Lunar diurnal variation in Earth‐currents at Huancayo and Tucson

Abstract

The lunar diurnal variation in earth‐current flow at Huancayo and Tucson has been determined from the records for the year 1932, near the sunspot‐minimum. Using the method employed by Chapman for the magnetic elements, a definite lunar diurnal variation of markedly double period is found in both components of the potential‐gradient at both stations. Harmonic analyses of the mean curves show that at Huancayo the amplitude of the second harmonic is about one‐sixth that of the solar diurnal variation for the same year, while the amplitudes of the first, third, and fourth harmonics are negligibly small. At Tucson the amplitude of the second harmonic is about one‐fifth that of the solar diurnal variation for the northward component which is the principal component at that station.The manner in which the lunar diurnal variation changes with the phase of the Moon was also examined. For both components at Huancayo, curves constructed for the four main phases of the Moon show a marked increase in activity during daylight hours and a corresponding diminution during the night so that the individual curves are no longer semi‐diurnal. For Tucson the curves for the individual phases of the Moon, as determined from this single year's record are too irregular to show the changes definitely, but do indicate that they are less pronounced at this higher‐latitude station. As a check on this point the lunar diurnal variation at Tucson was redetermined by the method applied by Egedal to the data from Ebro. In this method the lunar diurnal variation is obtained from the records for the individual hours of the solar day. Using the hours from 12 to 24 the lunar diurnal variation found is in good agreement with that obtained by the usual method. Comparing the lunar diurnal variation obtained separately by this method during daylight, 12h to 18h, with that obtained during the night, 18h to 24h, there is found only a slight decrease in amplitude from day to night and very little change in phase. These results are in agreement with those obtained by the first method and also with the revised conclusions reached by Egedal from his studies of the Ebro records.