The same phenomenon appears at Jan Mayen especially in November, December and January, and it is the normal state of matters in temperate latitudes, where the frequency is usually greatest between 8 and 10 p.m. An excess of evening over morning occurrences is also the rule, and it is not infrequently more pronounced than in Table III. Thus at Tasiusak (65° 37′ N. lat., 37° 33′ W. long.) the Danish Arctic Expedition (10) of 1904 found seventy-five out of every hundred occurrences to take place before midnight.
Table III.—Diurnal Variation.
| Hour. | Dec. | Nov. and Jan. | Feb., March, Sept. and Oct. |
Sept. to March (N. Lat.). March to Sept. (S. Lat.). | |||||
| A | B | A | B | A | B | A | B | C | |
| 1 2 3 4 5 6 7 8 9 10 11 Noon 1 2 3 4 5 6 7 8 9 10 11 Midnight |
14 10 9 10 13 11 9 5 7 10 9 10 10 14 18 16 12 14 16 15 14 12 10 9 |
7 6 4 5 5 3 2 1 2 0 0 0 0 0 1 7 11 10 13 12 15 15 12 9 |
14 15 15 21 20 15 13 6 9 5 6 4 6 10 20 19 22 21 23 22 18 19 18 13 |
8 6 5 7 3 4 3 1 0 0 0 0 0 0 3 7 10 16 16 18 17 15 17 11 |
27 20 15 14 10 2 1 0 0 0 0 0 0 0 0 1 5 8 20 24 27 31 33 28 |
23 25 21 18 10 3 2 0 0 0 0 0 0 0 0 1 2 5 9 24 28 25 26 22 |
55 45 39 45 43 28 23 11 16 15 15 14 16 24 38 36 39 43 59 61 59 62 61 50 |
38 37 30 30 18 10 7 2 2 0 0 0 0 0 4 15 23 31 38 54 60 55 55 42 |
24 23 10 4 2 1 0 0 0 0 0 0 0 0 0 0 3 3 14 25 31 29 26 26 |
| Totals | 277 | 140 | 354 | 167 | 266 | 244 | 897 | 551 | 221 |
| Percentages— Forenoon Afternoon |
42 58 |
28 72 |
42 58 |
25 75 |
39 61 |
46 54 |
41 59 |
35 65 |
35 65 |
8. The preceding remarks relate to auroras as a whole; the different forms differ considerably in their diurnal variation. Arcs, bands and, generally speaking, the more regular and persistent forms, show their greatest frequencies earlier in the night than rays or patches. Table IV. shows the percentages of e. (evening) and m. (morning) occurrences of the principal forms as recorded by the Arctic observers at Cape Thorsden, Jan Mayen and Tasiusak.
Table IV.
| Arcs. | Bands. | Rays. | Patches. | |||||
Cape Thorsden Jan Mayen Tasiusak |
e. | m. | e. | m. | e. | m. | e. | m. |
| 76 78 85 |
24 22 15 |
66 68 85 |
34 32 15 |
52 60 65 |
48 40 35 |
51 60 62 |
49 40 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:—
| Years of | 1764–1872. | 1764–1817. | 1818–1872. | |||
| Spots. | Auroras. | Spots. | Auroras. | Spots. | Auroras. | |
| Many sun-spots Few sun-spots |
93·4 13·4 |
99·9 61·5 |
86·7 13·6 |
70·7 51·6 |
100·1 13·1 |
129·1 71·3 |
