emission must exceed a certain minimum intensity to become visible.
There may be nebulæ, and they probably constitute the majority, which
we cannot observe because the number of electrically charged particles
rushing into them is far too insignificant. A confirmation of this
view was furnished by the flashing-up of the new star in Perseus on
February 21 and 22, 1901. This star ejected two different kinds of
particles, of which the one kind travelled with nearly double the
velocity of the other. The accumulations of dust formed two spherical
shells around the new star, corresponding in every respect to the two
kinds of comets’ tails of Bredichin’s first and second classes, which
we have sometimes observed together in the same comet (Fig. 35). When
these dust particles, on their road, hit against nebular masses, the
latter became luminescent, and we thereby obtained knowledge of the
presence of large stellar nebulæ of whose existence we previously
had not the faintest suspicion. Conditions, no doubt, are similar
in other parts of the heavens where “we have not so far discovered
any nebulæ—we believe, because of the small number of these charged
particles straying about in those parts. On the same grounds we may
explain the variability of certain nebulæ which formerly appeared quite
enigmatical.”
V
THE SOLAR DUST IN THE ATMOSPHERE—POLAR LIGHTS AND THE VARIATIONS OF
TERRESTRIAL MAGNETISM
We have so far dwelt on the effects which the particles expelled
from the sun and the stars exert on distant celestial bodies. It may
be asked whether this dust does not act upon our own earth. We have
already recognized the peculiar luminescence which on clear nights is
diffused over the sky as a consequence of electrical discharges of this
straying dust. This leads to the question whether the magnificent polar
lights, which according to modern views are also caused by electric
discharges in the higher strata of the atmosphere, are not produced
by dust which the sun sends to us. It will, indeed, be seen that we
can in this way explain quite a number of the peculiarities of these
mysterious phenomena which have always excited man’s imagination.
We know that meteorites and shooting-stars are rendered incandescent
by the resistance which they encounter in the air at an average height
of 120 km. (75 miles), sometimes of 150 and 200 km. In isolated cases
meteorites are supposed to have become visible even at still greater
altitudes. It would result that there must be appreciable quantities of
air still at relatively high elevation, and that the atmosphere cannot
be imperceptible at an altitude of less than 100 km., as was formerly
assumed. Bodies smaller than the meteorites as well as the solar dust
we have spoken of—which, owing to their minuteness and to the strong
cooling by heat radiation and conduction that they undergo in passing
through the atmosphere, could never attain incandescence—would be
stopped at greater heights. We will assume that they are arrested at a
mean height of about 400 km. (250 miles).
The masses of dust which are expelled by the sun are partly uncharged,
partly charged with positive or negative electricity. Only the latter
can be connected with the polar lights; the former would remain dark
and slowly sink through our atmosphere to the surface of the earth.
They form the so-called cosmical dust, of whose great importance
Nordenskiöld was so firmly convinced. He estimated that the yearly
increase in the weight of the earth by the addition of the meteorites
was at least ten million tons, or five hundred times more than we
stated above (page 108). Like Lockyer and, in more recent days,
Chamberlin, he believed that the planets were largely built up of
meteorites.
The dust reaching the earth from the sun would not, were it not
electrically charged, amount to more than 200 tons in a year. Although
this figure may be far too low, yet the supply of matter by these means
is certainly very small in comparison with the 20,000 tons which the
earth receives in the shape of meteorites and shooting-stars. But owing
to its extremely minute distribution, the effect of this dust is very
important, and it may constitute a much greater portion of the finely
distributed cosmical dust in the highest strata of the atmosphere than
the dust introduced by falling meteorites and shooting-stars.
That these particles exert a noticeable influence upon terrestrial
conditions, in spite of their relatively insignificant mass, is due to
two causes. They are extremely minute and therefore remain suspended
in our atmosphere for long periods (for more than a year in the case of
the Krakatoa dust), and they are electrically charged.
In order to understand their action upon the earth, we will examine how
the terrestrial conditions depend upon the position of the earth with
regard to the various active portions of the sun, and upon the change
of the sun itself in regard to its emission of dust particles. For this
examination we have to avail ourselves of extensive statistical data;
for only a long series of observations can give us a clear conception
of the action of solar dust.
