Thunderstorms and the Earth's Magnetism

Abstract

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 variations due to thunderstorms visiting near the Kakioka Magnetic Observatory 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 other kinds of thunderstorms. 212 thunder storms are obtained in the three summer months of June, July and August in the course of 1924-1933. If the mean hourly values of each element of the earth's magnetism of these thunderstorm days and those of the total summer days are designated by K_s and K₀ respe_??_ively, the difference ΔK_s=K_s-K₀ will be responsible for the investigation of the effects of thunderstorms. The results thus obtained are as follow.
(1) The mean daily range of the horizontal intensity on thunderstorms days is almost equal to that of total period, for example, 49.O^γ for the former and 49.1^γ for the latter in ten years' mean, indicating that the activity of the earth's magnetism on thunderstorm days is normal.
(2) Various kinds of Statistics show the similar type of the diurnal variation of ΔK_s which is a single wave with a maximum between 16^h and 18^h in local time, while the diurnal variation of the horizontal intensity H has a maximum at 14^h, 13^h for the declination D and 5^h for the vertical intensity V.
(3) The diurnal variation of ΔK_s of H, or 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 maximum time of (P), or H and D, when the thunderstorm passes overhead. It is reasonably expected that thunderstorm effects upon the earth's magnetism become larger with increasing (P).
(4) From the different periods of years, the absolute amplitudes of the diurnal variation of ΔK_s vary in the range of 1^γ_??_3^γ for H, 2^γ_??_5^γ for V and 0.'2_??_0.'4 for D.
(5) The absolute amplitudes of ΔK_s for the sun-spots maximum years of 1926-1930 is 2 or 3 times larger than that of minimum five years of 1924, 1925, and 1931-1933.
(6) Each horizontal component of ΔK_s for different groups of thunderstorms are almost perpendicularly to the direction of thunderstorms most frequently observed at this observatory; that is, in the other words, if the magnetic field of ΔK_s is derived from the earth-current flowing approximately uniformly around the observatory, the current flows into, or out of the thunderclouds.
(7) From ten years' mean, for example, the above current flows into the cloud. It is, therefore, resulted that the earth has gained an excess of negative charges from the thunderstorm in the course of ten years, and so-called Wilsontype of thunderstorm has been frequently observed, provided that the quantity of electric charges of the individual thunderstorm is approximately equal. Under reasonable assumptions, the order of magnitude of the current flowing into the clouds is estimated as 0.2amps./km², which is the same order obtained B. F. J. Schonland and F. J. W. Whipple from their atmospheric electric researches.