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Beyond the Atmosphere:
Early Years of Space Science
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- CHAPTER 6
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- THE HARVEST
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- [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.
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- 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.
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- 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.
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- 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
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- 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.
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- 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.
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- 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
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- The ionosphere was also receiving
immediate attention in the sounding rocket program. Among the
early experimenters, J. Carl Seddon undertook....
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- [77]
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- 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.
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- ....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.
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- The phase speed c , wavelength
X
[Greek letter Lambda] , and frequency f of a steady-state
radio signal satisfy the equation
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- c = Xf
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- while the relation between c and
co, the phase velocity in free space, is
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- c = c0/n
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- 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:
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- [Delta] f = - f(v/c)
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- = - fnv/c0
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- 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.
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- 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.
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- 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,....
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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.
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- [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.
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- 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
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- 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.
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- 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.
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- 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.
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- 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
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[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.
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* Ster, short
for steradian, a common measure of solid angle.
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