+-------+----------------------++-------+----------------------+
| | Frequency. || | Frequency. |
| Year. +----------------------++ Year. +----------------------+
| | Sun-spot. | Auroral. || | Sun-spot. | Auroral. |
+-------+-----------+----------++-------+-----------+----------+
| 1749 | 80.9 | 103 || 1829 | 67.0 | 93 |
| 1750 | 83.4 | 134 || 1830 | 71.0 | 132 |
| 1751 | 47.7 | 53 || 1831 | 47.8 | 89 |
| 1752 | 47.8 | 111 || 1832 | 27.5 | 54 |
| 1753 | 30.7 | 96 || 1833 | 8.5 | 79 |
| 1754 | 12.2 | 65 || 1834 | 13.2 | 81 |
| 1755 | 9.6 | 34 || 1835 | 56.9 | 58 |
| 1756 | 10.2 | 60 || 1836 | 121.5 | 98 |
| 1757 | 32.4 | 83 || 1837 | 138.3 | 137 |
| 1758 | 47.6 | 80 || 1838 | 103.2 | 159 |
| 1759 | 54.0 | 113 || 1839 | 85.8 | 165 |
| 1760 | 62.9 | 86 || 1840 | 63.2 | 82 |
| 1761 | 85.9 | 124 || 1841 | 36.8 | 75 |
| 1762 | 61.2 | 114 || 1842 | 24.2 | 91 |
| 1763 | 45.1 | 89 || 1843 | 10.7 | 66 |
| 1764 | 36.4 | 107 || 1844 | 15.0 | 81 |
| 1765 | 20.9 | 76 || 1845 | 40.1 | 26 |
| 1766 | 11.4 | 51 || 1846 | 61.5 | 50 |
| 1767 | 37.8 | 68 || 1847 | 98.5 | 63 |
| 1768 | 69.8 | 80 || 1848 | 124.3 | 107 |
| 1769 | 106.1 | 89 || 1849 | 95.9 | 131 |
| 1770 | 100.8 | 83 || 1850 | 66.5 | 95 |
| 1771 | 81.6 | 62 || 1851 | 64.5 | 60 |
| 1772 | 66.5 | 38 || 1852 | 54.2 | 92 |
| 1773 | 34.8 | 58 || 1853 | 39.0 | 65 |
| 1774 | 30.6 | 98 || 1854 | 20.6 | 64 |
| 1775 | 7.0 | 33 || 1855 | 6.7 | 49 |
| 1776 | 19.8 | 17 || 1856 | 4.3 | 46 |
| 1777 | 92.5 | 64 || 1857 | 22.8 | 38 |
| 1778 | 154.4 | 59 || 1858 | 54.8 | 88 |
| 1779 | 125.9 | 60 || 1859 | 93.8 | 131 |
| 1780 | 84.8 | 67 || 1860 | 95.7 | 119 |
| 1781 | 68.1 | 103 || 1861 | 77.2 | 127 |
| 1782 | 38.5 | 67 || 1862 | 59.1 | 135 |
| 1783 | 22.8 | 70 || 1863 | 44.0 | 135 |
| 1784 | 10.2 | 78 || 1864 | 47.0 | 124 |
| 1785 | 24.1 | 83 || 1865 | 30.5 | 119 |
| 1786 | 82.9 | 136 || 1866 | 16.3 | 130 |
| 1787 | 132.0 | 115 || 1867 | 7.3 | 127 |
| 1788 | 130.9 | 97 || 1868 | 37.3 | 144 |
| 1789 | 118.1 | 89 || 1869 | 73.9 | 160 |
| 1790 | 89.9 | 90 || 1870 | 139.1 | 195 |
| 1791 | 66.6 | 54 || 1871 | 111.2 | 185 |
| 1792 | 60.0 | 64 || 1872 | 101.7 | 200 |
| 1793 | 46.9 | 29 || 1873 | 66.3 | 189 |
| 1794 | 41.0 | 37 || 1874 | 44.7 | 158 |
| 1795 | 21.3 | 34 || 1875 | 17.1 | 133 |
| 1796 | 16.0 | 37 || 1876 | 11.3 | 137 |
| 1797 | 6.4 | 61 || 1877 | 12.3 | 126 |
| 1798 | 4.1 | 35 || 1878 | 3.4 | .. |
| 1799 | 6.8 | 28 || 1879 | 6.0 | .. |
| 1800 | 14.5 | 30 || 1880 | 32.3 | .. |
| 1801 | 34.0 | 34 || 1881 | 54.3 | .. |
| 1802 | 45.0 | 65 || 1882 | 59.7 | .. |
| 1803 | 43.1 | 73 || 1883 | 63.7 | .. |
| 1804 | 47.5 | 101 || 1884 | 63.5 | .. |
| 1805 | 42.2 | 85 || 1885 | 52.2 | .. |
| 1806 | 28.1 | 62 || 1886 | 25.4 | .. |
| 1807 | 10.1 | 42 || 1887 | 13.1 | .. |
| 1808 | 8.1 | 20 || 1888 | 6.8 | .. |
| 1809 | 2.5 | 20 || 1889 | 6.3 | .. |
| 1810 | 0.0 | 4 || 1890 | 7.1 | .. |
| 1811 | 1.4 | 13 || 1891 | 35.6 | .. |
| 1812 | 5.0 | 11 || 1892 | 73.0 | .. |
| 1813 | 12.2 | 18 || 1893 | 84.9 | .. |
