Aurora

A luminous display seen in the northern heavens in the northern hemisphere, where it is the Aurora Borealis, and seen in the southern heavens in the southern hemisphere, where it is called Aurora Australis, or indifferently for either, the Aurora Polar is. It takes the form of pale luminous bands, rays and curtains varying in color. Near the poles they are very numerous. A French commi ion observed 150 auroras in 200 days. Their height is variously estimated at from 90 to 460 miles; they are most frequent at the equinoxes and least so at the solstices. There is a secular variation also, they attain a maximum of occurrence every 11 years together with sun spots, with a minimum 5 or 6 years after the maximum. There is also a period of 60 years, coincident with disturbances in the earth's magnetism. Various attempts have been made to account for them. They have a constant direction of arc with reference to the magnetic meridian (q. v.) and act upon the magnetic needle; in high latitudes they affect telegraph circuits violently. There is a strong probability that they represent electric currents or discharges. De la Rive considers them due to electric discharges between the earth and atmosphere, which electricities are separated by the action of the sun in equatorial regions. According to Balfour Stewart, auroras and earth currents.(q. v.) may be regarded as secondary currents due to small but rapid changes in the earth's magnetism. The subject is very obscure. Stewart treats the earth as representing the magnetic core of an induction coil, the lower air is the dielectric, and the upper rarefied and therefore conducting atmosphere is the secondary coil. This makes the aurora a phenomenon of induced currents. Then the sun may be regarded as the instigator of the primary changes in the earth's lines of force representing the primary of an induction coil. [Transcriber's note: Solar wind, streams of electrons and protons, interacting with the earth's magnetic field causes aurora. Neither electrons (1897) nor protons (1920) were known in 1892. The Soviet satellite Luna first measured the solar wind in 1959. Even today increased understanding of solar and auroral phenomenon continues.] 54   STANDARD ELECTRICAL DICTIONARY.