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Reprinted from:
Journal of the Franklin Institute, Vol. 268, No. 6, December, 1959

Cosmic Thunderstorms

BY

C. E. R. Bruce <index.htm>

* Based on a series of Reports of the Electrical Research Association,
Leatherhead, England.

Summary

Applications of the writer's electrical discharge theory of some
astrophysical phenomena are discussed, and interesting
interrelationships are adduced between corresponding physical
processes in the laboratory and in the terrestrial, stellar and
galactic atmospheres. The building-up of electrostatic fields in
these atmospheres is discussed, and the breakdown of these fields in
electrical discharges is shown to account for the light emission
from, and gas movements in, the atmospheres of the long-period
variable and combination-spectra stars. The theory has a bearing on
the evolutionary process in, and chemical composition of, late-type
stars. It will explain the gas movements observed in extra-galactic
radio sources, and accounts for the magnetic fields and
"relativistic" electrons required by the synchrotron theory of the
radio noise itself, for which no other explanation has so far been
offered. The theory likewise suggests an explanation for the
existence in some galaxies of two stellar populations, which is in
agreement with observations of some of their major features. A new
theory of propagation of these cosmical electrical discharges is put
forward which offers a way out of the difficulty hitherto met in
explaining the short time lags of some magnetic storms on the
causative solar outbursts, and the correspondingly high average
velocities of the particles responsible for these storms. These are
much greater than any velocities so far observed at or near the
sun's surface. It is shown that in these large cosmic electrical
discharges thermonuclear reactions become important when the
discharge temperature reaches about 400,000,000 K.

Introduction

Some years ago the writer (1a) attempted to out-Franklin Franklin in the
extension of the field of electrical discharges in gases, by suggesting
a series of steps, the greatest of which may be as great as the universe
itself. However, the present survey will be limited to the presentation
of the evidence for some applications of the theory on the stellar and
galactic scales. The manifestations of a series of physical processes
will be studied in the laboratory, as well as in the terrestrial,
stellar and galactic atmospheres, in the hope that the consideration of
electric field-building and discharge phenomena on such a wide variety
of magnitudes may prove suggestive for meteorological and nuclear
physicists, as well as for astrophysicists and those interested in the
study of electrical discharges themselves. For, in the course of these
investigations an estimate has been obtained for the temperature
required for the engendering of thermonuclear reactions to quite a
marked degree in these extensive electrical discharges in cosmic
atmospheres. This is found to occur at a temperature of about 400
million degrees absolute.

Atmospheric Electric Field-building

In a letter to Dr. Lining of Charles Town, South Carolina, addressed and
dated "Philadelphia, March 18, 1755," Franklin wrote: "I wish I could
give you any satisfaction in the article of clouds. I am still at a loss
about the manner in which they become charged with electricity; no
hypothesis I have yet formed perfectly satisfying me." After over 200
years that last sentence might, and indeed can still be found in any
exhaustive discussion of the subject. For example a paper presented to
last year's U. S. Air Force Conference on Atmospheric Electricity and
entitled "'The Lightning Mechanism and its Relation to Natural and
Artificial Freezing Nuclei" opens with the sentence, "There is as yet no
generally accepted theory for the electric charge generation in
thunderstorms", while another paper refers to "the unsolved problem of
thunderstorm electricity."

Not surprisingly it is still more difficult to deal adequately with the
problem of charge separation and field-building in cosmic atmospheres,
in which the air, water and ice of the terrestrial atmosphere are
replaced mainly by hydrogen and helium and the oxides, hydrides etc. of
a variety of metals, such as titanium, zirconium, vanadium, etc. Indeed
the writer has often been told authoritatively, as at the Liège
astrophysical symposium in 1957, that it is "impossible" for
electrostatic fields to be built up, even in the relatively cold
atmospheres of the long-period variable stars. However he hopes to show
that far from it being "impossible," it would be quite surprising if
electrical effects were /not/ observed in the conditions existing in
these stellar atmospheres.

*Terrestrial Atmospheric Electric Fields*

Two names which should be better known to students of electricity than
they appear to be are those of Stephen Gray and the late Professor P. E.
Shaw of Nottingham University. The former first showed that electricity
could be conducted, and thus greatly extended the science of electricity
as it was known in 1729 (2), while the latter (3a) fundamentally changed
the subject of electrostatics by showing that in order to cause the
separation of electric charges by the rubbing together of two bodies it
is not necessary to start with two different materials, an experimental
fact which still causes surprise to most physics students when they are
informed of it. Two sticks of the same material will become oppositely
charged provided the rubbing is asymmetrical; for example, if a limited
section, say 1 cm. in length, of one rod, is rubbed along the whole
length of a similar rod, then the two rods will have opposite charges.

Such asymmetrical reactions obviously occur in wind-blown dusts and
powders, and Shaw showed (3b) that these also become charged, even
though the reactions are limited to those between particles of the same
material. Furthermore, he showed that the charging effect is of the same
order of magnitude with cold dry ice particles as it is with sand.

In view of the chief cause of asymmetry of the effects in these
conditions we may suppose that on an average larger and smaller
particles will become oppositely charged, and there is some experimental
evidence to support this conclusion. Their separation in wind-blown
clouds of dust in a gravitational field will then load to the generation
of electric fields in such clouds. It is well known that electric fields
are set up in such circumstances, and in terrestrial sand and dust
storms and in the ejectamenta from volcanoes the fields so generated can
lead to the electrical breakdown of air at atmospheric pressure.