These particles withdraw from the sun gases which they were able
to condense on their surface, and which had originally been in the
chromosphere and in the corona of the sun. The most important among
these gases is hydrogen; next to it come helium and the other noble
gases which Ramsay has discovered in the atmosphere, in which they
occur in very small quantities. As regards hydrogen, Liveing and
(after him) Mitchell have maintained that it is not produced in the
terrestrial atmosphere. Occasionally it is certainly found in volcanic
gases. Thus hydrogen escapes, for instance, from the crater of Kilauea,
on Hawaii, but it is burned at once in the atmosphere. If hydrogen were
present in the atmosphere, it would gradually combine with the oxygen
to water vapor; and we have to assume, therefore, that the hydrogen
must be introduced into our atmosphere from another source—namely,
from the sun. Mitchell finds in this view a strong support for the
opinion that solar dust is always trickling down through our atmosphere.
The quantity of solar dust which reaches our atmosphere will naturally
vary in proportion with the eruptive activity of the sun. The quantity
of dust in the higher strata influences the color of the light of the
sun. After the eruption of the volcano Rakata on Krakatoa, in 1883,
and again, though to a lesser degree, after the eruption of Mont Pelée
on Martinique, red sunsets and sunrises were observed all over the
globe. At the same time, another phenomenon was noticed which could be
estimated quantitatively. The light of the sky is polarized with the
exception of the light coming from a few particular spots. Of these
spots, one called Arago’s Point is situated a little above the antipode
of the sun, and another, Babinet’s Point, is situated above the sun.
If we determine the elevation of these points above the horizon at
sunset, we find in accordance with the theoretical deduction that this
elevation is greater when the higher strata of the atmosphere are
charged with dust (as after the eruption of Rakata) than under normal
conditions. Busch, a German scientist, analyzed the mean elevation
of these points (stated in degrees of arc) at sunset, and found the
following peculiar numbers:
│1886 │ ’87 │ ’88 │ ’89 │ ’90 │
Arago’s Point │20.1 │19.7 │18.4 │17.8 │=17.7=│
Babinet’s Point│23.9 │21.9 │17.9 │56.8 │=15.4=│
Sun-spot Number│21.1 │19.1 │ 6.7 │=6.1=│ 6.5 │
│ ’91 │ ’92 │ ’93 │ ’94 │ ’95 │Mean
Arago’s Point │20.6 │19.6 │ 20.2 │=20.7=│18.8 │19.4
Babinet’s Point│23.3 │21.5 │=24.2=│ 23.3 │19.0 │20.7
Sun-spot Number│35.6 │73.8 │=84.9=│ 78.0 │63.9 │40.0
There is a distinct parallelism in these series of figures. Almost
simultaneously with the sun-spot maximum the height of the two
so-called neutral points above the horizon attains its maximum at
sunset, and the same applies to the minimum. That the phenomena in the
atmosphere take place a little later than the phenomena on the sun
which caused them is perhaps only natural.
When the air is rich in dust, or when it is strongly ionized by kathode
rays, conditions are favorable for the formation of clouds. This can
be observed, for instance, with auroral lights. They regularly give
rise to a characteristic cloud formation, so much so that Adam Paulsen
was able to recognize polar lights by the aid of these clouds in
full daylight. Klein has compiled a table on the connection between
the frequency of the higher clouds, the so-called cirrus clouds, at
Cologne, and the number of sun-spots during the period 1850-1900. He
demonstrates that during this half-century, which comprises more than
four sun-spot periods, the sun-spot maxima fell in the years in which
the greatest number of cirrus clouds had been observed. The minima of
the two phenomena are likewise in agreement.
A similarly intensified formation of clouds seems also to occur on
Jupiter when sun-spots are frequent. Vogel states that Jupiter at such
times shines with a whiter light, while at sun-spot minima it appears
of a deeper red. The deeper we are able to peep into the atmosphere of
Jupiter, the more reddish it appears. During periods of strong solar
activity the higher portions of Jupiter’s atmosphere therefore appear
to be crowded with clouds.
The discharge of the charged solar dust in our atmosphere calls forth
the polar lights.
