Our Place in the Sky

DIRECTOR OF RESEARCH, THAYER SCHOOL OF ENGINEERING

A Little Perspective

Many men are awed, and many more stupefied, by the avalanche of scientific discovery and technological development which we are witnessing. More than anything else about it, we are worried about our lagging social adaptation. Perhaps we can take comfort in the knowledge that this is indeed a very old dilemma in our cultural development. Whereas one marvels that democracy was conceived and practiced by the Greeks more than two thousand years ago, consider the recent archeological deduction that an object, found in 1901 by Greek divers on a wrecked shipload of marble and bronze statues lying in two hundred feet of water off the little island of Antikythera, and reliably dated from the first century B.C., is in fact a sophisticated analog computer which reads out on three dials the motions of the planets, the rising and setting of the stars, the phases of the Moon, and the eclipses of the Sun. Derek Price, who finally solved the riddle of this disintegrated tangle bronze gears, inscribed plates, and graduated dials, said It was like finding a jet airplane in the tomb of King ‘Tut’!" Although we cannot help feeling privileged to have witnessed during our lives the first vehicles to leave the Earth altogether, certainly we must at the same time feel a measure of humility that from an auspicious position two thousand years ago, it has taken until now to do it. In this view, perhaps the scientific and technological progress which has now been made, does not completely outstrip the sociological progress which has also been made.

Now to see our planetary environment in astronomical perspective, let us begin with the galaxies. The strongest radio source observed in our sky is not a single radio star but two galaxies in the process of collision, a remarkable event when one considers that even though the galaxies of stars are of incomprehensible dimensions, the separations in space are vastly greater. Our own galaxy, known simply as the Galaxy with a capital G, contains in one of its spiral arms, an ordinary sort of star, the Sun, which like most visible stars is in the midst of a long stable period of dissipation by converting itself to radiation through a nuclear process figured out by Hans Bethe at Cornell. Just after World War II, vail de Hulst at Leiden predicted that neutral hydrogen in the tenuous expanses between the stars of the Galaxy would radiate at 1420.405 megacycles per second. This radiation was soon found and now much of the Galaxy has been mapped by observing it.

The Sun is accompanied by cold and solid planets, and cosmologists such as Fred Hoyle or Harlow Shapley tell us that undoubtedly there are many other stars in the Galaxy with such planets. The Sun is spinning and the planets rotate about it in the same direction in orbits which are nearly plane. The planets, which are of various sizes and at various distances from the Sun, are all spinning in the same direction as the Sun (anti-clockwise as viewed from the North). The Galaxy is a flat, spinning hub with spiral arms which we see as the Milky Way when we look across it. The plane containing the Sun and its planets is inclined at about sixty degrees to the galactic plane. A number of the planets in the Solar System have satellites or sub-planets associated with them. Ours, the Moon, rotates around us in the direction of the Earth's spin, always keeping one face towards us. The Moon is larger in relation to its parent body than the satellites of any of the other planets in the Solar System. The planets have atmospheres of various densities and chemical compositions which blend into the interplanetary environment.

The Sun's Pervading Atmosphere

Though the hydrogen gas which occupies the interplanetary environment is more tenuous than that remaining in the best vacuum we can produce in a vessel on Earth, it is nevertheless significant. Sydney Chapman has successfully pursued the thesis that it is the Sun's hydrogen atmosphere or corona which extends to a diminishing degree throughout the entire solar planetary system. From a temperature of about 5,000°K at the photosphere, which is the apparent sharp limit of the Sun, the temperature is known to rise in the overlying chromosphere to about a million degrees at a height of 40,000 kilometers above the photosphere. The corona probably has a varying supply of heat from below and can radiate little, but it is an excellent heat conductor. In this way, it continuously loses heat by downward flow into the photosphere and by outward flow into interplanetary space. Using balloons and aircraft, the solar corona has been observed out to twenty solar radii, or one-tenth the distance to the Earth, and the temperatures measured are in agreement with Professor Chapman's calculations. Extending the calculation to the distance of the Earth, he obtains a temperature of about 200,000°K.

Above 85 kilometers or so in the Earth's atmosphere, the temperature is found to rise steadily. Although the gradient is not known with certainty, it is perhaps not more than 10° centigrade per kilometer. It must decrease with increasing height because the same heat flows through a greater spherical surface and because the thermal conductivity of the gas increases with increasing temperature. Taking these factors into account, it seems likely that the radius of the boundary where the terrestrial atmospheric temperature reaches that of the ambient solar atmosphere may be about forty Earth radii or nearly half the distance to the Moon. Similar considerations apply in suitable measure to the other planets as we envision the tenuous but hot solar atmosphere pervading the entire Solar Planetary System with the relatively very cold planets imbedded in it as heat sinks. At Pluto, the ambient temperature should be about 70,000°K.