It seems to the writer significant that during a discussion of
thunderstorm problems at the Royal Meteorological Society (4), two of
the most active observers both averred that no electrical effects are to
be anticipated in thunderclouds until the anvil-shaped cap of cirrus
cloud is formed at the top of the thundercloud. This forms at about -30
C. and at a height of 30,000 to 40,000 ft., and is composed of dry ice
crystals. This view of the critical requirement for the occurrence of
electrification in thunderclouds is supported by the recent mass attack
on this problem in the U. S. (5). It was found that lightning only
occurs when the top of the thundercloud reaches heights of the order of
30,000 to 40,000 ft. and temperatures below - 20 C. Though the actual
physical processes involved in thundercloud charge separation are still
the subject of considerable discussion, it seems to the writer that
these observations in the laboratory, in sand and dust storms, and in
volcanic eruptions point to the adequacy of static electrification to
explain the phenomena (3b).

This is supported by other papers in the aforementioned volume of the
proceedings of the second U. S. Air Force on Atmospheric Electricity
where Chalmers (6) writes "... there seems to be support for the idea
that the charge separation is concerned with ice particles colliding
with one another," as Simpson and Scrase had earlier suggested. In an
investigation of charge generation on a mountain top it was noted that
"All strong charging rates are connected with ice crystals in the air (7)."

But perhaps the strongest evidence on the origin of thunderstorm
electricity afforded by that Symposium is the observation of the quite
remarkable intensity of the electrical effects in the electric storms
associated with tornadoes and at heights where ice particles alone exist
(8).

*Electric Fields in Stellar Atmospheres*

The most obvious extension of these ideas is to the atmospheres of the
long-period variable stars, the outstanding characteristics of which,
apart from their great cyclical variation in optical magnitude, are
their size and their extensive atmospheres, and their very low
temperatures; some of them hardly shine at all, and the highest of their
"surface" temperatures is under 4000 K. These cold "surfaces" -- if they
can be said to have a surface at all -- have radii approximating in some
cases to that of the Earth's orbit, and outside these "surfaces" extend
tenuous atmospheres which could in some cases envelop the whole solar
system.

These atmospheres would be, and are, relatively cold, apart from the
periodical outbursts, during which the nature of the light emitted shows
that some of it must originate in gas whose temperature has somehow been
raised to 5000 or 10,000 K., and in a few cases even to 500,000 K. or a
million degrees absolute. The vexed question has been, whence come these
high temperatures? To which the writer's reply is, from lightning
flashes in stellar thunderstorms (1a, c). For at minimum light the
temperatures of these extensive atmospheres fall far below their
"surface" temperatures of 1500 or 2000 K., and there would appear to be
nothing to prevent them reaching values at which the electrical
conductivity is sufficiently low to allow of the generation of
electrostatic fields.

At these low temperatures a number of materials, such as metallic
oxides, hydrides, and carbides will solidify out of the atmosphere. The
existence at minimum phase of these solid or liquid particles had indeed
already been deduced, as they offer the likeliest explanation of a large
proportion of the diminution of the star's light at minimum brightness.
To a large extent the nature of the light remains the same -- there is
just less of it. It is veiled by the cloud of particles.

It is also known from a spectroscopic analysis of the light that great
winds blow in these atmospheres, with velocities up to more than 10 km.
per second, so that the solid particles in these atmospheres will be
subject to the violent impacts required for the generation of static.

The conclusion would appear to be inevitable that there will be
considerable generation of static and of electric fields in these
stellar atmospheres. These fields will go on building up at an
increasing rate as the temperature falls towards minimum, so that,
unless some other, and hitherto quite unforeseen, cause of the outburst
becomes effective, electrical breakdown in discharges is bound to occur
sooner or later.

*Temporal Characteristics of Stellar Outbursts*

One can compare very roughly the time which will be required for the
build-up to breakdown by comparing the gas densities and velocities and
the gravitational forces in these stellar atmospheres with those
observed in thunderstorms. Whereas the build-up time in the thundercloud
is of the order of 100 seconds, the estimated time under these stellar
atmospheric conditions is of the order of 10^6 to 10^9 seconds,
according as the process of charge separation depends on the first or
second power of the relative velocity of the particles (1c). This agrees
as well as can be expected with the observed periods of these stars,
which range from about 100 to 600 days, or 10^7 to 10^8 seconds. The
writer has therefore suggested (1d) that meteorological physicists may
be able to elucidate the process of charge separation in thunderclouds,
by a more precise comparison of the conditions therein, with those
existing in the different types of long-period variable and
combination-spectra stars, to which more reference will be made later.

Another check on the times involved in these stellar outbursts is
obtained from a consideration of the duration of the period during which
bright emission lines are observed in the stars, spectra, indicating the
occurrence of the discharges. Apart from an effect to be discussed
later, which does not affect the present argument, the velocity of
propagation of electrical discharges will be independent of the gas
density, and equal to the velocity of propagation of the lightning
leader stroke at atmospheric pressure -- that is, 10^7 to 10^8 cm. per
second, the velocity of propagation of electrical breakdown in a
hydrogen atmosphere being probably slightly greater than in air (9).
Since the distances involved are of the order of 10^14 to 10^15 cm., the
duration of the discharge process will be of the order of 10^7 seconds,
again in good agreement with the observed periods of variation, during
about half of each of which the bright lines are observed in these
stellar spectra. Thus the temporal characteristics of this type of star
agree reasonably well with those to be expected on the "thunderstorm"
theory of their periodical outbursts.

