Pulsations in an ionized region at height of 650–800 KM during the appearance of giant pulsations in the geomagnetic records

During recent years the occurrence of some very regular pulsations in magnetograms, usually called “giant micropulsations,” has aroused considerable interest. During the International Polar Year 1932–33 and the succeeding years they were frequently recorded on the magnetograms at the Tromsö Observatory, but in the past two years an astonishingly low number of pulsations were recorded in the northern part of Scandinavia and in Iceland. For the first time since March 1938, giant pulsations of the normal character but with somewhat small amplitudes, were recorded on December 28, 1938 between 2 h 40 m and 4 h 10 m MET. The waves were clearly developed in D and V , but were not perceptible in the H ‐curve, with maximum double amplitudes in D and V of 3.3 and 2.5 gammas, respectively.