The polar lights occur, as the name indicates, most frequently in
the districts about the poles of the earth. They are, however, not
actually more frequent the nearer we come to the poles; but they
attain a maximum of frequency in circles which enclose the magnetic
and the geographical poles. The northern maximum belt passes, via
Cape Tscheljuskin, north of Novaja Semlja, along the northwestern
coast of Norway, a few degrees to the south of Iceland and Greenland,
right across Hudson Bay and over the northwestern extension of Alaska.
When we go to the south of this belt, the auroras, or boreal lights,
diminish markedly. They are four times less frequent in Edinburgh, and
fifteen times less frequent in London or New York, than in the Orkney
Islands or Labrador.
Paulsen divides the auroras into two classes, which behave quite
differently in several respects. The great difficulties which the
solution of the problems of polar lights has so far offered seem to a
large extent to be due to the fact that all polar lights were treated
as being of the same kind.
The polar lights of the first class do not display any streamers. They
cover a large portion of the sky in a horizontal direction. They are
very quiet, and their light is strikingly constant. As a rule, they
drift slowly towards the zenith, and they do not give rise to any
magnetic disturbances.
These polar lights generally have the shape of an arch whose apex is
situated in the direction of the magnetic meridian (Fig. 38). Sometimes
several arches are grouped one above another.
Nordenskiöld observed these arches quite regularly during the polar
night when he was wintering near Pitlekaj, in the neighborhood
of Bering Sound. Adam Paulsen has often seen them on Iceland and
Greenland, which are situated within the maximum belt spoken of, where
northern lights are very common. Occasionally auroras are also seen
farther from the poles, as circular arches of a milky white, which may
be quite high in the heavens.
Sometimes we perceive in the arctic regions that large areas of the
heavens are covered by a diffused light which might best be compared
to a luminous, transparent cloud; the darker portions in it probably
appear dark by contrast. This phenomenon was frequently observed
during the Swedish expedition of 1882-1883, near Cape Thordsen.
[Illustration: Fig. 38.—Arch-shaped auroræ borealis, observed
by Nordenskiöld during the wintering of the _Vega_ in Bering
Strait, 1879]
Masses of light at so low a level that the rocks behind them are
obscured have frequently been observed to float in the air, especially
in the arctic districts. Thus Lemström saw an aurora on the island
of Spitzbergen in front of a wall of rock only 300 m. (1000 ft.) in
height. In northern Finland he observed the auroral line in the light
of the air in front of a black cloth only a few metres distant. Adam
Paulsen counts these phenomena also as polar lights of the first class,
and he regards them as phosphorescent clouds which have been carried
down by convection currents to an unusually low level of our atmosphere.
Polar lights of the second class are distinguished by the
characteristic auroral rays or streamers. Sometimes these streamers are
quite separated from one another (see Fig. 39); as a rule they melt
into one another, especially below, so as to form draperies which are
so easily moved and unsteady that they appear to flutter in the wind
(Fig. 41.) The streamers run very approximately in the direction of the
inclination (magnetic dip) needle, and when they are fully developed
around the celestial dome their point of convergence is distinctly
discernible in the so-called corona (Fig. 40). When the light is at its
greatest intensity the aurora is traversed by numerous waves of light.
[Illustration: Fig. 39.—Aurora borealis, with radial streamers]
The draperies are very thin. Paulsen watched them sometimes drifting
over his head in Greenland. The draperies then appeared foreshortened,
in the shape of striæ or ribbons of light in convolutions. These polar
lights influence the magnetic needle. When they pass the zenith their
influence changes sign, so that the deviation of the magnetic needle
changes from east to west when the ribbon is moving from north to
south. Paulsen therefore concluded that negative electricity (kathode
rays) was moving downward in these rays. These polar lights correspond
to violent displacements of negative electricity, while polar lights
of the first class appear to consist of a phosphorescent matter which
is not in strong agitation. The streamers may penetrate down into
rather low atmospheric strata, at least in districts which are near the
maximum belt of the northern lights. Thus Parry observed at Port Bowen
an auroral streamer in front of a cliff only 214 m. (700 ft.) in height.