Light Emission From Long-period Variables

The general nature of the light itself during each increase in magnitude
of these stars -- which at maximum may reach 10,000 times their
brightness at light minimum, the average increase being by a factor of
about 100 -- and its regular "programme," is also quite in accord with
the electrical discharge theory (1c). Indeed it has so far proved
impossible to account for it in any other way. For in this low
temperature atmosphere, mainly hydrogen, there suddenly appear emission
lines of hydrogen, helium, including ionized helium, and ionized metals.
As we shall see later, in the closely associated combination-spectra
stars, the level of excitation reaches that of six times, and even
possibly nine or thirteen times, ionized iron atoms, representing an
excitation equivalent to temperatures of between five hundred thousand
and a million degrees, or more. Indeed so varied is the light emitted by
this last type of star at different phases of its cycle, that they have
been assumed to comprise a pair of stars, one very cold and one very
hot, and an associated nebula.

However, we shall consider simply one large cold star surrounded by an
extensive atmosphere. The rate of build-up of the electric field
increases with the square of the density, with the gravitational force,
and with the velocity; while the breakdown voltage is inversely
proportional to the gas density. It follows (1c) that the conditions
requisite for electrical breakdown will be reached first low down in the
star's atmosphere and the discharges will be propagated outwards towards
the star's peripheral layers.

The writer has emphasized (1b, e) that these long electrical discharges
will serve as "energy pumps," so to speak, just as does the lightning
leader stroke, causing energy generated in one place to be liberated at
another. In the lightning discharge, for example, whereas the electrical
energy generator is in the thundercloud, the highest current in the
discharge actually flows just at the Earth's surface (1f), several
kilometers away from the generator. The leader stroke acts as almost a
complete short-circuit of the space between the cloud and the ground
(1g, h), so that very high field concentrations occur around its
advancing tip. This effect is likely to be enhanced in an atmosphere in
which the discharge is propagated outwards through a decreasing gas density.

There are thus two effects to be looked for as the discharge proceeds.
In the early stages, since it starts low down in the star's atmosphere,
the light will be subject to considerable general and selective
absorption by the dust particles and molecules of the vapors which
abound, such as the oxides of titanium, zirconium, vanadium, etc., as
well as C_2 , CN, and other radicals. It is not to be expected,
therefore, that such series of emission lines as the Balmer series of
hydrogen, or the various multiplets in say the iron spectrum, will have
the relative intensities observed in the laboratory, or anything like
them. These relationships will be considerably mutilated by differential
absorption in the upper regions of the atmosphere. However, as the
discharge is propagated outwards, and as the energy liberated in it
causes dissociation of the molecules already referred to, then the
relative intensities of these series and multiplets will approach more
and more those observed in the laboratory.

This sequence of events has been observed repeatedly by Merrill and
other investigators. It is well illustrated, for example, by the
variation of the individual lines of multiplet (2) of Fe of which
Merrill writes(10a) :

"In the last column, phase + 162 days (i.e. 162 days after maximum
light) the relative intensities are the same as in the laboratory.
At earlier phases, the intensities are modified, probably by TiO
band absorption, as in R Leonis. The behaviour of this multiplet
presents another example of the general tendency of bright lines to
escape from the effects of the reversing layer as the phase advances." 

The same explanation will account for the wide variation in the relative
intensities of Hg: Hd at different phases of the brightness cycle (10b).
It is only some considerable time after maximum light that this ratio
approaches the value observed in the laboratory.

*Combination-spectra Spectra Stars*

As a result of the intensification of the field at the head of the
advancing discharge, and its projection outwards into regions of lower
gas pressure, referred to above, the excitation of the gas will be
increased. The spectrum of the gas will, therefore, change during the
outburst from one of high temperature at relatively high gas pressure,
to one of higher excitation at lower gas density, with forbidden lines
entering as the very low pressures of the outer regions of the star's
atmosphere are reached. The former of these two phases accounts for the
spectrum to account for which it was earlier assumed that the large cold
star was accompanied by a small hot "companion" star; while the later
stages of the discharge account for the '"nebular" contribution to the
spectrum. It was therefore suggested (1c) that the theory will account
for the combination spectra in such stars as R Aquarii, Z Andromedae, BF
Cygni and AX Persei. In these the initial bright line spectrum, that
usually attributed to the postulated "companion" star, comprising lines
of H, HeI, Fe II, Ti II, and Si II, gives place, after 100 or 200 days,
to a spectrum of higher excitation, containing lines of He II, N III, C
III, [O III], [Ne III] and [Fe III]. The nature of this last nebular
spectrum is in accord with the suggestion that it originates in regions
of very low pressure, far out in the star's atmosphere, towards the
completion of electrical neutralization.

The two spectra follow one another fairly regularly after periods of the
order of 100 to 200 days in different stars. One observer (11) summed up
his description of the sequence of the two different types of spectra by
concluding that it is just "... as though they occurred as a consequence
of the propagation of running waves over an extended medium." This will
be seen to be in accord with the electrical discharge theory, the
"running waves" being waves of electrical excitation.

Conditions for the Initiation of Long Electrical Discharges

Though the period of variability and brightness at maximum of the
long-period variable stars are fairly well defined, they are subject to
considerable variation in any one star. This may amount to about 10 days
in a period of say 200 days, and to one or two magnitudes in maximum
brightness. This variability may have an interesting analogy in the
variability of the current in different lightning flashes in the same
thunderstorm.

Some years ago (1g), when putting forward a new theory of the initiation
and propagation of lightning leader strokes, the writer showed that the
theory would explain the very wide variation in lightning currents. For
a lightning flash to occur two things are necessary: first, an average
electric field between the two charges in the cloud, or between one of
these charges and its image in the Earth, sufficient to maintain the
process of arc conduction in the leader stroke, when once it is
initiated, that is, an average field of 10 to 100 V/cm ; second, in that
relatively low average field there must exist a field concentration,
such as is caused by a tall grounded building on the Earth, or an
elongated volume of space charge in the cloud, sufficient to cause the
transition from the field-maintained corona discharge in the St. Elmo's
fire at its tip, to a thermally-ionized column of arc discharge. When
this transition occurs it was shown that the discharge will become
self-propagating, so to speak, and bridge the gap. The smaller the
initiating field concentration, the greater must be the average field
before the leader stroke is initiated, and the greater will be the
current in the discharge when it does occur.

