 Fig. 1. The sequence of nebular types. |
Session VII.6
[in Problems of Atmospheric and Space Electricity, 1963, ed. Coronti, S. C.]
Electrical Research Association, Leatherhead (Great Britain)
During his Kelvin Lecture on "The Sun" to the Institution of Electrical Engineers in May, 1941, CHAPMAN (1941) referred to a solar prominence which reached a height of 1,000,000 km in an hour. A little mental arithmetic established that its velocity of propagation must have been almost exactly that of a lightning leader stroke and that, since it moved "like lightning", it probably was lightning of some form.
The 22 years of research on atmospheric astrophysics since then have convinced me that not only this solar prominence, but also the universe is, so to speak, all-electric, and that electric fields and their breakdown in electrical discharges account for the observed phenomena and accelerate the process of universal evolution from universe to galaxies, from galaxies to stars, from stars to planets and, possibly, from large planets to satellites. In short, atmospheric astrophysics is merely an extension of atmospheric electricity.
There are good grounds for believing that the conditions necessary for the generation of these electric fields exist in cosmic atmospheres (BRUCE, 1955).
However, even if the exact nature of the process remains undetermined, as that in the earth's atmosphere has done for over 200 years, so that it is merely a hypothesis, it is the hypothesis to end cosmic hypotheses. These last are, at present, almost legion. They include the existences of magnetic fields, relativistic electrons (in atmospheres otherwise known to be at about absolute zero!), gas jets (to account for magnetic storms, etc.), processes of aggregation (to account for the galactic spiral arms), solar nuclear explosions, collisions between galaxies, collisions between matter and anti-matter, explosions in stars which detonate explosions in other stars, gravitational implosions, etc. The last four are only some of the suggestions put forward to account for cosmic radio sources alone, which, as we shall see, are accounted for in some detail by the discharge theory.
Two papers presented to the last Conference in 1958 (BRUCE, 1959a) echoed Franklin's statement in 1755, that he had no solution to offer to the problem of thundercloud electrification. I have suggested that the process involved in the terrestrial and cosmic atmospheres may be "static" electrification of dust or grains (BRUCE, 1955). In the long-period variable stars, for example, the additional dust formed in their atmospheres at light minimum is sufficient to dim the star's light by factors of ten or more. The atmospheric temperatures are therefore quite low, so that electric fields can evidently be built up to breakdown and can account for the bright emission lines which begin to appear in their spectra towards light minimum; i.e., the probability of the occurrence of the phenomenon evidently increases as the temperature decreases and as the amount of dust increases. Similarly, in galactic atmospheres, the ratio of grains to gas molecules is a million times the ratio of even the Aitken nuclei to gas molecules in the earth's atmosphere (ALLEN, 1963), and their temperatures are 0-100°K so that again there would appear to be every reason for electrification of the grains to occur, albeit very slowly. The time available is, however, long, on the order of 1010 years. In the largest galaxies, the field has still not had time to build up to the breakdown value, and no discharge channels, or spiral arms, are observed in them. The smallest of galaxies, on the other hand, as the theory predicts, are found to contain the most highly developed arms of types Sc and SBc (Fig. 1). It will be seen that the theory thus provides a categorical argument against continuous creation (BRUCE, 1961a).
Hubble himself wrote that there is evidence of the occurrence of some cataclysmic action at SO (HUBBLE, 1936). The theory provides this in the form of electrical discharges at 500,000,000°K which propagate with a velocity of 2500 miles/sec for a period of 10-100 million years (BRUCE, 1958a)! If you can imagine anything more cataclysmic than that...!
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Fig. 1. The sequence of nebular types. |
There is room for much work on such subjects as the formation and electrification of grains in the terrestrial, stellar and galactic atmospheres. The existence of times to breakdown of 102 sec, 107, sec, 109-1010 years or more in these three conditions will allow checks of any theories. I have already essayed a very rough comparison between the former two periods (BRUCE, 1955). Information regarding the nature of the grains in galactic atmospheres is afforded by the polarization of starlight which they cause, which shows that they are needle-shaped and orientated in one direction in one region, as they would be by an electric field. Thus it should be possible to calculate the magnitude of the field.
