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Thunderstorms and Earth's magnetism
The investigation of the earth's magnetic variation produced by thunderstorms is hitherto confined to its impulsive changes due to lightning discharges. A statistical investigation now performed is to find a rather slow or stationary variation due to the thunderstorm visiting near the Kakioka Magnetic Observatory, Japan, in the summer months. Most of them are heat-thunderstorms and their paths of travel are more or less regular, from the north-western part of the Kwanto-district to the east, or south-east, spreading over the ocean. Considering various modes of influencing mechanism, possible magnetic variations in the present case will be more conspicuous than those due to the other kinds of thunderstorms. 212 thunderstorms are obtained in the three summer months of June, July, and August in the course of 1924-1933. If the mean hourly value of each element of the earth's magnetism on these thunderstorm days and those values of the total summer period are designated by Ks and Ko respectively, the difference ΔKs=Ks-Ko will be responsible for the investigation on the effect of thunderstorms. The results thus obtained are tabulated as follows:
(1)The mean daily range of the normal horizontal magnetic intensity on thunderstorm days in almost equal to that of total period, for example, 49.0γ for former and 49.1 γ for the latter in the ten years' mean, indicating that the activity of the normal earth's magnetism on thunderstorm days is normal.
(2)Various kinds of statistics show the similar type of the diurnal variation of ΔKs which is a single wave with a max. between 16h and 18h in local time, while that of the horizontal intensity(H) has a max. at 14h, 13h for the declination(D) and 5h for the vertical intensity(V).
(3)The diurnal variation of ΔKs of H, of D is very similar to that of the frequency curve (P) of thunderstorms observed at the observatory, while V has generally the same tendency as the other elements, but markedly decreases near the max. time of (P), or H and D, when the thunderstorm passes over the head. It is reasonably expected that thunderstorm effects upon the earth's magnetism become larger with increasing (P), and each element gains an excess of positive or negative influences.
(4)From the different periods of years, the absolute amplitude of the diurnal variation of ΔKs vary in the range of 1γ-3γ for H, 2γ-5γ for V and 0.2'-0.4' for D.
(5) The absolute amplitude of ΔKs for the sun-spots max years of 1926-1930 is 2 or 3 times larger than that of min. five years of 1924,1925, and 1931-1933.
(6)Each horizontal components of ΔKs for different groups of thunderstorms is perpendicular to the direction of thunderstorms most frequently observed at this observatory; in other words, if the magnetic field of ΔKs is derived from the earth-current flowing approximately uniformly around the observatory, the very current flows into the thundercloud, or flows out it.
(7)For the ten years' mean, for example, the current flows into the cloud. It is, therefore, resulted that thunderstorms of so-called Willson-type have been frequently observed and the earth has gained an excess of negative charge from the thunderstorm in the course of ten years. Under reasonable assumptions, the order of magnitude of the current flowing into the cloud is estimated as 0.3 Amp./km2, which is the same order as that obtained by B.F.J. Schouland and F.J.W. Whipple from their atmospheric electric researches.
(8)If the electric current in the upper atmosphere is taken in place of the above earth-current, positive charges escaping from the cloud will take the role.
(9)The mountains ranging from the north to south about 6 km. west to the observatory seem to disturb the distribution of the earth-current affecting upon the earth's magnetism.
(10)The earth's magnetism at Toyohara, South Sakhalien, is independent of the thunderstorm observed at Kakioka.
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