Beyond the Atmosphere: Early Years of Space Science

 
 
CHAPTER 6
 
THE THRESHOLD TO SPACE
 
 
 
[58] Thirty thousand light-years from the center of a disk-shaped galaxy, itself measuring 100 000 light-years from edge to edge, planet Earth revolves endlessly around an average star, the sun, which with its attendant planets speeds toward remote Vega, brightest star in the northern skies. Although containing billions of stars, nebulas, and other celestial objects, most of the galaxy consists of empty, or nearly empty, space. To inhabitants of Earth the threshold to these outer voids is the upper atmosphere.
 
[59] One can easily show theoretically that the pressure and density of the atmosphere must decrease exponentially with increasing height above the ground, and experiment confirms this conclusion.1 Roughly, at least for the first hundred kilometers, pressure and density fall to one-tenth their former value for every 10-mile (16-km) increase in altitude. Hence, above 30 km only one percent of the atmosphere remains, while beyond 100 km lies only one-millionth of the atmosphere.
 
Interest in the lower atmosphere where people live and experience the continuous round of changes in weather and climate is obvious, but one might well ask what could possibly hold the attention, even of scientists, in a region so nearly empty as the upper atmosphere? The initial impression, however, is misleading. After closer study the upper atmosphere is found to exhibit many fascinating, often practically important phenomena-such as the ionosphere, which profoundly influences radio communications, especially shortwave; the auroras; electric currents, which at times cause magnetic effects that blank out both radio and telephone links; and the ozonosphere, which during the debate over fluorocarbon-propelled aerosols gained temporary stature in the public mind as the protecting layer that shields the earth's surface from life-killing ultraviolet rays of the sun. So interesting were the challenging phenomena of the upper atmosphere that by the time sounding rockets put in their appearance, scientists had already evolved from afar a coherent, comprehensive picture of the upper atmosphere and solar-terrestrial relationships. In the mid-1940s this remarkably complete picture formed a paradigm that hundreds of geophysicists around the world shared and used in reporting their continuing researches at scientific meetings and in the literature.
 
The main features of this paradigm were set forth in an article on the upper atmosphere by B. Haurwitz, first published in 1936 and 1937 and reissued with some updating in 1941.2 For those who began using sounding rockets in 1946 to explore the upper atmosphere, Haurwitz's concise review provided a helpful introduction, while a review paper by T. H. Johnson told much of what was known about cosmic rays from ground-based and balloon researches.3 A classic paper by Fred Whipple on the use of meteor observations to deduce atmospheric densities at altitudes between 50 and 110 km was one of the best examples of the ingenuity necessary in studying a region not yet accessible to them or their instruments.4 But the work that best described the state of knowledge of the earth's high atmosphere at the very time when the sounding rocket program was getting under way in the United States was a book of more than 600 pages, The Upper Atmosphere, by Indian scientist S. K. Mitra. Mitra furnished an exhaustive review of upper-atmospheric research, concluding with a chapter summarizing what had been learned and listing some of the most important problems needing further research. The very last paragraph noted that as the volume was going to press word had reached him:
 
[60] that experiments are being conducted in the U.S.A. with the V-2 rockets to study the cosmic rays and the ionospheric conductivity up to heights of 150 km (August, 1946). It is hoped that the scope of these experiments will be extended and that the records obtained therefrom will, on the one hand, give direct information of upper atmospheric conditions and on the other reveal the true picture of the intensity distribution of solar ultraviolet radiation and thus help to solve the many mysteries of the upper atmosphere which till now have resisted all attacks.5
 
Mitra's hopes paralleled the motivations of the sounding rocket experimenters, many of whom were entering what was to them an entirely new field.
 
Following is an elaboration of the extensive paradigm that the space scientists inherited from the ground-based researchers (fig. 1). The description is based on the works cited above, especially on Mitra's treatise.
 