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It is argued that galactic atmospheres, for example, are infinitely conducting, and that the electric fields therein are impossible. One eminent professor of mathematics claimed to be able to demonstrate this in five minutes by setting up and solving the wrong equations. At about 0°K, we have a perfectly insulating atmosphere with grains which can become charged and apply a field to the gas. The process will be exactly analogous to the build up to and onset of the Townsend mechanism in air at atmospheric pressure. If there are any free electrons formed, then they will be swept out by the field to one of the grain electrodes. The current will increase as the field increases, until all the free electrons are swept out (at a, Fig. 2). After this point, it will remain constant, until the field increases sufficiently to lead to ionization by collision (at b, Fig. 2). In. a galactic atmosphere with 1 atom or molecule per cm3; this will presumably be when the electric field reaches a value of about 10-14 V/cm. In the extensive stellar atmospheres of planetary-system dimensions of the long-period variables, with pressures of about 10-6 atmosphere, the breakdown fields will be about 10-2 V/cm.
The evidence for these cosmic electrical discharges is the same as that which would be afforded by terrestrial atmospheric discharges to an observer outside the earth, i.e., the sudden appearance of bright emission lines in its spectrum. In the essentially molecular band spectra of the long-period variable stars, for example, superimposed on a continum at 1500-4000°K, there begin to appear, towards minimum light, bright emission lines of hydrogen and neutral helium, indicating temperatures of 5000-10,000°K. In some associated stars, like AX Persei, the level of excitation rises until lines of FeX are observed. Similarly, there are many galaxies which show patches of bright emission, and we shall see that their spectra indicate the existence of discharge temperatures of about 500,000,000°K in an atmosphere which is normally quoted as being at 0-100°K! If there is any way of bringing about these conditions apart from an electrical discharge, then engineers and nuclear physicists would be very interested to hear of it.
In addition, there is the whole subject of radio astronomy, which may almost be regarded as a prediction of the electrical discharge theory which I failed to make.
There are two "thermometers" which can be used in estimating the temperatures of electrical discharges, whether laboratory, atmospheric, or cosmic. The first derives from an effect, the existence of which was probably first emphasized by BELLASCHI (1937). The pressure in the axial regions of an electrical discharge is increased by an amount proportional to the product of the current and the current density, as a result of the movement of the conducting particles in the discharge's own magnetic field. In an electric arc, the current density increases at and towards the electrode spots. Therefore, pressure gradients are set up which give rise to the anode and cathode jets (MAECKER, 1955). Their velocity will be that of sound in the gas; if it can be determined, we will have a measure of the temperature. Applied to the lightning discharge, the theory explains the existence of metallic lines in the spectrum of the discharge up to heights of 2m above the ground (BRUCE., 1958b), and applied to the welding arc by my colleague King, it explains the physical basis of arc welding (KING, 1959).
Table I
Temperature Measurements of Electrical Discharges in the Atmospheres of Three Stars(1)
| Star | Level of excitation (eV) |
Temperature (°K) |
Gas velocity (km/sec) |
Temperature (°K) |
| X Cygni | Fe I, FeII (8-10) |
9,000 | 12 | 10,000 |
| P Cygni | CIII, NIII 47 |
47,000 | 28 | 47,000 |
| AX Persei | CaXV 814 |
814,000 | 110 | 814,000 |
The second thermometer derives from an observed relationship between the level of excitation and the discharge temperature, which was first brought to my notice for the laboratory arc by King. If the level of excitation observed is k eV, then the discharge temperature is k.103 eV. These thermometers will be referred to as the "v-thermometer" and "I.P.-thermometer" respectively. Table I shows their application to discharges in the atmospheres of three stars (BRUCE, 1962a; for a fuller discussion, see BRUCE, 1962b). The application of the I.P.-thermometer to laboratory discharges has recently been extended to lines of Si XIII or about 2500 eV (BRUCE, 1963c), while the v-thermometer predicted the existence of an upper limit to cosmic gas velocities of about 5000 km/sec (BRUCE, 1958a), the existence of which has been commented on by astrophysicists. The reason for the upper limit is not hydro-dynamical as one enquirer asked (BRUCE, 1962a), but thermonuclear. The temperature of these discharges is limited by the onset of thermonuclear processes at about 500,000,000°K.