It may be noted in passing that this new conception had an important
bearing on the theory of the operation of a lightning conductor (1g),
probably the first major change since its introduction by Franklin
nearly two hundred years earlier. For the field concentration at the
advancing tip of the leader stroke will also vary with the average
predischarge field, so that upward streamers will be initiated from
grounded buildings earlier in the leader stroke's descent for high
average predischarge fields. Thus, heavy flashes will be attracted to
the conductor from much greater lateral distances than will light or low
current flashes. Previously it had been considered that the protective
range of a lightning conductor depended only on its height, and not at
all on the nature of the lightning flash.

Fortuitous variation in the distribution of space charge may be expected
to cause a similar variability in the conditions required for discharge
initiation in all long and purely atmospheric discharges. The longer the
initiation of the discharge is delayed in the stellar atmosphere, the
greater will be the cooling of the atmosphere after the previous
outburst, and the greater also will be the amount of dust solidified out
of the atmosphere during the minimum phase. This will have two results.
It will cause a greater dimming of the star's light, and hence a lower
light minimum, and there will also be a greater increase in the average
pre-discharge field, and hence a greater outburst and increase in
luminosity when the discharges do occur.

Thus, besides accounting for the irregularity of the periods and the
amplitudes of the variations of brightness observed in these stars
generally, the theory would also explain some observations made by
Merrill (10c) on the combination-spectra star R Aquarii. He has pointed
out that in a series of pulsations of this star in the early 1930's very
marked dimming of the main "cold" red star was associated with extra
bright outbursts of the "companion" star or discharge spectra. The idea
of a "companion" star was introduced, as we have seen, to account for
the early stages of the electrical discharge. Merrill is the leading
observer of and authority on this type of variable star, and it should
be recorded that, though the belief is generally held that in all cases
two stars and a nebula are required to account for the phenomena,
Merrill himself in his Monograph (10c) and papers has been careful to
emphasize that in many cases, including R Aquarii itself, there is no
positive evidence for the existence of the '"companion" star as he has
usually so written it, and that all might in fact come from one large
"'cold" star and its atmosphere. Summing up the discussion of this type
of star in his Monograph, Merrill wrote (10d) that "... it would be
inadvisable at the moment to accept without reserve the hypothesis of
actual duplicity for all combination spectra."

Evolution and Chemical Composition of Late-type Stars

The application of the discharge theory to the long-period variable
stars has a bearing on two questions of major interest in astronomy,
namely, stellar evolution, and the uniformity of the chemical
composition of matter throughout the universe, since the atmospheres of
these late-type stars are one of the few places where there is generally
considered to be a departure from this uniformity. The theory suggests
that the observations can be explained by differences in the physical
state of matter of the same general chemical composition.

Stars can be arranged in a series having decreasing "surface"
temperatures until temperatures of the order of 3500 to 4000 K. are
arrived at, that is, temperatures at which the freezing out of hydrides
and carbides and carbon itself from their atmospheres begins. Below
these temperatures the series apparently trifurcates, the three branches
being differentiated by the different absorbing molecules in their
atmospheres. The spectra of one group, the carbon stars of Classes R and
N, show the bands produced by the C_2 and CN molecules; another, group,
the titanium stars of Class M, shows mainly bands of titanium oxide;
while in the third group, the zirconium stars of Class S, the titanium
oxide bands are replaced by those of zirconium oxide. These differences
are usually attributed to actual differences in the chemical
constitution of the stars, atmospheres. The writer, however, has
suggested (1j) that the difference is mainly one of average physical
state of the atmospheric material external to the star's "surfaces" or
photospheres.

Let us assume that during the evolutionary process the average
temperature of this outer atmosphere is falling. The arguments adduced
can be reversed if in fact this average temperature rises as the stars
age. The first molecules to form will be those of C_2 and CN. At still
lower temperatures particles of carbon, carbides and hydrides will be
formed so that the molecules of C_2 will disappear, and with them the
C_2 bands will disappear from the spectrum. They will be replaced by the
bands of molecules which associate at lower temperatures, such as
zirconium oxide, which will begin to appear at temperatures of the order
of 3000 K. However, in its turn zirconium oxide will freeze out and
become solid particles at temperatures of around 2500 K., its place
being taken by titanium oxide and others which associate at around these
temperatures. Titanium oxide will remain in the vapor state, and give
rise to bands in the star's spectrum, until it too solidifies at
temperatures of around 1600 K.

At any one point in the evolution of the outer atmosphere of a star,
therefore, the oscillations in temperature of that atmosphere, due to
the occurrence of the discharges followed by a period of cooling and
electric field-generation, will occur within a given range. This range
will be appropriate for the appearance in its spectrum of each of the
three main sets of bands in turn as the average temperature falls.
First, the carbon molecules will disappear, having become particles of
carbon which will no longer be vaporized to any appreciable extent by
the discharges when they occur. The carbon bands will be replaced by
zirconium bands, until they too in turn disappear when the average
temperature falls so low that the solid particles of zirconium oxide are
no longer vaporized, and finally only titanium oxide bands will appear
in the star's spectrum throughout its cycle..

