+--------+----------------------------------------------+------------+
| Direc- | Absolute Number for each Type. | Percentage |
| tion. +-------+--------+-----------+-------+---------+ from all |
| | Arcs. | Bands. | Curtains. | Rays. | Patches.| Types. |
+--------+-------+--------+-----------+-------+---------+------------+
| N. | 9 | 16 | 5 | 15 | 4 | 10 |
| N.E. | 9 | 13 | 2 | 20 | 4 | 9 |
| E. | 3 | 11 | 2 | 26 | 3 | 9 |
| S.E. | 5 | 6 | 1 | 10 | 7 | 6 |
| S. | 45 | 43 | 1 | 16 | 15 | 24 |
| S.W. | 9 | 9 | 2 | 12 | 13 | 9 |
| W. | 3 | 11 | 2 | 22 | 6 | 9 |
| N.W. | 2 | 8 | 2 | 8 | 5 | 5 |
+--------+-------+--------+-----------+-------+---------+------------+
Table VI. accounts for only 81% of the total displays; of the remainder
15% appeared in the zenith, while 4% covered the whole sky. Auroral
displays generally cover a considerable area, and are constantly
changing, so the figures are necessarily somewhat rough. But clearly,
whilst the arcs and bands, and to a lesser extent the patches, showed a
marked preference for the magnetic meridian, the rays showed no such
preference.
At Cape Thorsden (7) in 1882-1883 auroras as a whole were divided into
those seen in the north and those seen in the south. The variation
throughout the twenty-four hours in the percentage seen in the south
was as follows:--
+--------+------+------+------+-------+
| Hour. | 0-3. | 3-6. | 6-9. | 9-12. |
+--------+------+------+------+-------+
| A.M. | 69 | 55 | 44 | 35 |
| P.M. | 55 | 70 | 65 | 65 |
+--------+------+------+------+-------+
The mean from the whole twenty-four hours is sixty-three. Between 3
A.M. and 3 P.M. the percentage of auroras seen in the south thus
appears decidedly below the mean.
17. The following data for the apparent angular width of arcs were
obtained at Cape Thorsden, the arcs being grouped according to the
height of the lower edge above the horizon. Group I. contained thirty
arcs whose altitudes did not exceed 11 deg. 45'; Group II. thirty arcs
whose altitudes lay between 12 deg. and 35 deg.; and Group III, thirty
arcs whose altitudes lay between 36 deg. and 80 deg.
+-----------------+----------+----------+----------+
| Group. | I. | II. | III. |
+-----------------+----------+----------+----------+
| Greatest width |11.5 deg. |12.0 deg. | 21.0 deg.|
| Least " | 1.0 deg. | 0.75 deg.| 2.0 deg.|
| Mean " | 3.45 deg.| 4.6 deg. | 6.9 deg.|
+-----------------+----------+----------+----------+
There is here a distinct tendency for the width to increase with the
altitude. At the same time, arcs near the horizon often appeared wider
than others near the zenith. Furthermore, Gyllenskold says that when
arcs mounted, as they not infrequently did, from the horizon, their
apparent width might go on increasing right up to the zenith, or it
might increase until an altitude of about 45 deg. was reached and then
diminish, appearing much reduced when the zenith was reached. Of
course the phenomenon might be due to actual change in the arc, but it
is at least consistent with the view that arcs are of two kinds, one
form constituting a layer of no great vertical depth but considerable
real horizontal width, the other form having little horizontal width
but considerable vertical depth, and resembling to some extent an
auroral curtain.
18. According to numerous observations made at Cape Thorsden, the
apparent angular velocity of arcs increases on the average with their
altitude. Dividing the whole number of arcs, 156, whose angular
velocities were measured into three numerically equal groups,
according to their altitude, the following were the results in minutes
of arc per second of time (or degrees per minute of time):--
+-------------------+---------+---------+----------+------+
| Group. | I. | II. | III. | All. |
+-------------------+---------+---------+----------+------+
| Mean altitude |10.5 deg.|34.6 deg.| 72.3 deg.| .. |
| Greatest velocity | 4.81 | 15.12 |109.09 | .. |
| Mean velocity | 0.48 | 2.42 | 8.67 | 3.86 |
+-------------------+---------+---------+----------+------+
Each group contained auroras which appeared stationary. The intervals
to which the velocities referred were usually from five to ten
minutes, but varied widely. The velocity 109.09 was much the largest
observed, the next being 52.38; both were from observations lasting
under half a minute.
