Lunar effect on the diurnal variation of the geomagnetic horizontal field near the magnetic equator

Abstract

Lunar effect on the diurnal variation of this geomagnetic horizontal field ( H) at Ibadan has been investigated using magnetograms of 28 months all in the season of Northern Winter. It has been found that lunar tide ( L) accounts for about 11 per cent of the daily range of H and its contribution to the departure of the hourly mean of H i.s about 14 per cent at 11 hr, 17 per cent at 13 hr, and 36 per cent at 10 hr local time at Ibadan. The asymmetry of the diurnal curve of H is found to be independent of L. A new method is described for estimating the intensity of the L-field at certain hours on individual days and this has shown that the ratio of intensities of L to S varies from hour to hour on the same day. The existence of “Big- L-days” reported by Bartels and Johnston (1940) is found to be doubtful. Evidence has been brought to show that if the existence of Big- L-days is accepted, then the existence of “Small-Todays” and “Inverse- L-days” also has to be accepted.