Diurnal-variation anomalies at Tucson

The seasonal changes in diurnal variation at Tucson are of unusual interest because of the location of the observatory in the transition-belt where the type of diurnal variation changes markedly during the year. Monthly hodograms of the diurnal variation in earth-current potential-gradient indicate that its changes are in general consistent with the recognized movement of the current-systems in the ionosphere northward and southward with the Sun. Two definite anomalies, less readily explained, are found to exist. The first is a marked increase in activity in January, appearing as a large increase in amplitude, and the second consists of a decrease and irregularity in the eastward component during March. Corresponding anomalies are found in the magnetic diurnal variation. In January, that of the total force, F, is increased to a value well above its mean winter level, with X, Y, and Z all showing approximately the same change in percentage. In March only X is abnormal and, like the eastward earth-current component, its diurnal variation is small and irregular. The winter anomaly might be accounted for by assuming the existence of a zone of unusually high conductivity in the ionosphere, into which the current-system passes at its southern-most position. The anomaly in March may be due to erratic shifting of the latitude of the current-center to the northward such as that deduced by Hasegawa from the records at Japanese stations.