Abstract The parallelism in the variations of the geomagnetic horizontal component ( H) and vertical component ( Z), so obvious in the mangetograms of Ibadan, has been investigated. During day-time when the effect of the electrojet is dominant, H and Z vary proportionately. Numerous indentations of varying period and amplitude are a prominent feature of day-time variation curves of H and Z at Ibadan. Six hundred of these indentations have been measured and analysed. The indentations attain their maximal values simultaneously in H and Z. Within the limits of experimental error, in 86 per cent of the cases during the day and 54 per cent of those during the night, the indentations also begin and end simultaneously in H and Z. The ratio r of the amplitude of an indentation in H to the amplitude of the same indentation in Z exhibits a remarkable diurnal variation, being constant for most of the day-time. The average value is 2·114 ± 0·021 during daytime and 4·112 ± 0·095 during the night. The ratio r is surprisingly independent of magnetic disturbance but decreases with increase in the period of duration T. For indentations of short periods T (e.g. not exceeding 15 min) the ratio r equals the ratio R of the diurnal range of H to the range of Z on undisturbed days. It is suggested that the indentations are caused by fluctuations in the quantity and quality of ionizing agents from the sun and that the decrease of r with increase in T is associated with vertical movements of the E-layer of the ionosphere (or the S q -layer).