There is one piece of evidence which strongly supports the new theory,
and which would appear entirely to negative the possibility that the
explanation lies in differing chemical constitutions. It is quite
possible on the view now proposed that stellar atmospheres will exist in
which at minimum only titanium oxide bands appear in their spectra, but
in which the rise in temperature caused by the discharges is so great
that, at maximum, all the titanium oxide molecules are dissociated, and
sufficient zirconium oxide particles are vaporized, to lead to the
replacement of the titanium oxide bands by those of zirconium oxide at
maximum brightness. In other words, the star will change from type M, at
minimum, to type S, at maximum, a change which would be quite impossible
if the difference between these two stellar types is one of chemical
composition.

In fact, however, stars /do/ exist in which this change occurs as the
result of a specially great outburst -- that is, when they reach what is
for them an exceptionally bright maximum, and consequently an unusually
high average temperature. One such star is c Cygni. It has been observed
to change from type M to type S at unusually bright maxima.

It would thus seem that these three types of late stars -- carbon stars,
zirconium stars and titanium stars of types N, R, S and M, respectively
-- are not necessarily in conflict with the uniformity of chemical
constitution of matter observed fairly generally throughout the
universe, as they are generally believed to be, nor do they necessarily
indicate a trifurcation of the stellar evolutionary sequence in the way
they are generally regarded as doing.

Gas Movements in Electrical Discharges

Perhaps the most intriguing inter-relationship so far brought to light
between the characteristics of these electrical discharges in the
laboratory, the atmosphere, and in stellar and galactic atmospheres, is
that existing between the gas movements engendered by the discharges. We
are not here concerned with movements analogous to the /explosive/
movement of the /surrounding/ gas, which results in the thunder of the
lightning discharge. It is, in contrast, a /continuous/ axial flow of
the hottest gas /along/ the /central regions/ of the discharge channel.
The latter acts like a hose-pipe squirting gas from regions of high
current and high current density towards regions where the product of
these two quantities is reduced.

*The Arc and Lightning Discharges*

R. C. Mason (12) showed that because the charged particles of the
electric discharge flow along the channel in its own magnetic field,
they will be constrained by the field to move inwards towards the axis
of the discharge. He showed that this would result in an axial increase
in gas pressure, which is proportional to the product of the current and
the current density.

Maecker (13) later drew the "obvious" conclusion that constrictions in
the discharge channel, such as exist at the anode and cathode spots of
the arc discharge, will give rise to high pressures, and therefore to
gas movement down the resulting pressure gradient. And so the anode and
cathode jets of the electric arc were explained satisfactorily for the
first time. King (14) has shown in these laboratories that these jets
account in large part for the transfer of metal in the arc welding
process, and explain why it is independent of gravity. (He has also
shown that the temperature of the welding arc is several times greater
than the 6000 or 7000 K. usually quoted for it. It is usually in the
range 15,000 to 20,000 K.)

The pressure will increase with the product of the current and the
current density, but the velocity of the gas flow cannot go on
increasing indefinitely. It is limited by the velocity of sound in the
gas at the temperature of the discharge. For example (1h), in the
lightning discharge the temperature will vary with the current in
different flashes, but will almost certainly lie between 50,000 and
100,000 K., for periods of hundreds of microseconds or a millisecond.
With these ranges of temperatures and times, the distance moved up the
lightning channel by the gas and vaporized material at the Earth's
surface will lie between 70 and 1000 cm. This agrees with, and indeed
explains, the observations of Israel and Wurm (15) that metal lines are
observed in the spectrum of a lightning flash up to a height of about 2
meters above the ground.

*The Long-Period Variable Stars*

The first extra-terrestrial application of these ideas is again to the
discharges in the atmospheres of the long-period variable stars (1k).
When the bright emission lines appear amid the molecular absorption
bands in the spectra of these stars, they are those of ionized and
neutral metal atoms, hydrogen, and helium, denoting gas temperatures of
between 5000 and 10,000 K. Since the gas is largely ionized hydrogen the
velocity of sound in it at these temperatures will lie between /8.5 and
12 km. per second/. This is an extremely narrow range of velocities when
one considers that, apart from the theory now being put forward, the gas
velocities might have been measured in miles per hour, miles per minute,
miles, tens, hundreds or thousands or more of miles per second. However,
extremely narrow though this theoretical range of gas velocities is,
relative to the whole gamut of possible cosmic velocities, it contains
both the average values obtained for these gas velocities by the two
leading authorities on this type of star at Mount Wilson. In these stars
the light absorption is so great that only that from the discharges on
the near side of the star's atmosphere is photographed, so that the
spectra show broadened emission lines displaced towards the violet
relative to the absorption lines produced by the relatively stationary
atmosphere. From the displacement of the emission lines towards the
violet in the spectra of 72 long-period variable stars, Merrill (10e)
obtained an average value for the velocity of the gas of /11km. per
second/, while from similar measurements in the spectra of seventeen
closely similar irregular variable stars Joy (16) obtained an average
velocity of /9 km. per second/.

*The Combination-Spectra Stars*

As has already been seen the combination-spectra stars are similar in
many respects to the long-period variable stars. It was therefore
somewhat disconcerting (11) from the point of view of this theory of
these gas movements, to find that in one of these stars, AX Persei,
Merrill (10f) had observed displacements of the emission lines relative
to the absorption lines which were equivalent to velocities of approach
of 110 km. per second. Since the velocity of sound in a gas only
increases as the square root of the absolute temperature, this meant
that in the very extensive cold atmosphere of this star the gas
temperature in the discharges must have reached 500,000 to 1,000,000 K.,
if the theory were to be saved. The theory /was/ saved, however, by an
equally surprising observation in another paper, by Swings and Struve
(17), in which they showed that some of the emission lines in the
spectra of AX Persei derived from Fe VI, Fe VII, and even possibly from
Fe X, that is, from five, six, or even possibly nine times ionized iron
atoms, which also require for their production the buffeting to be
expected in a gas at the temperature of about a million degrees
absolute, required to account for the high gas velocity.

