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Short-Period Geomagnetic Micropulsations with Period of about 1 Second in the Middle and Low Latitudes
Since the IQSY period, continuous observations of short-period geomagnetic and earth-current micropulsations have been carried out at two observatories, Memambetsu (43°55'N，144°12'E) and Kanoya (31°25'N，130°53'E), in addition to the usual ordinary and rapid-run observations. In the present paper, some characteristics of geomagnetic micropulsations with shortest periods ranging from 0.2 sec to 5 sec defined as pc-1 by IAGA-IUGG nomenclature are investigated, mainly statistically. It seems that the continuous micropulsation may be classified into two or more classes, the most peculiar one being PP, firstly found by Troitskaya's pioneering work (1957). It shows well known microstructure on the frequency-time display, while for the other classes the microstructures are not so evident.
It seems that the diurnal variation of the occurrence frequency and the mean period，the seasonal shift of the time of maximum occurrence as well as the Kp-dependency of the pulsation are certainly due to the ionospheric attenuation. Existence of the latitude dependency of the amplitude and the structure doubling at the lower-latitude station means the propagation along a path above the ionosphere and also some attenuation ascribed to the penetration through the ionosphere. The difference of the occurrence between the auroral or subauroral zone and the low latitudes will be ascribed mainly to the above mentioned two effects.
As the result, it is deduced that the micropulsation is hydromagnetic oscillation of the upper atmospheric origin closely related with magnetic storms. Particularly, the fine-structured micropulsation will be understood as the hydromagnetic wave bouncing along the magnetic line of force between the conjugate points with a speed of several hundred kilometers per second. The wave perhaps propagates from the auroral or subauroral zone to the lower latitudes almost simultaneously over a wide region through the paths above the ionosphere.
In addition to the continuous micropulsations, a brief description is made on some characteristics of short-period irregular micropulsations having the same range of periods (≦10sec). Micropulsations may be interpreted as the higher frequency components of either pi-1 or pi-2 with longer periods associated with a geomagnetic storm or bay disturbance.
With examples, the occurrence characteristics of the micropulsations accompanied by geomagnetic storms are shown.
Outline of the measuring apparatus of the pulsations is described.
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