Points on the same curve are supposed to have the same average number of auroras in the year, and this average number is shown adjacent to the curve.
In other words, auroras are much more numerous in the southern parts of Canada and in the United States than in the same latitudes of Europe.
Starting at the extreme north, we have a simple period with a well-marked maximum at midwinter, and no auroras during several months at midsummer.
In southern Europe - where, however, auroras are too few to give smooth results in a limited number of years - in southern Canada, and in the United States, the difference between the winter and summer months is much reduced.
The total number of auroras in the year is taken as 100, and t denotes the time, in months, that has elapsed since the middle of January.
Putting t=o, 1, &c., in succession, we get the percentages of the total number of auroras which occur in January, February, and so on.
These show how the frequency of visible auroras diminished as cloud increased from o (sky quite clear) to 10 (sky wholly overcast).
Loo 82 57 46 8 Out of a total of 1714 hours during which the sky was wholly overcast the Swedish expedition saw auroras on 17, occurring on 14 separate days, whereas 226 hours of aurora would have occurred out of an equal number of hours with the sky quite clear.
Whilst daylight is the principal cause of the diurnal inequality, it is not the only cause, otherwise there would be as many auroras in the morning (forenoon) as in the evening (afternoon).
The preceding remarks relate to auroras as a whole; the different forms differ considerably in their diurnal variation.
Gives contemporaneous data for the frequency of sun-spots and of auroras seen in Scandinavia.
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 annual means derived from the whole group, and the two sub-groups, of years of many and few sun-spots are as follows: 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.
Certainly point to this conclusion, unless we are prepared to believe that auroras have increased enormously in number.
Sun-spots 56.4 Auroras 77.5 1844-1876..
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.
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.
In high latitudes, however, where both auroras and magnetic storms are most numerous, the connexion between them is much less uniform.
In higher latitudes auroras are most often seen in the south.
The conditions, however, as regards pressure and temperature under which the hypothetical discharges take place must vary greatly in different auroras, or even sometimes in different parts of the same aurora.
Further, auroras are often possessed of rapid motion, so that conceivably spectral lines may receive small displacements in accordance with Doppler's principle.
At Cape Thorsden (7) in 1882-1883 auroras as a whole were divided into those seen in the north and those seen in the south.
The percentage of auroras seen in the south thus appears decidedly below the mean.
Dividing the whole number of arcs, 156, whose angular velocities were measured into three numerically equal groups, according to their altitude, the following were the results in minutes of arc per second of time (or degrees per minute of time): - Each group contained auroras which appeared stationary.
Such extremely bright auroras seem very rare, however, even in the Arctic. There is a general tendency for both bands and rays to appear brightest at their lowest parts; arcs seldom appear as bright at their summits as nearer the horizon.