Page:EB1911 - Volume 02.djvu/981

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931
AURORA POLARIS
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
1794-1826
1827-1859
65.6
20.3
56.1
76.1
39.5
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 
Frequency.
 Total Nights 
of Aurora.
1865-1868
1869-1872
1876-1879
 48
339
 23
274
138
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.


Table V.

Year. Frequency. Year. Frequency. Year. Frequency. Year. Frequency.
Sun-spot. Auroral. Sun-spot. Auroral. Sun-spot. Auroral. Sun-spot. Auroral.
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
 80.9
 83.4
 47.7
 47.8
 30.7
 12.2
  9.6
 10.2
 32.4
 47.6
 54.0
 62.9
 85.9
 61.2
 45.1
 36.4
 20.9
 11.4
 37.8
 69.8
106.1
100.8
 81,6
 66.5
 34.8
 30.6
  7.0
 19.8
 92.5
154.4
125.9
 84.8
 68.1
 38.5
 22.8
 10.2
 24.1
 82.9
132.0
130.9
103
134
 53
111
 96
 65
 34
 60
 83
 80
113
 86
124
114
 89
107
 76
 51
 68
 80
 89
 83
 62
 38
 58
 98
 33
 17
 64
 59
 60
 67
103
 67
 70
 78
 83
136
115
 97
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
118.1
89.9
66.6
60.0
46.9
41.0
21.3
16.0
 6.4
 4.1
 6.8
14.5
34.0
45.0
43.1
47.5
42.2
28.1
10.1
 8.1
 2.5
 0.0
 1.4
 5.0
12.2
13.9
35.4
45.8
41.1
30.4
23.9
15.7
 6.6
 4.0
 1.8
 8.5
16.6
36.3
49.7
62.5
 89
 90
 54
 64
 29
 37
 34
 37
 61
 35
 28
 30
 34
 65
 73
101
 85
 62
 42
 20
 20
  4
 13
 11
 18
 17
 10
 33
 60
 74
 43
 62
 37
 33
 13
 14
 40
 58
 79
 60
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
 67.0
 71.0
 47.8
 27.5
  8.5
 13.2
 56.9
121.5
138.3
103.2
 85.8
 63.2
 36.8
 24.2
 10.7
 15.0
 40.1
 61.5
 98.5
124.3
 95.9
 66.5
 64.5
 54.2
 39.0
 20.6
  6.7
  4.3
 22.8
 54.8
 93.8
 95.7
 77.2
 59.1
 44.0
 47.0
 30.5
 16.3
  7.3
 37.3
 93
132
 89
 54
 79
 81
 58
 98
137
159
165
 82
 75
 91
 66
 81
 26
 50
 63
107
131
 95
 60
 92
 65
 64
 49
 46
 38
 88
131
119
127
135
135
124
119
130
127
144
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
 73.9
139.1
111.2
101.7
 66.3
 44.7
 17.1
 11.3
 12.3
  3.4
  6.0
 32.3
 54.3
 59.7
 63.7
 63.5
 52.2
 25.4
 13.1
  6.8
  6.3
  7.1
 35.6
 73.0
 84.9
 78.0
 64.0
 41.8
 26.2
 26.7
 12.1
  9.5
  2.7
  5.0
 24.4
 42.0
 62.8
 53.8
 62.0
 48.5
160
195
185
200
189
158
133
137
126
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13. Auroral Meridian.—It is a common belief that the summit of an auroral arc is to be looked for in the observer’s magnetic meridian. On any theory it would be rather extraordinary if this were invariably true. In temperate latitudes auroral arcs are seldom near the zenith, and there is reason to believe them at very great heights. In high latitudes the average height is probably less, but the direction in which the magnetic needle