摘要：The day-to-day variation of the geomagnetic daily variation has been examined using hourly data of the Z component at Chichijima (geographic 27.15°N, 142.30°E; geomagnetic 18.11°N) in the northwestern Pacific Ocean. Principal component analysis is applied to the data from 1970 to 1992, and 2-day, 3-day, and 5-6-day oscillations were extracted from the day-to-day variation of the daily profile. The temporal variations of both the waveform and amplitude of each oscillation were clarified. The fluctuation associated with any of these oscillations is confined to the sunlit hours, which infers the oscillations are originated from ionospheric currents. Although the amplitudes of the oscillations are variable, they seem to be present almost continuously. Each oscillation is expressed primarily by the periodic increase and decrease of the amplitude of the quiet time daily variation, and the deviation from the normal level statistically amounts to 12% (3-day oscillation), 15% (5–cillation), and 17% (2-day oscillation) of the mean amplitude of the daily variation. There are cases, however, for which the magnitude is considerably amplified during a short duration. There also exist the oscillations that show the three periods in the pattern of the daily variation as well as in intensity. The result of analysis of day-to-day variations at other four stations indicates that the oscillations are phenomena having some latitudinal extent across the geomagnetic equator.
