Auroral electrojet activity index AE and its universal time variations

An index, denoted by AE, is derived as a measure of global electrojet activity. The basic data used are 2.5-minute readings of the H trace in the standard magnetograms from seven auroral-zone observatories. The readings are referenced to a level determined for each observatory from quiet intervals. All the data from the seven observatories are then plotted against UT, and two envelopes are drawn to embrace all the points. The index AE at any epoch is defined by the distance (or separation) between the upper and lower envelopes at that epoch. When viewed as functions of UT the upper and lower envelopes themselves show development and decay of positive and negative variations. It is found that a positive excursion usually accompanies a negative (larger) variation, confirming the well known feature of polar disturbance. The AE index for a 6-day period, February 10–15, 1958, shows that polar disturbances statistically repeat with a time interval of about 4 hours, and that the average duration of their most active phase is a little more than 1 hour. It is pointed out that the average repetition time of 4 hours is comparable with that of the electron flux enhancements observed by Anderson et al. in the magnetosphere tail. It is suggested that polar disturbances are directly related to Anderson's ‘electron islands’ in the magnetosphere tail.