Applied to solar discharges, the theory is the only one which has explained the solar streams (BRUCE, 1961b, 1963a), which have been postulated as the cause of magnetic storms since about 1896. The theory led to the conclusion that temperatures of well over a hundred million degrees must be reached in these outbursts (BRUCE, 1959b, c), a deduction which was confirmed by U.S. Navy Satellite observations (BRUCE, 1960a).
Another prediction concerned the Evershed effect. In 1909, Evershed observed an outflow of gas from sunspots at 1-2 km/sec, and only these velocities had been confirmed by subsequent investigation, whereas the v-thermometer suggested that they should reach 8 km/sec, the velocity of sound in ionized atomic hydrogen at 6000°K, the temperature of the photospheric arc discharges (BRUCE, 1963b). The observation of these predicted higher gas velocities by his colleague, Bumba, was announced by Severny at a recent I.A.U. Symposium (no. 12) on cosmic gas dynamics (SEVERNY, 1961).
The propagation of voltage breakdown is proportional to the product of the breakdown field and the mean free path. The former is proportional and the latter inversely proportional to the gas density so the velocity of propagation of cosmic electrical discharges should be the same as that of lightning discharges, namely about 3.107 cm/sec. The atmospheres of the long-period variables extend about 1014-1015 cm above their surfaces, so the duration of the discharges should be about (1014 ~ 1015/(3.107) or about 107 sec. This agrees reasonably well with the periods during which bright emission lines appear in their spectra.
However, as the temperature of the leader stroke increases, as it apparently and understandably does when there is a fairly steep density gradient, then there will come a point when the velocity of sound, and therefore the velocity of the jet of hot gas generated by the discharge, exceeds the velocity of voltage breakdown. The jet of hot gas then takes over the propagation process, and velocities of propagation up to about 4000 km/sec become possible. These higher velocities are reached at the temperatures observed in solar atmospheric discharges, and explained how the time lag between the solar flare and the resulting magnetic storms can be as low as one day. Protons have actually been observed entering the earth's upper atmosphere at velocities of up to 3500 km/sec during aurorae.
Early in these researches, I suggested (BRUCE, 1949) that certain effects observed in the earth's magnetic field may be the "static" radiated from discharges in the sun's atmosphere. The currents, 1014 A, and current densities, 10-5 -10-3, A/cm2, required to account for a disturbance in the earth's field rising to 40[gamma] in 15 min seemed reasonable. Further work on these lines might prove of interest.
It is of interest that a flare-star has recently been observed to be a radio source (LOVELL et al., 1963). In these stars, as in the sun, but not in the long-period variables, the discharges take place in an atmosphere having a rapid density gradient, in which high temperatures are built up which probably reach thermonuclear values.
Though most textbooks suggest that cosmic radio noise is the result of synchrotron radiation of relativistic electrons moving in a magnetic field, SMITH (1961) has recently pointed out that this theory does not yield the observed relationship which connects spectral energy distribution with wavelength. The observed relation lies between those given by ionization processes and Bremsstrahlung respectively; these two mechanisms will be active in electrical discharges. So far as existing theories are concerned, the necessary magnetic fields and relativistic electrons are mere postulations; there is no mechanism apart from an electrical discharge which will account for their origin (BRUCE, 1958c).