*Galactic Electrical Discharges*

This upward trend of the axial temperatures with increase in the scale
of these cosmic electrical discharges cannot go on indefinitely. There
will come a time when those temperatures are reached, which are being
eagerly pursued in the world's physical laboratories at the moment,
namely those at which thermonuclear reactions will occur. When the
latter are produced in sufficient degree then the increase in gas
pressure which they produce will balance the inward pressure of the
magnetic pinch effect, and further increase in temperature will be
prevented.

Instead of taking place in deuterium, as in the laboratory discharges
aimed at producing thermonuclear reactions, the cosmic discharges occur
in a gas which is probably about 80 per cent hydrogen, with two parts in
10,000 deuterium, and with about 20 per cent helium and fractional
percentages of the other atoms. The experts will probably agree that in
the conditions of these large cosmic discharges temperatures of 10^8 to
10^9 K. will be required to cause nuclear reactions on a large scale. On
the gas velocity thermometer, so to describe it, maximum velocities of
1750 to 5400 km. per second, the velocity of sound in ionized hydrogen
at these temperatures, will therefore be observed in these larger
electrical discharges when these temperatures are reached. These
galactic discharges have indeed been investigated by Seyfert (18) at
Mount Wilson, for he has examined the spectra emitted by bright emission
patches in some extra-galactic nebulae. In these discharges velocities
of recession are observed, as well as velocities of approach, so that
the emission lines are ,broadened, rather than displaced. The velocities
which Seyfert has recorded are in the range 1800 to 4250 km. per second,
in good agreement with the above "theoretical" range of velocities.

Cosmic Radio Sources

An interesting observation from the new point of view is Baade and
Minkowski's (19) determination of the gas velocities in the well-known
radio source, NGC 1275, illustrated in Fig. 1. They find that the gas in
the well-defined arms is moving at a velocity of about 5250 km. per
second, while that in the less well-defined patches of the back-ground
gas is moving at about 8250 km. per second. They have therefore
suggested that the source is a collision between two nebulae or
galaxies, moving with these two velocities. The writer has suggested
(1m) that at least some of these extra-galactic radio sources are
galaxies in which the galactic radial electric field is breaking down
and being. neutralized in electrical discharges, which ultimately result
in the formation of the spiral arms, for which last there is still no
satisfactory theory. On this view the channels in NGC 1275 are these
discharge channels, and the gas in them has been accelerated to a
velocity of about 3000 km. per second in the line of sight by the
pressure gradient caused by the magnetic pinch effect in the galactic
discharge.

Radio source of Peculiar Galaxy NGC 1275
*Fig. 1. Photograph of the radio source NGC 1275 taken with the 200-in.
telescope at Mount Palomar Observatory.(ll3600-5000 Å.)*

Radio source NGC 4486 (M87)
*Fig. 2. Photograph of the radio source NGC 4486 taken with the 200-in.
telescope at Mount Palomar Observatory. (ll3600-5000 Å)*

The light from the discharges on the other side of the galaxy may well
be lost in the nebula's dusty atmosphere. The difficulty of
photographing these discharge channels, even on the near side of a
galaxy is illustrated in Figs. 2 and 3. The length of the discharge
channel in that radio source, NGC 4486, is 300 parsecs and its diameter
about 30 parsecs -- a parsec being about 19 million million miles (19).

As regards the actual mechanism producing the radio waves, Shklovsky
(20) showed that this could be explained in terms of synchrotron
radiation emitted by extremely high speed electrons moving in a magnetic
field. However, the world's astrophysicists recently assembled in the
U.S.A. (21) had no clue to offer as to the origin of /either/ the
magnetic field /or/ the "relativistic" electrons, so that, as it stood,
the "'explanation" left something to be desired. This something would
appear to be supplied by the electrical discharge theory of the
phenomena. The current in the discharge obviously produces the required
magnetic field. As regards the high speed electrons, the gas velocities
being 1800 to 5400 km. per second, the corresponding electronic
velocities will be over 40 times these values or over 7.2 x 10^9 to 2.16
x 10^11 cm. per second. The theoretical values are therefore in the
range required by Shklovsky's theory.

Center of NGC 4486 (M87)
*Fig. 3. Photograph of the central regions of NGC 4486 taken with the
100-in. telescope at Mount Wilson Observatory. (l < 4000Å)

*

At an earlier Symposium of the I.A.U. (22) prominence was given to the
prediction made by Shklovsky that the radiation from NGC 4486 should be
polarized on the synchrotron radiation theory, and to Baade's
observations confirming this prediction. However, many years ago (1p)
the writer pointed out that the radiation from these large single
electrical discharges should be polarized, and that this could be looked
for in the initial stages of novae, for example. As a result of this
suggestion this observation was put on the observing program of Mount
Wilson Observatory for the next bright nova outburst.

In the galactic radio source, the Crab Nebula, from which the radiation,
both optical and radio, is similar to that from NGC 4486, the phenomena
can be subjected to more detailed investigation. As a result of such an
examination Woltjer (23) has recently deduced that the varying
directions of the polarization can be accounted for if electric currents
flow along the gaseous filaments. The conclusion that these filaments
are electrical discharge channels would appear to be inevitable, and the
observed gas velocity of over 1,000 km. per second enables their
temperature to be determined as about 3 x 10^7 K.

