On Rapid Periodic Variations of Terrestrial Magnetism

The period of the magnetic pulsations has no sharply defined value, varying from about 20 seconds to nearly 1 hour. Nor is it exactly constant even in a coherent train. 2. During the day time, waves of 0.5-1 minute periods predominate, whereas during the night hours longer periods 1.5-2.5 minutes are most frequent. 3. A periodicity of 25-30 days is suspected in the daily frequency of the pulsations. 4. The vertical component of the waves is a reduced reproduction of the NS-component except the phase retardation. The shorter the period, the more remarkable is the reduction of the amplitude as well as the phase retardation. 5. The azimuth of the linearly pulsating magnetic field undergoes a remarkable diurnal variation, showing maximum deviations from the meridian a few hours before noon and midnight. 6. The disturbing field is generally more or less rotatory. The sense of rotation shows a semidiurnal variation. The clock-wise rotation is most frequent during the hours between sunrise and noon, as well as between sunset and midnight, while the opposite rotation falls most frequently in the remaining hours. 7. The observed results may probably be explained by the fluctuations of the horizontal electric current existing in the upper atmosphere, and causing the diurnal variation of the terrestrial magnetism. 8. Two diverging lines of theoretical considerations intended for the explanation of the phenomena in question, are given; the one based on the assumption of the simultaneity of the phenomena in a wide area, and the other on the assumption of a progressive nature of the pulsations, though some evidences at hand seems to speak rather strongly against the latter assumption. The results of the discussions turn out rather favourable for the hypothesis of simultaneous disturbances than for that of progressive waves. If the simultaneity be universally established, the phenomena may probably be accounted for by the fluctuation of the atmospheric current, in its intensity as well as in its location. The fluctuation must then very probably be attributed to the more or less vertical oscillation of limited portions of the upper atmosphere. If such be actually the case, we have in the phenomena of the magnetic pulsations a very valuable clue for studying the physical conditions of the upper atmosphere unattainable by the usual means, and then, it may be hoped, for following the hourly or daily changes occurring in the remotest part of our atmosphere.