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It was suggested in the original summary of these researches (BRUCE, 1944) that breakdown of the atmospheric electric field built up in a globular nebula, like that of N.G.C. 4486 shown in Fig. 3, led to the formation of the irregular or spiral arms. N.G.C. 4486 is the radio source, Virgo A, and in order to see what is going on in it, the photographic enlargement of the central regions shown in Fig. 4 was also obtained at Mount Wilson and Palomar Observatories. It shows the discharge in its early stages, a mere 300 parsecs long, as compared with the average length of tens of kiloparsecs for the spiral arms. This explains why Burbidge's estimate of the total energy radiated by a radiogalaxy, derived from observations on this radio source, was only 1058 ergs (BURBIDGE, 1961), as compared with Heeschen's later value of 1060 ergs (HEESCHEN, 1962), which agrees with that derived from the discharge theory (BRUCE, 1961c).
The theoretical value derives from the observed change in the atomic constitution of the gas effected by its being exposed to thermonuclear temperatures on the order of 500 million °K for 10-100 million years. This changes the proportion of heavy atoms in the gas from 0.003, that of the original gas which went to the formation of the older stars of Population II, to 0.03, the proportion observed in the younger stars of Population I, which were later formed from the gas collected along the spiral arms which had been subjected to the discharge.
The energy liberated during the radio-galaxy phase is therefore the thermonuclear energy represented by the loss of mass involved of 1039 gm or 1060 ergs.
Fig. 5. Photographs of planetary nebulae showing pairs of old discharge channels. Note: The images of nebulae from this original article were too poor to reproduce, and have beenreplaced by alternative images from a variety of sources. The captions remain unchanged | ||||||||||||||||||||||
When the corresponding phenomenon occurs on a stellar atmospheric scale, the result is a miniature spiral nebula or straight-armed nebula (BRUCE, 1960b, 1961b; Fig. 5), the existence of which was predicted by the discharge theory. The confirmatory photographic evidence was found in a paper published 42 years earlier at Lick Observatory (CURTIS, 1918), thanks to a suggestion from Merrill of Mount Wilson Observatory, one of the astronomers whose help was sought in the search. It had been realized that the spectra of stars with pairs of bright emission lines could be explained if discharges in the two arms caused jets of gas in opposite directions. The result of a more detailed study of the gas velocities in two of these nebulae was quoted earlier. These had proved quite intractable to Campbell and Moore, whose investigations of the gas velocities were published at the same time as Curtis, photographic survey (CAMPBELL and MOORE, 1918).
I wrote in my contribution on Terrestrial and Cosmical Lightning Discharges to the last conference on Atmospheric Electricity (BRUCE, 1958b) that "One purpose of the present paper is to introduce to meteorological physicists a new field in which the writer thinks that they may well find the answer to some of their thunderstorm problems, namely that of cosmic (electric fields and) electrical discharges". It is gratifying that five years later, a whole section of this conference is to be devoted to this extension of the subject of atmospheric electricity which I then proposed.
In the present short talk, it has only been possible to refer to a few more developments of the theory. Some of the others are, however, summarized in the accounts circulated (BRUCE 1960c, 1963d) before the conference, and may come up in the discussion; a forthcoming E.R.A. Report will give a list of about 75 papers, notes and articles in which the subject is discussed further.
The suggestion is that the introduction of a new major variable (atmospheric electric fields) leads to a correlation of astrophysical phenomena and a consistent theory of universal evolution, without any additional ad hoc hypotheses, which are so necessary in astrophysics at present. The process of electric field building and the evidence for cosmic electrical discharges, including the whole new subject of radio astronomy, are referred to, and the writer's two cosmic gas thermometers discussed. It is shown that the theory offers an explanation of radio galaxies, and accounts for their occurrence in nebulae of Hubble's Type SO, and for others of their characteristics. Since the paper was presented the radio haloes of galaxies and the recently discovered extremely intense radiosources have been explained in letters to the Journal of the Institution of Electrical Engineers.
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