Page:Handbook of Meteorology.djvu/123

  in the photosphere, or envelope of the sun, are coincident with magnetic storms and earth currents leads to the belief that solar influences at times are factors in atmospheric electricity. It is not safe to infer, that because of this fact, the electricity of the earth and its atmosphere are derived from the sun. Practically all evidence is contrary to such an assumption; nevertheless, there seems no reason to doubt that high-frequency waves generated in the sun reach the earth.

The phenomenon known as the aurora borealis {aurora polaris), more commonly called “northern lights,” is most frequently observed during great disturbances in the sun’s photosphere. But it is by no means certain that the display, which is electrical, is due to solar causes. The belief that the aurora is of solar cause, however, is held by many physicists.

The height of the aurora above the earth does not vary much from 60 miles. It is rarely visible in the latitude of New Orleans, occasionally in the latitude of New York, and rather more frequently in the latitude of Quebec; its maximum frequency is in the latitude of Norway and the southern part of Alaska.

The time of frequency varies. At Hammerfest, Norway, it is not visible during the summer months, presumably because of daylight. In New York, the spring and fall months are the periods of greatest frequency. Records from 1764 show that auroras are much more frequent during the periods in which sun spots are most frequent; this is one reason why the aurora is thought to be due to solar influence.

The work of the observer is to watch carefully and to note faithfully whatever is visible. Information is desired concerning the position, direction and extent of the arch, if one appears—otherwise the position of the patch or patches of light. It is desirable to know whether the arch takes the form of a curtain, a luminous band, or a corona. It is also desirable to note whether the light occurs in rays with dark spaces between them, or is a diffuse illumination without definite outlines, or takes the form of dancing streaks of light, changing rapidly in color, form, and intensity. When possible, it is well to compare the aurora with illustrations in any known publication, especially with those in the Encyclopaedia Britannica.