It is estimated that as much as 1017 kilograms of matter may be emitted by the Sun per year. The density of this coronal hydrogen gas must decrease with distance from the Sun because of the uniform radial outflow and at the distance of the Earth, Professor Chapman has calculated the density to be about 1000 protons per cubic centimeter accompanied by an equal number of electrons. This approximate density figure is confirmed by observation of a number of divers phenomena: the intensity and polarization of the sunlight that is backscattered onto the dark side of the Earth by the interplanetary gas (the zodiacal light); the existence of whistling atmospherics, that is, the very low frequency radio waves which are emitted from lightning flashes and guided along the flux of the Earth's magnetic field out to distances of several Earth radii and back to Earth again in the opposite hemisphere; the intensity of auroras; and the acceleration of comets' tails. Recently the Soviet Academy of Sciences has announced that their cosmic rocket Mechta (or "Lunik I"), making a direct observation with ion probes, found the atmospheric density to be down to 1000 positive ions per cc at 1500 km and to 500 at 2000 km but that it held to this value beyond that distance.

The Terrestrial Atmosphere

The Earth's atmosphere reflects about one-third of the radiation falling on it from the outside. The transmitted portion is partly absorbed by the atmosphere and partly transmitted to the ground. Transmission occurs only in certain parts of the spectrum, notably the optical window which is but an octave wide (4-8000 Å) and the radio window which is about three decades wide (1cm—10m). The narrow optical window includes the somewhat narrower, visible portion of the spectrum, which unique fact certainly has significant human evolutionary implications. All wavelengths shorter than the optical window are molecularly absorbed. All wavelengths longer than the radio window are turned back by the conducting ionosphere. In the infra-red region between the optical and radio windows, there are some narrow transmission bands.

The upper atmosphere is ionized by the solar radiation, mainly by soft x-rays. This process, which strips electrons from atoms and molecules, extends down to about 85 km, that is, to the height above which the temperature rises steadily with height. The maximum concentration of free electrons is produced at about 250 km and is called the F-layer of the ionosphere. A typical value is about 106 electrons/cc. A very important subsidiary maximum occurs at 100 km, the ionospheric E-layer, where the mid-day value in middle latitudes is about 105 electrons/cc. At 100 km the temperature is approximately 200°K but at 300 km it has already reached 1400°K. lonization however is not complete until the temperature has reached about 30,000°K in the outer hydrogen atmosphere. Of course, the gas density is then so low that the total number of free electrons produced is much smaller than at lower levels.

lonization equilibrium requires a balance of the ionization and recombination rates. At 100 km, ionization disappears very rapidly when the ionizing radiation is removed as at night. However at 250 km, because the mean free path is so much longer, the ionization diminishes very slowly. The presence of significant residual ionization at 100 km at night has been a long-standing enigma. We have also long wondered what happens at greater heights in the prolonged polar night and have now learned from the South Pole scientific station that the ionization does not undergo any large decrease at that time. What is more, it exhibits a strong diurnal variation, as at lower latitudes, even though it is not alternately illuminated and darkened by the earth's rotation. This presents a new puzzle.

The Geomagnetic Field

It was first recognized by William Gilbert, a contemporary of Shakespeare, that the Earth possesses a significant magnetic field. On the basis of potential-theory, it has been demonstrated that the main field rises from sources within the Earth. The nature of the source has not been determined but the most plausible theory is that it is an ordered turbulence within the liquid core. The recent finding by the Russian rocket "Lunik III" that the Moon has no substantial magnetic field is consistent with this theory and with the belief that the Moon has no liquid core, although the latter point must be reconciled with the observation by the Soviet astronomer Nikolai Kozyrev of what he interpreted to be a volcanic eruption on the Moon. The external magnetic field of the Earth approximates that of a uniformly magnetized sphere or, what is equivalent, that of a small dipole placed at the center of the Earth. The axis of this field is inclined 11.4° to the spin or geographic axis. There are marked perturbations in the field near the Earth's surface, presumably due to non-uniformity in the permeability of the Earth's crust. The effect of these perturbations is significant well into the ionosphere. The magnetic or dip poles are the places where the field is perpendicular to the surface. Because of the surface perturbations, these do not coincide with the geomagnetic poles defined for the unperturbed field. The field undergoes long-term (secular) changes which are barely significant from year to year.

Now imagine a bar magnet being rotated, not about an axis along its length, but about an axis inclined at 11.4° to that direction. Then the magnetic field wobbles as it turns in space. This is precisely the way the Earth's field looks when seen from interplanetary space.

