Semiannual Component of the Earth's Magnetic Field & Its Generating Mechanism

Abstract

The nature of the semiannual oscillation in the low-latitude geomagnetic field has been examined by a new method due to the author [<i>Planet Space Sci., </i> (1972),(in press], Analyses of extensive time series of horizontal intensity observed at Honolulu, Alibag and San Juan over a period of about 45 years establish that in low latitudes the semiannual component assumes largest magnitudes at the same local time twice a day, first between 0700 and 0900 hrs local time (LT) and again in the evening-night sector. During a brief interval centred around local noon the semiannual oscillation in the field practically vanishes. Power spectra computed from the series of mean monthly horizontal intensity, derived from observations on all days and those derived from five international quiet days, suggest a two-component generating mechanism. A major part of the semiannual variation appears to arise from modulation of the field by disturbance. The existence of an appreciable increase in power density around 0800 hrs LT in the spectra derived from quiet day observations suggests a small but significant component of ionospheric origin.