What Space Satellites Have Discovered

Press, Volume CIII, Issue 30389, 13 March 1964, Page 5

What Space Satellites Have Discovered

[By

BRUNO FRIEDMAN)

Scientific instruments in satellites and space vehicles launched since the onset of the space age in 1957 have added greatly to our knowledge of the earth’s space environment, ranging from the upper atmosphere out to the stars themselves. A summary of current knowledge of space physics was made in papers presented during a survey meeting on space physics information held during the fourteenth International Astronautical Congress in Paris.

Professor V. I. Krassovsky, (Russia) discussed the particles of the upper atmosphere and particularly the phenomenon of auroras. While these wavering ghostly curtains are most commonly seen visually in upper latitudes, particularly around the 65th geomagnetic parallels, modern advanced scientific instruments, sensitive to infra-red radiation are able to detect auroral phenomenon not visible to the eye. They have detected non-visible auroras in much lower latitudes and even near the equator. The auroras are caused by massive streams of lowenergy (10 kilo-electron volt) electrons and protons penetrating the earth’s upper atmosphere to within a hundred'kilometres of the earth’s surface. The most familiar auroras, the curtain type, are composed of rapidly flickering “rays” which are as small as a hundred metres in diameter. The mechanism of aurora formation is as yet unknown. While auroras follow after increased solar activity, they cannot be due directly to the greatly increased flow of charged particles streaming from the sun and striking the earth’s upper atmosphere. The earth’s magnetic field forms an effective barrier preventing this, except for protons of very high energy which can only penetrate in the near-polar regions where usually there are no intensive auroras.

The question remains, how are the particles of low energy produced which arrive in the upper atmosphere to cause the appearance of auroras? This question ties in very closely with the structure of the heterosphere, a subject which was discussed by Professor Marcel Nicolet of Belgium. All planetary atmospheres are divided into a “homosphere” and a “heterosphere.” The homosphere is the lower region of the atmosphere where the main gaseous and ionic constitare found in constant proportions. in the heterosphere, which lies above the homosphere and, for the earth, commences at about 85 km., the proportions of the constituents, which are different at different levels, will vary according to such factors as temperature, pressure and the rate of diffusion of the various gases in the earth’s gravitational field. Thus successive layers of the heterosphere will show diff-er-nt dominant atomic or molecular species and different dominant ion species and will have different mechanisms for the formation of the dominant ion species. The temperature and pressure (thus, gas densities) in the heterosphere are dependent on the ultra-violet radiation received from the sun and on cooling resulting from heat conduction. Since the intensity of ultra-violet light from the sun will vary during the course of the solar cycle, there will be considerable variations in the structure of the heterosphere. It will also show considerable variation between day-time when atmosphere heating is taking place and night-time when cooling is occurring. Van Allen Belt The nature of that belt of high intensity radiation surrounding the earth like a band lying between the two 40 deg. parallels and commencing about 500 kilometres up, called the inner Van Allen belt, was discussed by Dr. S. Fred Singer, of the United States. Dr. Singer also discussed the effect of the inner Van Allen belt on space exploration. One theory is that this hard radiation belt is produced by the action of cosmic rays. According to this theory, independently developed in 1958 by Dr. Singer and by Soviet scientists, cosmic rays strike the earth’s atmosphere producing high-energy neutrons which are reflected upward to higher levels. There they decay into protons and electrons which are trapped by the earth’s magnetic field. At about 6,000 km. up, the magnetic field is too weak to keep the particles trapped and this level is roughly the upper limit of the inner Van Allen belt.

Enough is now known of the nature of the belt so that astronauts in space craft and space platforms can be shielded against its radiation. However, beyond the belt, astronauts will be subject to unpredictable hard radiation composed mainly of high energy protons which result from solar bursts. This radiation, because of its intensity and unpredictable nature, will be much more difficult to protect against. Meteoric Dust

An interesting fact revealed by Dr. Singer in discussing the earth’s radiation belts, going beyond the inner Van Allen belt, was that the flux of high energy protons reached maxima at tw’o different levels above the earth. One maximum is at 1.5 earth radL and a second, the lower maximum at about 2.2 earth radii. The first is found where it would be expected on the basis of current knowledge. The reason for the second is unexplained. However, Dr. Singer proposed the very speculative hypothesis that there might be a belt of meteoric dust at about 2 earth radii <13.000 kilometres) . which would, absorb protons, thus creating a “valley” of reduced proton intensity with a second maximum existing as a peak only by reference to this

valley. This hypothesis bore some relation to an interesting discussion on meteoric dust in the neighbourhood of the earth made by Professor Frantisek Link of Czechoslovakia. Professor Link emphasised the role of rockets and satellites in taking meteoric dust counts in the upper atmosphere where there is no contamination from earth-origin-ated dust One way of measuring the density of the dust layer surrounding the earth is by observing the diflfusion of the earth’s shadow as seen on the moon during moon eclipses. The earth’s shadow on the moon is 3 per cent broader at its equator than would be expected if our atmosphere were dust-free. Artificial satellites, like the passive large diameter Echo satellite, can be used in the same way as the moon to make estimates of atmosphere dust content. Artificial satellites, particularly the Mariner II Venus satellite, have done much to

confirm a theory proposed 30 years ago on the nature of the solar plasma, sometimes called the "solar wind." This was pointed out by Professor Ludwig Biermann, of the German Federal Republic. Based on observations by astronomers that comets’ tails always pointed away from the sun, the theory stated that the solar plasma is a “wind” composed of electrons, protons, ions and some electromagnetic radiation, originating in the sun’s corona, and streaming outward radially in all directions. A solar wind detector on Mariner II provided some confirmation for this theory. The average velocity of the solar wind is about 450 kilometres a second.

Professor Evry Schatzman of France summarised current knowledge of the ultra-violet and x-ray spectra of the stars. The frequency distribution curve of the electromagnetic radiation from a star will shift towards higher frequencies with increasing star activity. Thus, all eruptions of surfaces of stars will probably be accompanied by an enormous in-i crease in output of X-rays. This suggests, said Professor v Schatzmann. that the gamma-ray portion of the electromagnetic spectrum could be used in searching the galaxies for supernovae. Ordinary stars emit very little gamma radiation, whereas supernovae emit tremendous quantities of it. Gamma photon detectors, if sensitive enough, could detect supernovae in the galaxies far more readily than do current methods.—(UNESCO)