When an electric charge moves across a magnetic field it experiences a force proportional to the magnitude and speed of the charge and to the strength of the magnetic field. The direction of the force is mutually perpendicular to the motion of the charge and to the direction of the magnetic field. In the ionosphere, this is of basic importance because there are many charged particles produced by ionization and the geomagnetic field is present throughout. Consequently, winds, drifts, and turbulence in the ionosphere are very complicated. Positive and negative charges experience opposite forces so that charges are separated and the region is polarized (the Hall effect). This in turn produces an electric field which produces additional forces on the charges. Furthermore, moving charges constitute electric currents which give rise to their own magnetic fields. A study of this very complicated situation is called magneto-hydrodynamics. It is particularly essential to the study of stellar atmospheres such as that of the Sim.

Solar Activity

Solar activity undergoes a cyclic variation with an eleven-year period and the number of sunspots to be seen is a measure of the activity. The first spots of a new cycle appear at high latitudes on the Sun. As the cycle increases in intensity, the spots appear in greater numbers and size, and at lower and lower latitudes. Sometimes the first spots of a new cycle appear at very high latitudes while some from the dying cycle are still to be seen near the equator. Sunspots have magnetic fields which are all in one direction for spots associated with one cycle and all in the other for spots associated with the preceding or following cycle. Strictly speaking then, the sunspot cycle is a twenty-two-year cycle. When solar activity is high, the general level of radiation is raised and there are also then great outbursts of gas which spray out from the turning Sun like streams of water from a lawn sprinkler. When one of these engulfs the Earth, many phenomena occur. The charged particles are guided by the turning and wobbling Earth's magnetic field into very complicated orbits including the trapped orbits confirmed by James Van Allen's observations with artificial satellites. As the particles precipitate into the atmosphere, they excite the air by collision and cause it to glow, producing the aurora. The fortuitous observation of artificial aurora at Apia produced by the first nuclear explosions in the ionosphere presaged the more exciting Argus shots which injected electrons into trapped orbits in the ionosphere above the South Atlantic and produced artificial aurora near the Azores at the other end of the trapped orbit.

The sequence of events commencing with a solar outburst has received intense study by a handful of scientists for more than sixty years. In 1903 Kristian Birkeland conducted a magnificent laboratory experiment in which he shot a stream of electrons, or cathode rays as they were then called, at a magnetic dipole. The complicated trajectories which the particles revealed as he changed their speed and aspect attracted his colleague at the University of Oslo, Carl Stormer, to calculate them. As Professor Störmer became more ensnared by the complexity of the problem he began to take photographs of the aurora to supplement his calculations, making many thousands of exposures. Fifty years after starting on the problem, he published a collation of his work in a 400-page volume. Even now, with the wealth of synoptic data taken during the International Geophysical Year, the whole sequence of events on the Earth which may follow the observation of a solar flare has not been unravelled. The light from the flare reaches the Earth in eight minutes. A strong burst of ultraviolet radiation ionizes the atmosphere below the usual level and causes almost instant blackout of ionospherically reflected radio waves because of the short mean free path for electrons at the lower level. A burst of radio noise arrives more or less simultaneously. The frequency of the maximum intensity of the noise declines rapidly, corresponding to the upward progress of the disturbance on the Sun. About twenty hours later, the gas stream arrives and particles begin to precipitate into the atmosphere and circulate within it causing strong magnetic variations and prolonged radio blackouts. Very low frequency radio waves (the "dawn chorus") are induced. Particles arriving from the Sun in twenty hours must travel at about 1000 km/sec. But auroral effects require particles moving at ten times that speed. This is another unsolved puzzle. Perturbations in the motions of artificial satellites of the Earth have been found as they pass through these streams, directly confirming the existence of the streams.

A Parting Thought

Although we have created nuclear explosions and sent vehicles into interplanetary space, let us remember that we are a long way from controlling our natural environment. Consider, for example, the genetic effect of cosmic rays. These "rays" are charged particles of unknown cosmic origin which move with enormous kinetic energy and permeate the interplanetary space. Those of lower energy are deflected by the Earth's magnetic field and by the solar streams, but some penetrate deeply into the atmosphere producing nuclear showers. Two Russian scientists, V. I. Krasssovski and I. S. Sklovski, have recently adduced evidence that the Solar System may pass through regions in which the intensity of cosmic radiation resulting from the explosion of supernovae can for many hundreds of years exceed the mean level by several orders of magnitude. They show that the period of recurrence of these occasions may be something like 300,000 years. Doubling the mutation frequency in long-lived species requires only a three to ten times increase in intensity in cosmic radiation. Hence a few millennia can produce catastrophic consequences for many long-living specialized animal species with comparatively small populations. It is possible that the great dying out of reptiles at the end of the Cretaceous period was caused in this way. On the other hand, the evolution of other species may have been enhanced and it is suggested that this could account for the magnificent prosperity of vegetation in the Carbonic period. In fact, the formation of intricate complexes from simple organic compounds at the very beginning of life on Earth may have been stimulated in this way.