Universal time variations of the auroral electrojet indices

Using the hourly mean AE indices for the past 20 years, amounting to a tota l of 175,296 hours, we examine how the longitudinal station gaps of the pre sent AE network affect the ability to monitor accurately the auroral electr ojets. The latitudinal shift of the auroral electrojet location with magnet ic activity also affects the reliability of the AE indices, These combined effects would result in pronounced universal time (UT) variations of the AE indices. By counting the number of occurrences recorded during the given r anges of activity, say every 100 and 200 nT for the AU and AL indices, resp ectively, for each hour of universal time, the UT variations of the two ind ices are examined separately. The result demonstrates clearly that they are strongly dependent upon UT. Furthermore, it is noted that the equatorward expansion of the auroral electrojets is more responsible for the UT variati on than are the longitudinal station gaps. For the range of the magnetic ac tivity levels examined in this study, i.e., 0 to 500 nT and 0 to -1000 nT f or the AU and AL indices, the centers of the eastward and westward electroj ets seem to be located within the latitudinal ranges of 71 degrees-65 degre es and 68 degrees-62 degrees, respectively. The seasonal change of ionosphe ric conductance also contributes to the UT variation, particularly that of the AL index. While maintaining a similar variation pattern, the amplitude of the variation increases during winter and decreases during summer. It in dicates that the UT variation of the AL index is more serious during winter than summer. With more RE stations being located within the former range t han the latter, it is easily understood why the AL index is more strongly d ependent on UT than is the AU index. Considering such a latitudinal distrib ution, it is highly probable that the present AL indices often underestimat e disturbed conditions during specific universal time intervals, particular ly 0200-0800 UT.