A study of the average daily variation of the AL index by months for the years 1966--1974, inclusive, shows that near the winter solstice there is a diurnal oscillation with an amplitude of 45% of the mean. The amplitude diminishes to about 10% of the mean near the summer solstice. The maximum and minimum of the winter oscillation occur at the hours when the stations contributing to the AL index have their maximum and minimum latitudinal coverage, respectively. In an attempt to interpret these results, a physical model based upon observational data concerning auroral electron precipitation and ionospheric conductivity has been developed. It is postulated that electron precipitation is the dominant cause of ionization responsible for the conduction of the westward electrojet throughout the year. In the fall and winter months the electrojet is confined to the band of precipitation, while in the spring and summer months the contributions of solar ionization permit a latitudinal spreading of the currents. The model predictions are consistent with the observed pattern of change of the average value of AL for the individual AE stations between the winter and summer solstices. It is also consistent with the change in the frequency with which the stations contribute to the determinations of the AL index between the solstices. The results indicate that AL is best suited for estimates of the westward electrojet on a continuous basis during the spring and summer months. An observed increase in the average value of AL with decreasing corrected magnetic latitude is interpreted as being due to larger electrojet currents associated with larger magnetic disturbances that produce an equatorward displacement of the polar cap boundary.