. Compendium of meteorology. Meteorology. 350 THE UPPER ATMOSPHERE limit precisely for all forms. In Fig. 3 the lower limits of all forms of polar aurorae are shown. The measure- ments are the result of three series of observations. 150 140 inl30 tr 1^120 iii ;IIO J :ioo 90 80, â ! \ HARANG iâ \ ST0 , (19 1 \ \ \/EGARD 1 AND ^ TONSBERG _k,(l929/30) X 13) ^ (1913/14) â\ k \ --- -> V â ^ -= " ( ⺠^ ' t^ y y r 0 50 100 200 0 50 100 200 0 50 100 150 Fig. 3.âHeight statistics. Lower limits of polar aurorae, all forms. (The number of points appearing in 2-km height inter- vals are used as a
. Compendium of meteorology. Meteorology. 350 THE UPPER ATMOSPHERE limit precisely for all forms. In Fig. 3 the lower limits of all forms of polar aurorae are shown. The measure- ments are the result of three series of observations. 150 140 inl30 tr 1^120 iii ;IIO J :ioo 90 80, â ! \ HARANG iâ \ ST0 , (19 1 \ \ \/EGARD 1 AND ^ TONSBERG _k,(l929/30) X 13) ^ (1913/14) â\ k \ --- -> V â ^ -= " ( ⺠^ ' t^ y y r 0 50 100 200 0 50 100 200 0 50 100 150 Fig. 3.âHeight statistics. Lower limits of polar aurorae, all forms. (The number of points appearing in 2-km height inter- vals are used as abscissae.) The lower limits lie between 80 and 140 km, with a pronounced maximum at about 106 km. Stormer has made an extensive study of the lower limits of different forms and has also compared the lower limits of the polar aurorae and aurorae appearing at more southerly latitudes. There appear to be only small systematic changes in the lower limits for the various types and latitudes. There is a wide range of variation in the vertical extent of the luminosity. In polar regions the rays extend to heights up to 200-250 km. In lower latitudes the vertical extent may increase to as much as 800- 1000 km. Aurorae appear only at these latitudes during very intense magnetic storms, and radio-echo measure- ments indicate that there is a considerable increase in the reflection heights of the ionosphere during the storm phase. The increases in vertical extent of the aurorae at lower latitudes may therefore be due partly to the special conditions of the ionosphere during strong magnetic storms. Sunlit Aurome. Height measurements in arctic re- gions near the auroral zone have shown that the rays and draperies in the dark atmosphere very seldom attain heights over 250-300 km. Stormer has shown, however, that faint rays, often of a grayish color, appear-. FiG. 4.âAurorae situated in the border region between the dark and sunlit atmosphere. The horizontal line indicates the shadow li
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