Night-time geomagnetic variations at low latitudes

Using hourly values of the magnetic elements H, D and Z for 1964, 1965, their variation during night-time hours is examined from both their monthly means and from a previously used harmonic analysis method. The data set used represents quiet magnetic conditions. Consistent changes during the night are often found. Seasonal changes are also examined and it seems necessary to modify the Malin—Isikara hypothesis of a moving ring current by including a seasonal modulation of ring current strength with equinoctial maximum and a local time-varying component or partial ring current. The night-time D component shows considerable asymmetry between North and South hemispheres and this might be due to field-aligned current structure. There is a small amount of evidence for a night-time westward equatorial electrojet enhancement.