THE aurora borealis is produced by the action of electric radiation from outer space upon the upper atmosphere. Height measurements have shown that such radiation usually penetrates to a height of 100-105 km. Most auroral forms have their maximum light intensity at a height of 10-20 km. from the bottom edge, and by far the greater part of the light emitted from the aurora comes from a height interval of 100-130 km. The upper limit varies very much. Usually we can only follow an aurora up to an altitude of 130-150 km., but the ray-forms may be seen much higher. The greatest altitude at which isolated auroral “rays” may appear seems to increase towards lower latitudes. Near the auroral zone no rays are observed higher than 300-350 km. At Christiania, however, Stormer finds that isolated rays may reach altitudes of 750 km.
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Il Nuovo Cimento (1928)