Auroras Sentence Examples
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.
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).
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.
There has been at least one 33-year period during which the mean value of sun-spot frequency has been exceptionally low, and, as we shall see, there was a corresponding remarkable scarcity of auroras.
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.
AdvertisementIn higher latitudes auroras are most often seen in the south.
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.
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.
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.
AdvertisementThe magnetic disturbances can also dump particles from space into the upper air, where they cause Auroras.
Charged particles and energy, focused into the zone surrounding the magnetic poles, provide key scientific signals as well as spectacular Auroras.
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.
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