [Delta]f and v could be measured, and c0 would be a known constant. Hence n could be calculated. Since n depended on the electron and ion concentrations, their collision frequencies, and the strength and direction of the [78] magnetic field, one could thus get an equation relating, the very quantities to be determined.43 Seddon arranged his experiment so as to get several such equations, which could be solved simultaneously to give electron densities as a function of height, and sometimes some of the other quantities such as collision frequencies.
 
Although transmitting the probing signal from the flying rocket was supposed to reduce the complexity, many of the difficulties experienced by the ground-based probers remained. Inhomogeneities in the ionosphere, multiple reflections of the propagated wave, splitting of the signal into ordinary and extraordinary rays, and not knowing the identities of the ambient ions made the reduction and interpretation of the data a challenge. Nevertheless, Seddon was able to improve upon electron density curves obtained from the ground and to furnish some information about the lowdensity regions that had been hidden from the probing of the ground-based investigators. Figure 7 shows a curve of electron density changing with altitude, drawn by John E. Jackson from a composite of NRL data and measurements by other groups.
 
Other experimenters preferred to avoid the problems inherent in propagation experiments by using various kinds of ionization gauges. Even though the rocket introduced complications of its own, such as exuding gases carried from the ground and distorting the ambient electric field,....
 
 

Figure 7. Ionospheric charge densities Summary as of August 1958 of ionospheric data corresponding to summer noon, middle latitudes, and sunspot maximum. Courtesy of J. E. Jackson, CSAGI meeting, Moscow, 1958.

 
[79]....such gauge measurements were felt to be more "direct" than those obtained from the propagation experiments. Both techniques made their contributions, with the result that ground-based experimenters were provided with a standard, one might say, against which they could calibrate the methods of deducing results from their cheaper, more widespread observations.
 
Since the vexing question of composition continually entered into discussions of the upper atmosphere, particularly special regions like the ionosphere and the exosphere or fringe region at the top of the atmosphere, investigators soon tackled the problem of identifying atmospheric constituents as a function of altitude. At altitudes up to the bottom of the E region, workers from the University of Michigan tried sampling the air by opening evacuated glass vials or steel bottles in the upper atmosphere and immediately resealing them to lock in the sample before the rocket descended It was tricky, because one had to ensure that the bottles weren't sampling gases carried by the rocket itself and also that the sampling procedure was not somehow altering the composition of the sample. While these experiments provided some hints of diffusive separation of helium over limited ranges above the stratosphere, by and large they confirmed that the atmosphere was thoroughly mixed, up to the E region.44
 
The most powerful technique to be brought to bear upon the problem of atmospheric composition was that of the mass spectrometer.45 This device separates out the atmospheric particles in accordance with their molecular masses-or, more properly, in accordance with the ratios of these to their charges in the ionized state in which they are fed to the spectrometer's analyzer. While there can be some ambiguity, one can feel considerable confidence in the identifications achieved. With such an instrument John W. Townsend, Jr., and his colleagues at the Naval Research Laboratory produced a considerable amount of data on upper atmospheric composition above White Sands, New Mexico, and over Churchill, Canada.46 They confirmed that there was little diffusive separation below 100 km; but above 120 km separation processes, at least as indicated by the separation of argon relative to nitrogen, became quite effective. The changeover from molecular oxygen to atomic oxygen appeared to be slower than had been supposed. Neutral nitric oxide, NO, was shown to be a negligible constituent of the E region and above, since its presence would have been apparent in a pronounced absorption in the ultraviolet. No such absorption was observed in rocket solar spectrograms. On the other hand, NO+ turned out to be a major positive ion in the E region of the ionosphere. In northern latitudes, during the daytime above Fort Churchill, as altitude increased from 100 to 150 to 200 km the relative abundances of positive ions changed from (O+2, NO+) to (NO+, O+2, O+) to (O+, NO+, O+2). In the United States above White Sands the results were similar except that the nitric oxide ion NO+ was the predominant ion in the E region. In all cases O+ was the predominant positive ion above 250 km, while according to Soviet data N+ [80] was never more than about 7% of the O+ for altitudes up to more than 800 km. On several flights the negative nitrogen dioxide NO2- was detected in the E region.
 
Some of the uncertainties concerning the heights of emission of the night airglow were removed by rocket experiments.47 The atomic oxygen green line at 5577 Å was found to have its maximum at about 95 km, to show a sharp lower cutoff at 90 km, and to trail off at 120 km on the upper side. The sodium D lines (5890 Å - 5896 Å) came primarily from the region between 75 and 100 km, peaking at about 90 km. The red oxygen lines (6300 A - 6364 Å) came from above 163 km, while the 6257 Å Meinel hydroxyl, OH, band was emitted in the region from 50 to 100 km.
 
Although some measurements were made of the earth's magnetic field at high altitude and of associated current flows,48 this aspect of the high atmosphere received less attention during the first decade of rocket sounding than it would later when satellites became available. Cosmic rays were, however, a matter of intense interest to a few researchers. Of the many aspects of this fascinating subject to pursue, two topics in particular stood out: (1) What was the cosmic ray intensity above the atmosphere? (2) What was the composition of the cosmic rays? James A. Van Allen tackled these questions in a rather straightforward way. By sending a single geiger counter into the upper atmosphere, he was able to trace out a counting rate curve that rose to a Pfotzer maximum at a height of about 19 km, above which the remaining atmosphere corresponded to about 56 g/cm2 of material (fig. 8).49 With increasing altitude beyond that level the counting rate declined until it leveled off at a constant rate at and above 55 km. After several flights Van Allen was able to estimate the vertical intensity of cosmic rays at high altitude above White Sands to be 0.077 ± 0.005 particles per sec-cm2-ster,* close to the value that workers at the Naval Research Laboratory obtained. With rather poor statistics the rocket experimenters estimated that the primary cosmic rays consisted of protons and alpha particles in the ratio of about 5 to 1, with less than one percent heavier nuclei.50 These figures differed somewhat from better measures being obtained from balloon observations.
 
The energy spectrum of the cosmic rays had suggested a distinct lower bound for the cosmic ray particles. Van Allen began to investigate the lower energy end of the cosmic ray spectrum. He sent counters aloft at latitudes ranging from the geomagnetic equator to the polar regions. During these investigations, Van Allen noted a pronounced increase in the numbers of soft radiation particles encountered above the stratosphere in the auroral zone, particles that were not found at either lower or higher latitudes.51
 

[81] Figure 8. Cosmic ray flux. Smoothed composite curve of Applied Physics Laboratory single-counter counting rates above White Sands, New Mexico, geomagnetic latitude 41°N. Gangnes, Jenkins, and Van Allen in Physical Review 75 (1949). 57-69, courtesy of J. A. Van Allen and Physical Review.
 

* Ster, short for steradian, a common measure of solid angle.

 
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