Statistics of heights of various auroral forms from Southern Norway: Second communication

(1) Introduction—In a short paper published in this Journal in December, 1946, I gave the results of 12,330 measurements of height of polar aurora observed in Southern Norway during 1911–44. The present paper deals with the statistical work undertaken since that publication. If we compare the diagram of height (Fig. 1) in the above‐mentioned paper with the corresponding one from my expedition to Northern Norway in 1913 [see 1 of “References” at end of this paper], one of the essential differences is that the aurora in Southern Norway occur at much greater heights than those in Northern Norway. There are two reasons for this. First, certain aurora occurring in the Earth's shadow during years of great solar activity as rays and red diffuse surfaces [2], often reach great heights (from 300 to 700 km) and are seldom observed in Northern Norway. Second, the auroral rays in that part of the atmosphere lying in sunshine—which have only once been measured in Northern Norway [3]—have a position very much higher than aurora in shadow, as already pointed out in my paper on sunlit auroral rays published in 1929 [4]. This difference is clearly seen in Figure 1, where the 12,330 heights from my first paper of 1946 are split into heights of aurora in shadow (left) and aurora partly or completely in sunlight (right) [5]. For each kilometer the measured heights are distributed as points along the corresponding horizontal line at approximately equal distances. The same remark applies to the other Figures in this paper.