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 80.9 103 1789 118.1 89 1829 67.0 93 1869 73.9 160
1750 83.4 134 1790 89.9 90 1830 71.0 132 1870 139.1 195
1751 47.7 53 1791 66.6 54 1831 47.8 89 1871 111.2 185
1752 47.8 111 1792 60.0 64 1832 27.5 54 1872 101.7 200
1753 30.7 96 1793 46.9 29 1833 8.5 79 1873 66.3 189
1754 12.2 65 1794 41.0 37 1834 13.2 81 1874 44.7 158
1755 9.6 34 1795 21.3 34 1835 56.9 58 1875 17.1 133
1756 10.2 60 1796 16.0 37 1836 121.5 98 1876 11.3 137
1757 32.4 83 1797 6.4 61 1837 138.3 137 1877 12.3 126
1758 47.6 80 1798 4.1 35 1838 103.2 159 1878 3.4 ..
1759 54.0 113 1799 6.8 28 1839 85.8 165 1879 6.0 ..
1760 62.9 86 1800 14.5 30 1840 63.2 82 1880 32.3 ..
1761 85.9 124 1801 34.0 34 1841 36.8 75 1881 54.3 ..
1762 61.2 114 1802 45.0 65 1842 24.2 91 1882 59.7 ..
1763 45.1 89 1803 43.1 73 1843 10.7 66 1883 63.7 ..
1764 36.4 107 1804 47.5 101 1844 15.0 81 1884 63.5 ..
1765 20.9 76 1805 42.2 85 1845 40.1 26 1885 52.2 ..
1766 11.4 51 1806 28.1 62 1846 61.5 50 1886 25.4 ..
1767 37.8 68 1807 10.1 42 1847 98.5 63 1887 13.1 ..
1768 69.8 80 1808 8.1 20 1848 124.3 107 1888 6.8 ..
1769 106.1 89 1809 2.5 20 1849 95.9 131 1889 6.3 ..
1770 100.8 83 1810 0.0 4 1850 66.5 95 1890 7.1 ..
1771 81.6 62 1811 1.4 13 1851 64.5 60 1891 35.6 ..
1772 66.5 38 1812 5.0 11 1852 54.2 92 1892 73.0 ..
1773 34.8 58 1813 12.2 18 1853 39.0 65 1893 84.9 ..
1774 30.6 98 1814 13.9 17 1854 20.6 64 1894 78.0 ..
1775 7.0 33 1815 35.4 10 1855 6.7 49 1895 64.0 ..
1776 19.8 17 1816 45.8 33 1856 4.3 46 1896 41.8 ..
1777 92.5 64 1817 41.1 60 1857 22.8 38 1897 26.2 ..
1778 154.4 59 1818 30.4 74 1858 54.8 88 1898 26.7 ..
1779 125.9 60 1819 23.9 43 1859 93.8 131 1899 12.1 ..
1780 84.8 67 1820 15.7 62 1860 95.7 119 1900 9.5 ..
1781 68.1 103 1821 6.6 37 1861 77.2 127 1901 2.7 ..
1782 38.5 67 1822 4.0 33 1862 59.1 135 1902 5.0 ..
1783 22.8 70 1823 1.8 13 1863 44.0 135 1903 24.4 ..
1784 10.2 78 1824 8.5 14 1864 47.0 124 1904 42.0 ..
1785 24.1 83 1825 16.6 40 1865 30.5 119 1905 62.8 ..
1786 82.9 136 1826 36.3 58 1866 16.3 130 1906 53.8 ..
1787 132.0 115 1827 49.7 79 1867 7.3 127 1907 62.0 ..
1788 130.9 97 1828 62.5 60 1868 37.3 144 1908 48.5 ..

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 points changes rapidly with change of latitude and longitude, and has a large diurnal variation. Thus there must in general be a difference between the observer’s magnetic meridian—answering to the mean position of the magnetic needle at his station—and the direction the needle would have at a given hour, if undisturbed by the aurora, at any spot where the phenomena which the observer sees as aurora exist.