19. In 1882-1883 the direction of motion of arcs was from north to
south in 62% of the cases at Jan Mayen, and in 58% of the cases at
Cape Thorsden. This seems the more common direction in the northern
hemisphere, at least for stations to the south of the zone of maximum
frequency, but a considerable preponderance of movements towards the
north was observed in Franz Joseph Land by the Austrian Expedition of
1872-1874. The apparent motion of arcs is sometimes of a complicated
character. One end only, for example, may appear to move, as if
rotating round the other; or the two ends may move in opposite
directions, as if the arc were rotating about a vertical axis through
its summit.
20. _Height._--If an auroral arc represented a definite self-luminous
portion of space of small transverse dimensions at a uniform height
above the ground, its height could be accurately determined by
observations made with theodolites at the two ends of a measured base,
provided the base were not too short compared to the height. If a very
long base is taken, it becomes increasingly open to doubt whether the
portions of space emitting auroral light to the observers at the two
ends are the same. There is also difficulty in ensuring that the
observations shall be simultaneous, an important matter especially when
the apparent velocity is considerable. If the base is short, definite
results can hardly be hoped for unless the height is very moderate.
Amongst the best-known theodolite determinations of height are those
made at Bossekop in Norway by the French Expedition of 1838-1839 (16)
and the Norwegian Expedition of 1882-1883, and those made in the latter
year by the Swedes at Cape Thorsden and the Danes at Godthaab. At
Bossekop and Cape Thorsden there were a considerable proportion of
negative or impossible parallaxes. Much the most consistent results were
those obtained at Godthaab by Paulsen (15). The base was 5.8 km. (about
3-1/2 miles) long, the ends being in the same magnetic meridian, on
opposite sides of a fiord, and observations were confined to this
meridian, strict simultaneity being secured by signals. Heights were
calculated only when the observed parallax exceeded 1 deg., but this
happened in three-fourths of the cases. The calculated heights--all
referring to the lowest border of the aurora--varied from 0.6 to 67.8
km. (about 0.4 to 42 m.), the average being about 20 km. (12 m.).
Regular arcs were selected in most cases, but the lowest height obtained
was for a collection of rays forming a curtain which was actually
situated between the two stations.
In 1885 Messrs Garde and Eherlin made similar observations at
Nanortalik near Cape Farewell in Greenland, but using a base of only
1250 metres (about 3/4 m.). Their results were very similar to
Paulsen's. On one occasion twelve observations, extending over half an
hour, were made on a single arc, the calculated heights varying in a
fairly regular fashion from 1.6 to 12.9 km. (about 1 to 8 m.). The
calculated horizontal distances of this arc varied between 5 and 24
km. (about 3 and 15 m.), the motion being sometimes towards, sometimes
away from the observers, but not apparently exceeding 3 km. (nearly 2
m.) per minute. Heights of arcs have often been calculated from the
apparent altitudes at stations widely apart in Europe or America. The
heights calculated in this way for the under surface of the arc, have
usually exceeded 100 m.; some have been much in excess of this figure.
None of the results so obtained can be accepted without reserve, but
there are several reasons for believing that the average height in
Greenland is much below that in lower latitudes. Heights have been
calculated in various less direct ways, by observing for instance the
angular altitude of the summit of an arc and the angular interval
between its extremities, and then making some assumption such as that
the portion visible to an observer may be treated as a circle whose
centre lies over the so-called auroral pole. The mean height
calculated at Arctic stations, where careful observations have been
made, in this or analogous ways, has varied from 58 km. (about 36 m.)
at Cape Thorsden (Gyllenskold) to 227 km. (about 141 m.) at Bossekop
(Bravais). The height has also been calculated on the hypothesis that
auroral light has its source where the atmospheric pressure is similar
to that at which most brilliancy is observed when electric discharges
pass in vacuum tubes. Estimates on this basis have suggested heights
of the order of 50 km. (about 31 m.). There are, of course, many
uncertainties, as the conditions of discharge in the free atmosphere
may differ widely from those in glass vessels. If the Godthaab
observations can be trusted, auroral discharges must often occur
within a few miles of the earth's surface in Arctic regions. In
confirmation of this view reference may be made to a number of
instances where observers--e.g. General Sabine, Sir John Franklin,
Prof. Selim Lemstrom, Dr David Walker (at Fort Kennedy in 1858-1859),
Captain Parry (Fort Bowen, 1825) and others--have seen aurora below
the clouds or between themselves and mountains. One or two instances
of this kind have even been described in Scotland. Prof. Cleveland
Abbe (20) has given a full historical account of the subject to which
reference may be made for further details.
