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.

Pulsating describes a low‐intensity aurora that undergoes rapid alternating increases and decreases in luminosity. Extensive new data available from ground‐based low‐light‐level television cameras and satellite scanners have allowed a detailed study of the pulsating aurora phenomenon. Intensity variations in pulsating auroras may be repetitive, quasi‐periodic, or occasionally periodic with a time scale ranging from less than 1 s to several tens of seconds. The maximum intensity does not exceed approximately 10 kR in N2+ 1 NG (first negative group) 4278 Å, and a background luminosity is often observed in association with the pulsating forms. Pulsations occur in auroral arcs, arc segments, and patches of fixed and variable area. The temporal and spatial characteristics are highly variable over a broad and continuous spectrum; rapid changes from one set of characteristics to another frequently occur, as do reversible changes from pulsating to nonpulsating auroras. Diffuse, slowly pulsating arcs occur in the evening sector immediately before and after the passage of the westward traveling surge. In the midnight sector, both arc segments and patches occupy a broad region behind the westward traveling surge. Poleward stretching torchlike structures containing pulsating patches and arc segments are often observed at the poleward boundary of the diffuse auroral oval in the midnight sector. A narrow region of pulsating arcs and eastward drifting patches occupies the morning sector, the arcs generally being located at the poleward boundary. A 3 ± 1 Hz modulation appears in more than 50% of all pulsating auroras in the midnight and morning sectors, the amplitude of modulation ranging up to 20%.

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