Near 1 cps the natural background consists of a comparatively steady, low‐level, ‘white’ continuum relieved occasionally by the intrusion of either regular or impulsive geomagnetic signals, as well as by the normal occurrence of sferics. The amplitude and frequency of the intrusions depend on latitude. Characteristic, regularly repeating signal‐reinforcement patterns are sometimes observed in austral and boreal regions. Such patterns are displaced in time at conjugate points by one‐half their repetitive period.
Micropulsation fields measured simultaneously at pairs of stations are compared in order to describe their recognizable geographical extent and the perturbations due to local terrain. Some characteristics of coastal and inland sites are shown. The micropulsation field gradients were too small to be measured over a homogeneous earth but were clearly evident near the seacoast. Estimates are made of the extent of the anomaly at the seacoast, and of the fraction of the signal accounted for by geology of the site. Separations between sites range from 1 km up to 1750 km. The distribution of the micropulsation amplitude in different directions and its dependence on period are shown for each site. Its correlation with Kp is also shown.The data refer only to regular micropulsations of period from about 500 seconds down to 2 seconds, observed simultaneously at pairs of stations situated in mid-latitudes.
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