21. _Brightness._--In auroral displays the brightness often varies
greatly over the illuminated area and changes rapidly. Estimates of
the intensity of the light have been based on various arbitrary
scales, such for instance as the size of type which the observer can
read at a given distance. The estimate depends in the case of reading
type on the general illumination. In other cases scales have been
employed which make the result mainly depend on the brightest part of
the display. At Jan Mayen (8) in 1882-1883 a scale was employed
running from 1, taken as corresponding to the brightness of the milky
way, to 4, corresponding to full moonlight. The following is an
analysis of the results obtained, showing the number of times the
different grades were reached:--
+------------+------+------+------+------+------------+
| Scale of | 1. | 2. | 3. | 4. | Mean |
| Intensity. | | | | | Intensity. |
+------------+------+------+------+------+------------+
| Arcs | 27 | 53 | 13 | 1 | 1.87 |
| Bands | 46 | 83 | 49 | 22 | 2.24 |
| Rays | 30 | 116 | 138 | 28 | 2.21 |
| Corona | 3 | 14 | 12 | 12 | 2.81 |
+------------+------+------+------+------+------------+
On one or two occasions at Jan Mayen auroral light is described as
making the full moon look like an ordinary gas jet in presence of
electric light, whilst rays could be seen crossing and brighter than
the moon's disk. Such extremely bright auroras seem very rare,
however, even in the Arctic. There is a general tendency for both
bands and rays to appear brightest at their lowest parts; arcs seldom
appear as bright at their summits as nearer the horizon. It is not
unusual for arcs and bands to look as if pulses or waves of light were
travelling along them; also the direction in which these pulses travel
does not seem to be wholly arbitrary. Movements to the east were twice
as numerous at Jan Mayen and thrice as numerous at Traurenberg as
movements to the west. In some cases changes of intensity take place
round the auroral zenith, simulating the effect that would be produced
by a cyclonic rotation of luminous matter. In the case of isolated
patches the intensity often waxes and wanes as if a search-light were
being thrown on and turned off.
22. _Colour._--The ordinary colour of aurora is white, usually with a
distinct yellow tint in the brighter forms, but silvery white when the
light is faint. When the light is intense and changing rapidly, red is
not infrequently present, especially towards the lower edge. Under these
circumstances, green is also sometimes visible, especially towards the
zenith. Thus a bright auroral ray may seem red towards the foot and
green at its summit, with yellow intervening. In some cases the green
may be only a contrast effect. Other colours, e.g. violet, have
occasionally been noticed but are unusual.
23. _Spectrum._--The spectrum of aurora consists of a number of lines.
Numerous measurements have been made of the wave-lengths of the
brightest. One line, in the yellow green, is so dominant optically as
often to be described as the auroral line. Its wave-length is probably
very near 5571 tenth-metres, and it is very close to, if not absolutely
coincident with, a prominent line in the spectrum of krypton. This line
is so characteristic that its presence or absence is the usual criterion
for deciding whether an atmospheric light is aurora. The Swedish
Expedition (17) of 1899-1902, engaged in measuring an arc of the meridian
in Spitsbergen, were unusually well provided spectrographically, and
succeeded in taking photographs of aurora in conjunction with artificial
lines--chiefly of hydrogen--which led to results claiming exceptional
accuracy. In the spectrograms three auroral rays--including the principal
one mentioned above--were pre-eminent. For the two shorter wave-lengths,
for whose measurement he claims the highest precision, the observer, J.
Westman, gives the values 4276.4 and 3913.5. In addition, he assigns
wave-lengths for 156 other auroral lines between wave-lengths 5205 and
3513. The following table gives the wave-lengths of the photographically
brightest of these, retaining four significant figures in place of
Westman's five.