The atmosphere extended to great heights, auroras being observed on occasion to more than 1000 km. Pressure and density were calculated and observed to fall off in exponential fashion. If the temperature were uniform
throughout the atmosphere, the decline in these quantities would be given by
 
p = p0exp(-h/H)
 
and
 
p =p0exp(-h/H)

where p and p denoted pressure and density respectively, the subscript zero indicated values at the ground, and h was altitude. The quantity H, known as the scale height, was given by

 
H = kT/Mg
 
where
 
k = Boltzmann constant = 1.372X 10-16 erg/degree
T = temperature in kelvins
M = mean molecular mass of the air = 4.8 X 10-23 gm
g = acceleration of gravity.6
 
 
In (1) and (2) the value of g was assumed to be constant, whereas in actuality gravity varies inversely as the square of the distance from the....
 

[
61Figure 1. The upper atmosphere as visualized in the mid-1940s.
 
[62] .....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 1,000 K or possibly 1,500 K at 300 km, with much higher temperatures beyond. Calculations from auroral observations were, however, not always consistent with this picture, often [63] indicating considerably lower temperatures than those deduced from radio observations. Atmospheric composition near the ground was known to be:
 
 

(percentage by volume)

Nitrogen

78.08

99.9

Oxygen

20.95

Argon

0.93

Carbon dioxide

0.03

Neon

1.8 x 10-3

Helium

5 x 10-4

Krypton

1 x 10-4

Xenon

1 x 10-5

Ozone

Variable, > 1 x 10-6

Radon (average near ground)

6 x 10-18

Hydrogen

Doubtful, < 1 x 10-3

 
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 to 1760 , 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 2,550 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 [64] 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.8 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.11 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 E1 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 E1 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 E2 and F1 [65] 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....
 

Chapman Layer
 
Figure 2. Chapman layer. The parabolic distribution was estimated to be within 5 percent of the actual distribution of charge densities to a distance of one scale height H ( = kT/mg) above and below the level of maximum ionization.
 

[66]
Earth's magnetic field
 
Figure 3. Earth's magnetic field. The broken lines depict the lines parallel to the direction of magnetic force. As became increasingly clear over the years, the actual magnetic field of the earth differs considerably from this idealized picture of a dipole field.
 
....Chapman, Ferraro, and others. Then the gradual recovery from this "main phase" of the magnetic storm, as it was called, signified the gradual dissipation of the ring current and a return to normal conditions-or so it was thought.
 
Among the most notable of high-altitude phenomena, and among the earliest to be studied in detail, were the auroras, the northern and southern lights. These were seen most frequently at heights from 90 to 120 km, but also occurred at both lower and much greater heights. That the auroras correlated strongly with activity on the sun and appeared in an auroral belt at high latitude suggested that they must be due to charged particles from the sun. Charged particles would be steered by the earth's magnetic field, whereas neutral particles or solar photons would not be affected by the earth's field. Experimenting with cathode rays and small magnetized spheres, K. Birkeland in 1898 and others demonstrated how electrified particles approaching a magnetized sphere from a distance would be guided by the magnetic field toward the poles. Starting from Birkeland's concepts and experiments, over many years Carl Stormer developed a theory of how electrons or protons from the sun would be deflected by the earth's magnetic field into the auroral zones to produce the auroras as the particles impacted on the atmospheric molecules, causing them to glow.16 The spectrum of the aurora was observed to exhibit primarily lines and bands of atomic oxygen and molecular nitrogen, with the forbidden green lines of atomic oxygen at 5577 Å being particularly strong.
 
At nighttime the high atmosphere was seen to emit a very faint light, sometimes called the permanent aurora, also consisting of the forbidden lines of atomic oxygen and of bands of the nitrogen molecule. This air-glow [67] was estimated to come from well above 100 km, perhaps from as high as 400 to 500 km, very likely from F-region ions as they were neutralized during the night. The yellow sodium D lines were also seen emanating from the lower part of the E region, and were particularly intense at twilight. From a distant cloud of material particles of some sort, the zodiacal light, with a spectrum similar to that of the sun, contributed to the light of the night sky. In the mid-1940s it was not known whether this radiation came from within the high atmosphere or from interplanetary space.
 