[Illustration: Fig. 40.—Aurora with corona, observed by
Gyllenskiöld on Spitzbergen, 1883]
Polar lights of the first order may pass into those of the second
order, and _vice versa_. We frequently see rays suddenly flash out from
the arch of the aurora, mostly downward, but, when the display is very
intense, also upward. On the other hand, the violent agitation of a
“drapery light” may cease, and may give way to a diffused, steady glow
in the sky. The polar light of the first class is chiefly observed in
the arctic regions. To it corresponds, in districts farther removed
from the pole, the diffused light which appears to be spread uniformly
over the heavens and which gives the auroral line.
[Illustration: Fig. 41.—Polar-light draperies, observed in
Finnmarken, northern Norway]
The usually observed polar lights (speaking not only of those seen on
arctic expeditions) belong to the second class, which comprises also
all those included in the subjoined statistics, with the exception
of the auroral displays reported from Iceland and Greenland. While
the streamer lights distinctly conform to the 11.1 years’ period, and
become more frequent at times of sun-spot maxima, this is not the case,
according to Tromholt, with the auroras of Iceland and Greenland. Their
frequency, on the contrary, seems to be rather independent of the
sun-spot frequency. Not rarely auroral maxima corresponding to sun-spot
maxima are subdivided into two by a secondary minimum. This phenomenon
is most evident in the polar regions, but it can also be traced in the
statistics from Scandinavia and from other countries.
Better to understand the nature of auroras, we will consider the sun’s
corona during the time of a minimum year, taking as an example the year
1900 (compare Fig. 30). The rays of the corona in the neighborhood
of the poles of the sun are laterally deflected by the action of the
magnetic lines of force of the sun. The small, negatively charged
particles have evidently only a low velocity, so that they move quite
close to the lines of force in the neighborhood of the solar poles
and are concentrated near the equator. There the lines of force are
less crowded—that is to say, the magnetic forces are weaker—and the
solar dust can therefore be ejected by the radiation pressure and will
accumulate to a large disk expanding in the equatorial plane. To us
this disk appears like two large streams of rays which project in the
direction of the solar equator. Part of this solar dust will come near
the earth and be deflected by the magnetic lines of force of the earth;
it will hence be divided into two streams which are directed towards
the two terrestrial magnetic poles. These poles are situated below the
earth’s crust, and therefore not all the rays will be concentrated
towards the apparent position of the magnetic poles upon the surface of
the earth. It is to be expected that the negatively charged particles
coming from the sun will chiefly drift towards that district which is
situated somewhat to the south of the magnetic north pole, when it is
noon at this pole. When it is midnight at the magnetic pole, most
of the negatively charged particles will be caught by the lines of
force before they pass the geographical north pole, and the maximum
belt of the auroras will for this reason surround the magnetic and
the geographical poles, as has already been pointed out (compare page
122). The negatively charged solar dust will thus be concentrated in
two rings above the maximum belts of the polar lights. Where the dust
collides with molecules of the air, it will produce a phosphorescent
glow, as if these molecules were hit by the electrically charged
particles of radium. This phosphorescent glow rises in the shape of a
luminous arch to a height of about 400 km. (250 miles)—according to
Paulsen—and the apex of this arch will in every part seem to lie in
the direction where the maximum belt is nearest to the station of the
observer. That will fairly coincide with the direction of the magnetic
needle.
The solar corona of a sun-spot maximum year is of a very different
appearance (Fig. 31). The streamers radiate straight from the sun in
almost all directions; and if there be some privileged directions,
it will be those above the sun-spot belts. The velocity of the solar
dust is evidently so great that the streamers are no longer visibly
deflected by the magnetic lines of force of the sun. Nor is this
charged dust influenced to any noticeable degree by the magnetic lines
of force of the earth. It will in the main fall straight down in that
part of the atmosphere in which the radiation is most intense: As
these “hard” rays of the sun[8] seem to issue from the faculæ of the
sun which are most frequent in maximum sun-spot years, some polar
lights will also be seen in districts which are far removed from the
maximum belt of the auroras, especially when the number of sun-spots
is large. The opposite relation holds for the “soft” streams of solar
dust which fall near the maximum belt of the polar lights. These
streams occur most frequently with low sun-spot frequency, as we know
from observations of the solar corona. Possibly they are carried
along by the stream of harder rays in maximum years. The polar lights
corresponding to these rays therefore attain their maximum with few
sun-spots. Hard and soft dust streams occur, of course, simultaneously;
but the former predominate in maximum sun-spot years, the latter in
minimum years.
