Beyond the Atmosphere: Early Years of Space Science

CHAPTER 6

PROBLEMS TO SOLVE

[69] Thus, the scientific paradigm for the earth's upper atmosphere in the mid-1940s was rich in ideas accumulated over more than half a century of observation and theoretical study. It had been possible to explain to a considerable degree a wide range of phenomena, many of which proved to be extraordinarily complex; but many uncertainties, unanswered questions, and problems remained.

Consider the problem of estimating atmospheric densities in the E region of the ionosphere around 100-km altitude. In the 1920s F. A. Lindemann and G. M. B. Dobson approached this problem by using observational data on the heights of appearance and disappearance of visual meteors. Intuitively it seemed reasonable that the density of the gas traversed by a speeding meteor should play a role in determining where the meteor would glow and be visible. The challenge was to develop a suitable theory to relate the observed meteor trails to the atmospheric density. Lindemann and Dobson assumed that as the meteor rushed into the atmosphere, a hot gas cap formed because of compression of the air. Heat from the gas cap was transferred to the meteor, and if the object were small enough it became incandescent. Making a number of assumptions about how heat was transferred from the gas cap to the meteor and using kinetic theory, Lindemann and Dobson derived expressions for pa, the density of the atmosphere at the height of appearance, and pd, the density at the height of disappearance of the meteor. The equations are reproduced here to emphasize the large number of quantities involved, uncertainties in which could cause errors in the derived atmospheric densities.

and

[70] where

Pm = density of the meteor
s = specific heat of the meteoric material
T2 = temperature of the surface of the meteor
r = radius of the meteor
X = angle of the meteor path to the vertical
g = acceleration of gravity
Mo = molecular weight of the air
k = (V1 - V2)/3v = calculated efficiency factor of heating
V1 = velocity of the compressed gas molecules in front of the meteor
V2 = velocity of the gas molecules at the temperature of the meteoric surface
= latent heat of vaporization of meteoric material
v = velocity of the meteor, assumed constant
R = universal gas constant
To = temperature of the atmosphere, assumed isothermal throughout the
range of consideration
L = total length of the meteor trail
(Delta) h = projection of L on the vertical.20

From the apparent brightness of the meteor the rate at which energy was being emitted could be calculated, which multiplied by the time of visibility gave the total amount of energy radiated. Setting this equal to the kinetic energy 1/2mv 2 yielded the mass m of the meteor. If one then assumed that the meteor was iron and essentially spherical, one got from the expression

mass = density times volume

m = Pm . (4[Greek letter Pi]r3/3)

which gave the radius r. The other quantities in the expressions for the atmospheric density could be either measured directly or estimated from plausible assumptions, thereby giving densities at two altitudes, that of appearance and that of disappearance.

The chain of reasoning was lengthy, with many assumptions. The results obtained by the investigators immediately put some of the assumptions into question. For example, the air densities obtained proved three times too high to correspond to an isothermal atmosphere at the stratospheric temperature of 220 K, requiring instead temperatures around 300 K. Between the stratosphere and the E region of the ionosphere, then, there had to be a significant variation in temperature. Moreover, other observations [71] 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:

(4[Greek letter Pi]Noe e2)mp 2 = 1

where

Noe = value of the electron density at the point of reflection
e = the electronic charge
m = mass of the electron
p = angular frequency of the radio signal.24

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 [72] 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.....

[73]

Figure 5. Radio wave reflections from the ionosphere. The time required for a signal to go to the ionosphere and return to ground gives a measure of the reflecting layer's height.

.....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

[74] 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.