Shklovsky (20) has estimated that if all metagalactic radio noise is to
be accounted for as originating in such "jets" or discharge channels as
that in NGC 4486, then at present about one per cent of all galaxies
must be passing through this phase. From this the velocity of
propagation of these discharges can be calculated, since, on the
discharge theory, the time scale of these phenomena is determined by
this velocity. The age of the nebulae is 10^9 to 10^10 . years, so that
if at any one time one per cent are passing through a particular phase
of their life, this phase must last for 10^7 to 10^8 years in any one
nebula. As the length of the discharges is of the order of 10^4 to 10^5
light years, it follows that the velocity of propagation is of the order
of 10^-3 times the velocity of light, or of the same order as the
velocity of propagation of electrical discharges in the terrestrial and
stellar atmospheres. This is a result to be expected /a priori/ on
theory (1n), since the expression for the velocity of propagation of
electrical breakdown depends on the product of the mean free path and
the breakdown potential gradient. One is directly and the other
inversely proportional to the gas density, so the velocity of
propagation may be expected to be independent of the gas density, even
over the range of about 10^20 to 1 in density, embraced by the range of
atmospheres considered.

Actually another factor enters in these galactic nebulae, which changes
the nature of the discharge propagation process; however, it does not
materially alter the above argument, as will be seen later.

Stellar Populations I and II

Another major question on which the discharge theory would appear to
have an important bearing is that of the origin of the two stellar
Populations in the galaxies (1q). Globular and elliptical nebulae, which
are those in which the main galactic discharge has still to occur (1a),
contain stars of Population II. These, on the view now proposed, are the
oldest stars which have been formed contemporaneously with the
development of the rotational form of the nebula, and with the building
up of the generally radial electric field in the nebula's gaseous
atmosphere, which envelops the stars of Population II. The electrical
breakdown of this atmospheric electric field results in the development
of either an irregular nebula, from a globular nebula, or a more or less
well defined spiral nebula, from a more or less markedly elliptical
nebula. The older Population II stars in the nebula will be little
affected by the occurrence of the discharges. The latter will, however,
have a considerable effect on the disposition of gas and dust in the
nebula. This will be collected into the discharge channels -- the spiral
arms -- by the magnetic pinch effect, a deduction which has been in fact
amply confirmed by various observations, optical and radio. There, in
gas of greatly increased density, a second population of younger smaller
stars will be formed relatively quickly.

On this view, therefore, this second population of stars, which
corresponds to Baade's Population I, should be formed along discharge
channels, superposed on, or threading through, the general aggregate of
Population II stars, which had been formed earlier in the original
globular or elliptical phase of the nebula.

That this conclusion agrees well with observation will be seen from the
following description (24) of what is actually observed, in which the
italics are the writer's:

"A spiral galaxy combines the properties of irregular and elliptical
nebulae. The flattened spiral arms are populated by the same objects
that characterize irregular systems -- dust, gas and blue super
giants. /The spiral structure is imbedded in, and rotates within, a
structureless sub-stratum that resembles an elliptical galaxy in
general features and, in particular, in the objects that populate it./" 

A New Theory of Propagation of Cosmical Electrical Discharges

A main idea behind the present account, as expressed in the introductory
section, has been the study of various phenomena -- atmospheric
electrostatic field-building, electrical discharge characteristics, etc.
-- on a wide variety of scales. The new theory of discharge propagation
now to be considered applies, however, only to the breakdown of
electrostatic fields in cosmical atmospheres, and does not apply at all
in normal long sparks or terrestrial lightning discharges. For the
latter the theory that breakdown to a thermally ionized column of arc
discharge is complete during the leader stroke still applies (lg, h).
The theory now proposed is merely a development of that conception which
becomes applicable when the temperature in the leader stroke reaches a
sufficiently high value -- of the order of 8 million degrees.

The writer has emphasized above, and in a recent note (1n), that, so far
as the normal process of voltage breakdown is concerned, there is no
reason to expect that the velocity of propagation of the breakdown
process will vary with gas density. However, in these long cosmical
electrical discharges a point will be reached at which a radical change
will occur in the whole propagation process. In the discharge channel
already formed a jet of gas will be generated, which will flow along the
axis of the channel towards its advancing head. As the temperature of
the channel rises, so also will the velocity of this jet. When this
velocity reaches about 5 x 10, cm. per second, that is, when the axial
gas temperature reaches about 8 million degrees absolute, then the
velocity of the jet of hot gas will exceed that of the normal process of
voltage breakdown in a hydrogen atmosphere, which is probably less than
5 x 10^7 cm. per second. Thereafter the propagation will depend on the
jet of hot gas, and the velocity of propagation will depend upon its
temperature. Velocities of propagation of up to about 4000 km. per
second will thus become a possibility.

Magnetic Storms

The last remark in the previous section may help to solve an outstanding
difficulty which confronts even the electrical discharge theory of those
magnetic storms which are observed to follow events at the sun's surface
by periods of 1 to 4 days. Whereas no gas velocities greater than 600 or
700 km. per second have been observed at or near the sun's surface, the
shorter of these two periods, 1 day, represents an average velocity of
the jet of particles causing the magnetic storm of over 2000 km. per
second. This situation has been rendered even more perplexing by
Meinel's recent observation (25), that during aurorae and the
accompanying magnetic storms protons enter the Earth's upper atmosphere
at velocities of over 3,500 km. per second, or about five times the
maximum velocity so far observed in outbursts near the sun's surface.

It will be seen that, applied to the theory (1a) that the time interval
represents the time required for the propagation of an electrical
discharge through a tenuous solar atmosphere, the new developments on
discharge propagation offer a possible solution. For, as has already
been shown, electrical discharges can accelerate particles up to just
about the maximum velocities so far observed in these particles
comprising magnetic storms.