Very elaborate observations have been made during several Arctic expeditions of the azimuths of the summits of auroral arcs. At Cape Thorsden (7) in 1882-1883 the mean azimuth derived from 371 arcs was 24° 12′ W., or 11° 27′ to the W. of the magnetic meridian. As to the azimuths in individual cases, 130 differed from the mean by less than 10°, 118 by from 10° to 20°, 82 by from 20° to 30°, 21 by from 30° to 40°, 14 by from 40° to 50°; in six cases the departure exceeded 50°, and in one case it exceeded 70°. Also, whilst the mean azimuths deduced from the observations between 6 A.M. and noon, between noon and 6 P.M., and between 6 P.M. and midnight, were closely alike, their united mean being 22.4° W. of N. (or E. of S.), the mean derived from the 113 arcs observed between midnight and 6 A.M. was 47.8° W. At Jan Mayen (8) in 1882-1883 the mean azimuth of the summit of the arcs was 28.8° W. of N., thus approaching much more closely to the magnetic meridian 29.9° W. As to individual azimuths, 113 lay within 10° of the mean, 37 differed by from 10° to 20°, 18 by from 20° to 30°, 6 by from 30° to 40°, whilst 6 differed by over 40°. Azimuths were also measured at Jan Mayen for 338 auroral bands, the mean being 22.0° W., or 7.9° to the east of the magnetic meridian. Combining the results from arcs and bands, Carlheim-Gyllensköld gives the “anomaly” of the auroral meridian at Jan Mayen as 5.7° E. At the British Polar station of 1882, Fort Rae (62° 23′ N. lat., 115° 44′ W. long.), he makes it 15.7° W. At Godthaab in 1882-1883 the auroral anomaly was, according to Paulsen, 15.5° E., the magnetic meridian lying 57.6° W. of the astronomical.

14. Auroral Zenith.—Another auroral direction having apparently a close relation to terrestrial magnetism is the imaginary line drawn to the eye of an observer from the centre of the corona—i.e. the point to which the auroral rays converge. This seems in general to be nearly coincident with the direction of the dipping needle.

Thus at Cape Thorsden (7) in 1882-1883 the mean of a considerable number of observations made the angle between the two directions only 1° 7′, the magnetic inclination being 80° 35′, whilst the coronal centre had an altitude of 79° 55′ and lay somewhat to the west of the magnetic meridian. Even smaller mean values have been found for the angle between the auroral and magnetic “zeniths”—as the two directions have been called—e.g. 0° 50′ at Bossekop (16) in 1838-1839, and 0° 7′ at Treurenberg (17) (79° 55′ N. lat., 16° 51′ E. long.) in 1899-1900.

15. Relations to Magnetic Storms.—That there is an intimate connexion between aurora when visible in temperate latitudes and terrestrial magnetism is hardly open to doubt. A bright aurora visible over a large part of Europe seems always accompanied by a magnetic storm and earth currents, and the largest magnetic storms and the most conspicuous auroral displays have occurred simultaneously. Noteworthy examples are afforded by the auroras and magnetic storms of August 28-29 and September 1-2, 1859; February 4, 1872; February 13-14 and August 12, 1892; September 9, 1898; and October 31, 1903. On some of these occasions aurora was brilliant in both the northern and southern hemispheres, whilst magnetic disturbances were experienced the whole world over. In high latitudes, however, where both auroras and magnetic storms are most numerous, the connexion between them is much less uniform. Arctic observers, both Danish and British, have repeatedly reported displays of aurora unaccompanied by any special magnetic disturbance. This has been more especially the case when the auroral light has been of a diffused character, showing only minor variability. When there has been much apparent movement, and brilliant changes of colour in the aurora, magnetic disturbance has nearly always accompanied it. In the Arctic, auroral displays seem sometimes to be very local, and this may be the explanation. On the other hand, Arctic observers have reported an apparent connexion of a particularly definite character. According to Paulsen (18), during the Ryder expedition in 1891-1892, the following phenomenon was seen at least twenty times by Lieut. Vedel at Scoresby Sound (70° 27′ N. lat., 26° 10′ W. long.). An auroral curtain travelling with considerable velocity would approach from the south, pass right overhead and retire to the north. As the curtain approached, the compass needle always deviated to the west, oscillated as the curtain passed the zenith, and then deviated to the east. The behaviour of the needle, as Paulsen points out, is exactly what it should be if the space occupied by the auroral curtain were traversed by electric currents directed upwards from the ground. The Danish observers at Tasiusak (10) in 1898-1899 observed this phenomenon occasionally in a slightly altered form. At Tasiusak the auroral curtain after reaching the zenith usually retired in the direction from which it had come. The direction in which the compass needle deviated was west or east, according as the curtain approached from the south or the north; as the curtain retired the deviation eventually diminished.

Kr. Birkeland (19). who has made a special study of magnetic disturbances in the Arctic, proceeding on the hypothesis that they arise from electric currents in the atmosphere, and who has thence attempted to deduce the position and intensity of these currents, asserts that whilst in the case of many storms the data were insufficient, when it was possible to fix the position of the mean line of flow of the hypothetical current relatively to an auroral arc, he invariably found the directions coincident or nearly so.

16. In the northern hemisphere to the south of the zone of greatest frequency, the part of the sky in which aurora most generally appears is the magnetic north. In higher latitudes auroras are most often seen in the south. The relative frequency in the two positions seems to vary with the hour, the type of aurora, probably with the season of the year, and possibly with the position of the year in the sun-spot cycle.