At some height, probably around 800 or 1,000 km, the atmosphere was expected to cease acting like a normal gas. In this region collisions between atmospheric particles would be infrequent, and a molecule might rise along an elliptic orbit to an apogee and fall back without colliding with another molecule until returning to the denser atmosphere at lower altitudes. If the molecule had sufficient velocity it might even escape into interplanetary space. Indeed, it was supposed that hydrogen and helium had to be escaping continuously through this fringe region, even though neither had been detected in the upper atmosphere. Helium was known to be entering the atmosphere from the ground-where it was produced by the decay of radioactive elements-at a small but measurable rate; but the percentage of helium in the lower atmosphere remained constant over time. The natural conclusion was that this light gas had to be diffusing up through the atmosphere to the highest levels where the very, high temperature permitted a ready escape of the gas.
 
Somewhere in this fringe region, or exosphere, the transition from the earth's atmosphere to the medium of interplanetary space was assumed to lie. One was hard put to it to define the boundary. Presumably where the atmospheric density had dropped to the few particles per cubic centimeter expected in interplanetary space the boundary must already have been crossed. But long before then the atmosphere had ceased to exist in the usual sense of the term. Across this ill-defined interface, radiations from the sun entered the earth's environs to cause the auroras, magnetic storms, ionization, and heating of the atmosphere.
 
Across this interface also came the cosmic rays.17 These highly energetic particles from outer space were more the concern of the high-energy physicist than of the geophysicist. Discovered between 1911 and 1914 from balloon experiments on atmospheric ionization, cosmic rays quickly became a subject of intense interest. It was soon accepted that the rays came from outside the earth. Measurements of the ionizing power of the rays at various depths below the surfaces of mountain lakes revealed both a soft component and a hard, or extremely penetrating, component to the rays. Balloon experiments showed that the intensity of the radiation increased steadily with altitude until a maximum-called the Pfotzer maximum-was reached at about 20 km in mid-lititudes. The shape of these intensity-altitude curves is shown in figure 4a.18 Figure 4b shows schematically that....
 

[
68]
Figure 4(a), at left. Cosmic rays at high geomagnetic latitudes. Figure 4(b), at right. Geomagnetic effect on cosmic rays. A schematic drawing showing how, according to experimental measurements, cosmic ray intensities vary with geomagnetic latitude. See Bowen, Millikan, and Neher in Physical Review 53 (1938): 855-61.
 
.....the earth's magnetic field has a distinct effect upon the radiation, leading to the conclusion that the rays are charged particles, not photons.
 
The shape of the intensity-altitude curve was explained as follows. The primary rays, whatever they might be, upon striking the atmosphere produced a shower of secondary rays, which, added to the primary rays, caused the initial increase in total ionization observed at high altitude. Eventually, however, an equilibrium was reached, with the atmosphere absorbing enough energy from both the primary and secondary particles to decrease the total ionizing power with further depth into the atmosphere. Such a transition curve, as it was called, would be observed not only in air, but also in lead or other substances, the principal difference being the spatial extent of the transitions, which depended on the density and nature of the material.
 
The early idea that the primary cosmic rays might be high-energy electrons was soon rejected. It could be shown that to penetrate the entire atmosphere and reach the ground, electron showers would have to be caused by primary electrons with such high energy that they would be completely unhindered by the earth's magnetic field. They would accordingly [69] not exhibit the magnetic field effect already shown to exist. In 1938 T. H. Johnson concluded that the primary radiation consisted of protons, as theorists had guessed somewhat earlier. In 1941 balloon observations revealed that the cosmic rays within the atmosphere at high altitude were mostly mesotrons (mesons), presumably generated by the primary protons.19 No significant component of electrons was observed at high altitude, supporting the conclusion that there could be no significant component of electrons in the primary radiation. But the soft component observed near the ground was believed to be electrons, decay products of the mesons generated at high altitude.
 

 
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