Universal Time Variations in the ap and Dst Indices and Their Possible Cause

The universal time (UT) variation in the ap index for the years 1932-1956 and 1957-1986 is analyzed. Results are compared with the UT variation in the Dst index and with that in the rate of energy input into the ring current estimated from the Dst index. It is found that the UT variation of the ap index does not agree with that of the Dst index, but is in agreement with the UT variation in the rate of energy input into the ring current. Results on the seasonal variation of the ap index indicate that the UT variation has mostly a minimum around 1030 UT for different seasons, though the average values have features similar to the well-known semiannual variation in geomagnetic activity. The averaged ap values and the numbers of events of the ap greater than 30, 50, or 100 reach a minimum around 1030 UT or during the UT time interval 0900-1200. These values are anti-correlated with the UT variation of the magnetic flux that would occupy the nightside auroral oval approximated by offset circles in corrected geomagnetic coordinate system. The modulation of E × B drift speed in the magnetosphere by the UT variation of the oval magnetic flux could be the source of the UT variation in the ap and the Dst index.