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.
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.
The preceding remarks relate to auroras as a whole; the different forms differ considerably in their diurnal variation.
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.
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.
The percentage of auroras seen in the south thus appears decidedly below the mean.
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.
Sun-spots 56.4 Auroras 77.5 1844-1876..
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.
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.