/Indeed the existence of this upper limit of about 3000 or 4000 km. per
second to these relative velocities in a wide variety of discharge
conditions in cosmical atmospheres suggests that the corresponding
discharge temperature, namely about 400 million degrees absolute, is
that at which thermonuclear processes become of paramount importance in
these cosmical electrical discharges./

Epilogue

An attempt has been made to show that a great extension of the field of
electrical discharges in gases may result from a reassessment of many
astrophysical phenomena from the point of view outlined in the preceding
pages.

In the letter referred to earlier in this paper, Benjamin Franklin
quoted a passage from the "Minutes" he kept of his experiments, in which
he had enumerated twelve particulars in which the "electrical fluid
agrees with lightning." He continued:

"The electric fluid is attracted by points. We do not know whether
this property is in lightning. But since they agree in all the
particulars wherein we can already compare them, is it not probable
that they agree likewise in this? Let the experiment be made" 

The last sentence is surely one of the most pregnant in the history of
electricity, and one wonders if perchance it was known to Marconi! In
suggesting a step of still greater ratio in the study of this same field
of electricity in gases, the writer cannot unfortunately end this note
on the intercomparison of the various fields with a similar suggestion.
He can only suggest that the observations made in some branches of the
wider field of astrophysics should be studied from the new point of
view, and hopes he has demonstrated that the first fruits of so doing
are at least promising.

Notes

1. C. E, R. Bruce:
1. "A New Approach in Astrophysics and Cosmogony <astro.htm>,"
London, 1944
2. /Engineer/, Aug. 17th (1956)
3. /Phil. Mag./, Vol. 46, p. 1123 (1955)
4. /Quart. J. Roy. Met. Soc./, Vol. 81, p. 265 (1955)
5. /Compl. Rend./, Vol. 242, p. 2101 (1956)
6. /J.I.E.E./, Vol, 88, (II), p. 487 (with R. H. Golde)(1941)
7. /Proc. Roy. Soc. A/, Vol. 183, p. 228 (1944)
8. "Recent Advances in Atmospheric Electricity", edited by L.
G. Smith, London, Pergamon Press, 1958, p. 461
9. /Observatory/, Vol. 75, p. 82 (1954)
10. /Observatory/, Vol. 77, p. 107 (1957)
11. /Observatory/, Vol. 77, p. 153 (1957)
12. /Phil. Mag./, Vol. 3, pp. 539-1328 (1958)
13. /J. I. E. E./, Vol, 6, p. 315 (1959)
16. /Observatory/, Vol. 69, p. 193 (1949)
17. E. R. A. Report, Ref. Z/T117, "Evolution of Extra-galactic
Nebulae and the Origin of Metagalactic Radio Noise," 1958. 
2. Stephen Gray, /Phil. Trans. Roy. Soc./, Vol. 37, p. 18 (1731).
3. P. E. Shaw:
1. /Nature/, Vol. 118, p. 659 (1926)
2. /Proc. Roy. Soc. A/, Vol. 122, p. 49 (1928). 
4. /Roy. Met. Soc./, Discussion, 18 May, 1955.
5. "The Thunderstorm," edited by H. R. Byers, Washington, U. S. Dept.
of Commerce, 1949, p. 89.
6. J. A. Chalmers, in "Recent Advances in Atmospheric Electricity",
edited by L. G. Smith, London, Pergamon Press, 1958, p. 309.
7. J. Kuettner and R. Lavoie, /ibid/., p. 391.
8. B. Vonnegut and C. B. Moore, /ibid/., p. 399.
9. T. E. Allidone, private communication to the author.
10. P. W. Merrill:
1. /Astrophysical J./, Vo1. 106, p. 274 (1947)
2. /Ibid./, Vol., 71, p. 285 (1930)
3. "Spectra of Long-Period Variable Stars," Chicago, University
of Chicago Press, 1940, p. 84
4. /Ibid./, p. 105
5. /Astrophysical J./, Vol. 93, p. 397 (1941)
6. /Ibid./, Vol. 99, p. 481 (1944). 
11. L. H. Aller, /Pub. Dom. Astrophysical Obs./, Vol. 9, p. 353 (1954)
12. See P. L. Bellaschi, /Electrical Engr./, Vol. 56, p. 1256 (1937).
13. H. Maecker, /App. Sci. Res. B/, Vol. 5, p. 231 (1955).
14. L. A. King, Paper to Physical Society's Conference on Discharges
in Gases, Swansea, Sept. 1958.
15. H. Israel and K. Wurm, /Wiss. Arb. Deutsch. Met. Dien./, Vol. I,
p. 48 (1947).
16. A. H. Joy, /Astrophysical J./, Vol. 96, p. 141 (1942).
17. P. Swings and O. Struve, Ibid., Vol. 91, p. 546 (1940).
18. C. K. Seyfert, /Ibid./, Vol. 97, p. 28 (1943).
19. W. Baade and R. Minkowski, /Ibid./,Vol. 119, p. 215 (1954).
20. I. S. Shklovsky, Proc. /I. A. U./, 1956, Paper No. 36, Cambridge
University Press, 1957, p. 205.
21. /Rev. Modern Phys./, Vol. 30, pp. 1042 and 938 (1958). (See also
p. 925 regarding the failure of current theories to account for
the interactions between extra-galactic nebulae some of which are
at least qualitatively explained by the discharge theory.)
22. "Radio Astronomy," Symposium No. 4 of the I. A. U., edited by H.
C. van de Hulst, Cambridge University Press, 1957, p. 207.
23. L. Woltjer, /Bull. Astronomical Inst. Netherlands/, Vol. 14,
(483), p. 39 (1958).
24. C. Payne-Gaposchkin, "Variable Stars and Galactic Clusters,"
London, Athlone Press, 1954.
25. A. B. Meinel, /Astrophysical J./, Vol. 113, p. 50 (1951). 

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