+----------------+-----------+-----------+-----------+-----------+
| | Arcs. | Bands. | Rays. | Patches. |
+----------------+-----+-----+-----+-----+-----+-----+-----+-----+
| | e. | m. | e. | m. | e. | m. | e. | m. |
| +-----+-----+-----+-----+-----+-----+-----+-----+
| Cape Thorsden. | 76 | 24 | 66 | 34 | 52 | 48 | 51 | 49 |
| Jan Mayen. | 78 | 22 | 68 | 32 | 60 | 40 | 60 | 40 |
| Tasiusak | 85 | 15 | 85 | 15 | 65 | 35 | 62 | 38 |
+----------------+-----+-----+-----+-----+-----+-----+-----+-----+
At Cape Thorsden diffused auroral light had percentages e. 65, m. 35,
practically identical with those for bands. At Tasiusak, 8 P.M. was
the hour of most frequent occurrence for arcs and bands, whereas
patches had their maximum frequency at 11 P.M. and rays at midnight.
9. _Lunar and other Periods._--The action of moonlight necessarily gives
rise to a true lunar period in the visibility of aurora. The extent to
which it renders aurora invisible depends, however, so much on the
natural brightness of the aurora--which depends on the time and the
place--and on the sharpness of the outlook kept, that it is difficult to
gauge it. Ekholm and Arrhenius (11) claim to have established the
existence of a true tropical lunar period of 27-32 days, and also of a
26-day period, or, as they make it, a 25.929-day period. A 26-day period
has also been derived by J. Liznar (12), after an elaborate allowance
for the disturbing effects of moonlight from the observations in
1882-1883 at Bossekop, Fort Rae and Jan Mayen. Neither of these periods
is universally conceded. The connexion between aurora and earth magnetic
disturbances renders it practically certain that if a 26-day or similar
period exists in the one phenomenon it exists also in the other, and of
the two terrestrial magnetism (q.v.) is probably the element least
affected by external complications, such as the action of moonlight.
10. _Sun-spot Connexion._--The frequency of auroral displays is much
greater in some years than others. At most places the variation in the
frequency has shown a general similarity to that of sun-spots. Table V.
gives contemporaneous data for the frequency of sun-spots and of auroras
seen in Scandinavia. The sun-spot data prior to 1902 are from A.
Wolfer's table in the _Met. Zeitschrift_ for 1902, p. 195; the more
recent data are from his quarterly lists. All are observed frequencies,
derived after Wolf's method; maxima and minima are in heavy type.
The auroral data are from Table E of Tromholt's catalogue (5), with
certain modifications. In Tromholt's yearly data the year commences with
July. This being inconvenient for comparison with sun-spots, use was
made of his monthly values to obtain corresponding data for years
commencing with January. The Tromholt-Schroeter data for Scandinavia as
a whole commenced with 1761; the figures for earlier years were obtained
by multiplying the data for Sweden by 1.356, the factor being derived by
comparing the figures for Sweden alone and for the whole of Scandinavia
from July 1761 to June 1783.
In a general way Table V. warrants the conclusion that years of many
sun-spots are years of many auroras, and years of few sun-spots years of
few auroras; but it does not disclose any very definite relationship
between the two frequencies. The maxima and minima in the two phenomena
in a good many cases are not found in the same years. On the other hand,
there is absolute coincidence in a number of cases, some of them very
striking, as for instance the remarkably low minima of 1810 and 1823.
11. During the period 1764 to 1872 there have been ten years of
maximum, and ten of minimum, in sun-spot frequency. Taking the three
years of greatest frequency at each maximum, and the three years of
least frequency at each minimum, we get thirty years of many and
thirty of few sun-spots. Also we can split the period into an earlier
half, 1764 to 1817, and a later half, 1818 to 1872, containing
respectively the earlier five and the later five of the above groups
of sun-spot maximum and minimum years. The annual means derived from
the whole group, and the two sub-groups, of years of many and few
sun-spots are as follows:--
+-----------------+------------------+------------------+------------------+
| | 1764-1872. | 1764-1817. | 1818-1872. |
| Years of +--------+---------+--------+---------+--------+---------+
| | Spots. | Auroras.| Spots. | Auroras.| Spots. | Auroras.|
+-----------------+--------+---------+--------+---------+--------+---------+
| Many sun-spots. | 93.4 | 99.9 | 86.7 | 70.7 | 100.1 | 129.1 |
| Few " | 13.4 | 61.5 | 13.6 | 51.6 | 13.1 | 71.3 |
+-----------------+--------+---------+--------+---------+--------+---------+
In each case the excess of auroras in the group of years of many
sun-spots is decided, but the results from the two sub-periods do not
harmonize closely. The mean sun-spot frequency for the group of years
of few sun-spots is almost exactly the same for the two sub-periods,
but the auroral frequency for the later group is nearly 40% in excess
of that for the earlier, and even exceeds the auroral frequency in
the years of many sun-spots in the earlier sub-period. This
inconsistency, though startling at first sight, is probably more
apparent than real. It is almost certainly due in large measure to a
progressive change in one or both of the units of frequency. In the
case of sun-spots, A. Schuster (13) has compared J.R. Wolf and A.