[Footnote 8: The designations “hard” and “soft” streams of
solar dust correspond to the terms used with regard to kathode
rays. The soft rays have a smaller velocity, and are therefore
more strongly deflected by external forces, as, for instance,
magnetic forces.]
That the periodicity of the polar lights in regions without the maximum
belt follows very closely the periodicity of the sun-spots was shown by
Fritz as early as 1863. The length of the period varies between 7 and
16 years, the average being 11.1 years. The years of maxima and minima
for sun-spots and for northern auroras are the following:
MAXIMUM YEARS
Sun-spots 1728 ’39 ’50 ’62 ’70 ’78 ’88 1804 ’16 ’30
1837 ’48 ’60 ’71 ’83 ’93 1905
Northern lights 1730 ’41 ’49 ’61 ’73 ’78 ’88 1805 ’19 ’30
1840 ’50 ’62 ’71 ’82 ’93 1905
MINIMUM YEARS
Sun-spots 1734 ’45 ’55 ’67 ’76 ’85 ’98 1811 ’23 ’34
1844 ’56 ’67 ’78 ’89 1900
Northern lights 1735 ’44 ’55 ’66 ’75 ’83 ’99 1811 ’22 ’34
1844 ’56 ’66 ’78 ’89 1900
There are, in addition, as De Mairan proved in his classical memoir of
the year 1746, longer periods common to both the number of sun-spots
and the number of auroras. According to Hansky, the length of this
period is 72 years; according to Schuster, 33 years. Very pronounced
maxima occurred at the beginning and the end of the eighteenth century,
the last in the year 1788; afterwards auroras became very rare in the
years 1800-1830, just as in the middle of the eighteenth century. In
1850, and particularly in 1871, there were strong maxima; they have
been absent since then.
The estimates of the heights of the polar lights vary very
considerably. The height seems to be the greater, on the whole, the
nearer the point of observation is to the equator, which would well
agree with the slight deflection of the kathode rays towards the
surface of the earth in regions which are farther removed from the
pole. Gyllenskiöld found on Spitzbergen a mean height of 55 km.;
Bravais, in northern Norway, 100 to 200 km.; De Mairan, in central
Europe, 900 km.; Galle, again, 300 km. In Greenland, Paulsen observed
northern lights at very low levels. In Iceland he fixed the apex of
the northern arch which may be considered as a point where the charged
particles from the sun are discharged into the air at about 400 km. Not
much reliance can be placed upon the earlier determinations; but the
heights given conform approximately to the order of magnitude which we
may deduce from the height at which the solar dust will be stopped by
the terrestrial atmosphere.
The polar lights possess, further, a pronounced yearly periodicity
which is easily explicable by the aid of the solar dust theory. We have
seen that sun-spots are rarely observed near the solar equator, and the
same applies to solar faculæ. They rapidly increase in frequency with
higher latitudes of the sun, and their maximum occurs at latitudes of
about fifteen degrees. The equatorial plane of the sun is inclined by
about seven degrees towards the plane of the earth’s orbit. The earth
is in the equatorial plane of the sun on December 6th and June 4th,
and most distant from it three months later. We may, therefore, expect
that the smallest number of solar-dust particles will fall on the earth
when the earth is in the equator of the sun—that is, in December and
June—and the greatest number in March and September. These relations
are somewhat disturbed by the twilight, which interferes with the
observation of auroras in the bright summer nights of the arctic
region, while the dark nights of the winter favor the observation
of these phenomena. The distribution of the polar lights over the
different seasons of the year will become clear from the subjoined
table compiled by Ekholm and myself:
Iceland and United Southern
Sweden Norway Greenland States auroræ
(1883-96) (1861-95) (1872-92) (1871-93) (1856-94)
January 1056 =251= 804 1005 =56=
February 1173 331 734 1455 126
March =1312= =335= 613 1396 =183=
April 568 90 128 =1724= 148
May 170 6 1 1270 54
June =10= =0= =0= =1061= 40
July 54 0 0 1233 =35=
August 191 18 40 1210 75
September 1055 209 455 =1735= 120
October =1114= =353= 716 1630 =192=
November 1077 326 811 1240 112
December =940= 260 =863= 912 81
Average number 727 181 430 1322 102