Quiet day variation of geomagnetic H-field at low latitudes.

The paper discusses the daily variation of the geomagnetic H-field averaged over the Five International Quiet days, Sq(H), at low latitude stations. The Sq(H) at a station near the dip equator during a low sunspot year represents an increase from about 0600 LT to 1600 LT and an almost constant value from 1600 LT throughout the night time. During a high sunspot year the H-field continues to decrease from noon to evening and through the night time up to sunrise, there being no period in the night when the field remains constant.The harmonic analysis of Sq(H) curves shows that the local time of maximum of the diurnal as well as the semidiurnal wave advances towards local noon with the increase of the average sunspot number. During minimum sunspot years the daily maximum in H occurs almost at 1100 LT while during maximum sunspot years the maximum occurs at about 1200 LT.The amplitudes (C1) of the diurnal and (C2) of the semidiurnal waves are shown to increase linearly with sunspot number. For any of the stations, the rate of increase of the diurnal component with sunspot number is about 2.5 times which is remarkably larger than the corresponding rate of about 1.8 times for the semidiurnal component, such that the ratio C1/C2 at the low latitude stations is about 1.7 in 1964 and about 2.0 in 1959.The monthly mean daily range, rSq(H), shows a predominant semiannual variation with equinoxial peaks at the equatorial stations and a predominant annual variation with summer maximum at the non-equatorial station Alibag. A linear increase of the annual mean daily range with Zurich mean sunspot number Rz is established, the rate of increase being larger towards the equator.The dependence of the diurnal range on the sunspot number is due to enhanced E-region ionization with increasing sunspot number. The delay in the time of diurnal maximum with increasing Rz, the semiannual variation of rSq(H) at equatorial stations and the annual variation of rSq(H) at nonequatorial stations are suggested to be due to the effects of E-region electric fields.