Wolfer's frequencies with data obtained by other observers for areas
of sun-spots, and his figures show unquestionably that the unit in one
or other set of data must have varied appreciably from time to time.
Wolf and Wolfer have, however, aimed persistently at securing a
definite standard, and there are several reasons for believing that
the change of unit has been in the auroral rather than the sun-spot
frequency. R. Rubenson (14), from whom Tromholt derives his data for
Sweden, seems to accept this view, assigning the apparent increase in
auroral frequency since 1860 to the institution by the state of
meteorological stations in 1859, and to the increased interest taken
in the subject since 1865 by the university of Upsala. The figures
themselves in Table V. certainly point to this conclusion, unless we
are prepared to believe that auroras have increased enormously in
number. If, for instance, we compare the first and the last three
11-year cycles for which Table V. gives complete data, we obtain as
yearly means:--
1749-1781 Sun-spots 56.4 Auroras 77.5
1844-1876 " 55.8 " 112.2
The mean sun-spot frequencies in the two periods differ by only 1%,
but the auroral frequency in the later period is 45% in excess of that
in the earlier.
The above figures would be almost conclusive if it were not for the
conspicuous differences that exist between the mean sun-spot
frequencies for different 11-year periods. Schuster, who has
considered the matter very fully, has found evidence of the existence
of other periods--notably 8.4 and 4.8 years--in addition to the
recognized period of 11.125 years, and he regards the difference
between the maxima in successive 11-year periods as due at least
partly to an overlapping of maxima from the several periodic terms.
This cannot, however, account for all the fluctuations observed in
sun-spot frequencies, unless other considerably longer periods exist.
There has been at least one 33-year period during which the mean value
of sun-spot frequency has been exceptionally low, and, as we shall
see, there was a corresponding remarkable scarcity of auroras. The
period in question may be regarded as extending from 1794 to 1826
inclusive. Comparing it with the two adjacent periods of thirty-three
years, we obtain the following for the mean annual frequencies:--
+-----------------+------------+----------+
| 33-Year Period. | Sun-spots. | Auroras. |
+-----------------+------------+----------+
| 1761-1793 | 65.6 | 76.1 |
| 1794-1826 | 20.3 | 39.5 |
| 1827-1859 | 56.1 | 84.4 |
+-----------------+------------+----------+
12. The association of high auroral and sun-spot frequencies shown in
Table V. is not peculiar to Scandinavia. It is shown, for instance, in
Loomis's auroral data, which are based on observations at a variety of
European and American stations (_Ency. Brit._ 9th ed. art.
METEOROLOGY, Table XXVIII.). It does not seem, however, to apply
universally. Thus at Godthaab we have, according to Adam Paulsen (15),
comparing 3-year periods of few and many sun-spots:--
+----------------+----------------+--------------+
| 3-Year Period. | Total Sun-spot | Total Nights |
| | Frequency. | of Aurora. |
+----------------+----------------+--------------+
| 1865-1868 | 48 | 274 |
| 1869-1872 | 339 | 138 |
| 1876-1879 | 23 | 273 |
+----------------+----------------+--------------+
The years start in the autumn, and 1865-1868 includes the three
winters of 1865 to '66, '66 to '67, and '67 to '68. Paulsen also gives
data from two other stations in Greenland, viz. Ivigtut (1869 to 1879)
and Jakobshavn (1873 to 1879), which show the same phenomenon as at
Godthaab in a prominent fashion. Greenland lies to the north of
Fritz's curve of maximum auroral frequency, and the suggestion has
been made that the zone of maximum frequency expands to the south as
sun-spots increase, and contracts again as they diminish, the number
of auroras at a given station increasing or diminishing as the zone of
maximum frequency approaches to or recedes from it. This theory,
however, does not seem to fit all the facts and stands in want of
confirmation.