| 1814 | 13.9 | 17 || 1894 | 78.0 | .. |
| 1815 | 35.4 | 10 || 1895 | 64.0 | .. |
| 1816 | 45.8 | 33 || 1896 | 41.8 | .. |
| 1817 | 41.1 | 60 || 1897 | 26.2 | .. |
| 1818 | 30.4 | 74 || 1898 | 26.7 | .. |
| 1819 | 23.9 | 43 || 1899 | 12.1 | .. |
| 1820 | 15.7 | 62 || 1900 | 9.5 | .. |
| 1821 | 6.6 | 37 || 1901 | 2.7 | .. |
| 1822 | 4.0 | 33 || 1902 | 5.0 | .. |
| 1823 | 1.8 | 13 || 1903 | 24.4 | .. |
| 1824 | 8.5 | 14 || 1904 | 42.0 | .. |
| 1825 | 16.6 | 40 || 1905 | 62.8 | .. |
| 1826 | 36.3 | 58 || 1906 | 53.8 | .. |
| 1827 | 49.7 | 79 || 1907 | 62.0 | .. |
| 1828 | 62.5 | 60 || 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 deg. 12' W., or 11 deg. 27' to the W. of the magnetic meridian. As
to the azimuths in individual cases, 130 differed from the mean by
less than 10 deg., 118 by from 10 deg. to 20 deg., 82 by from 20 deg.
to 30 deg., 21 by from 30 deg. to 40 deg., 14 by from 40 deg. to 50
deg.; in six cases the departure exceeded 50 deg., and in one case it
exceeded 70 deg. 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 deg. W. of N. (or E. of S.), the mean derived from the 113
arcs observed between midnight and 6 A.M. was 47.8 deg. W. At Jan
Mayen (8) in 1882-1883 the mean azimuth of the summit of the arcs was
28.8 deg. W. of N., thus approaching much more closely to the magnetic
meridian 29.9 deg. W. As to individual azimuths, 113 lay within 10
deg. of the mean, 37 differed by from 10 deg. to 20 deg., 18 by from
20 deg. to 30 deg., 6 by from 30 deg. to 40 deg., whilst 6 differed by
over 40 deg. Azimuths were also measured at Jan Mayen for 338 auroral
bands, the mean being 22.0 deg. W., or 7.9 deg. to the east of the
magnetic meridian. Combining the results from arcs and bands,
Carlheim-Gyllenskold gives the "anomaly" of the auroral meridian at
Jan Mayen as 5.7 deg. E. At the British Polar station of 1882, Fort
Rae (62 deg. 23' N. lat., 115 deg. 44' W. long.), he makes it 15.7
deg. W. At Godthaab in 1882-1883 the auroral anomaly was, according to
Paulsen, 15.5 deg. E., the magnetic meridian lying 57.6 deg. 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 deg. 7', the magnetic inclination being 80 deg. 35', whilst the
coronal centre had an altitude of 79 deg. 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 deg. 50' at Bossekop (16) in
1838-1839, and 0 deg. 7' at Treurenberg (17) (79 deg. 55' N. lat., 16
deg. 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 deg. 27' N. lat., 26 deg. 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.
At Jan Mayen (8) in 1882-1883, out of 177 arcs whose position was
accurately determined, 44 were seen in the north, their summits
averaging 38.5 deg. above the northern horizon; 88 were seen in the
south, their average altitude above the southern horizon being 33.5
deg.; while 45 were in the zenith. At Tasiusak (10) in 1898-1899 the
magnetic directions of the principal types were noted separately. The
results are given in Table VI.