On the UT and seasonal variations of the standard and SuperMAG auroral electrojet indices

The standard auroral electrojet (AE) indices are based on magnetic disturbance data from 10 to 12 northern auroral observatories. Recently, Newell and Gjerloev (2011a) computed equivalent SuperMAG electrojet (SME) indices using data from around 100 mid latitude to high latitude observatories in the Northern Hemisphere. The SME indices certainly have advantage over the AE indices in terms of number as well as temporal resolution of substorm onsets due to better latitudinal and longitudinal coverage. The UT and seasonal variations of geomagnetic activity have been extensively examined in the past. However, particularly for the AE indices, these variations have remained elusive due to sparse distribution of the AE observatories. In this study, we examine what effect the inclusion of large number of stations would make on the UT and seasonal variations of the auroral electrojets activities. For this purpose, data for years 1997-2009 have been considered when consistently many stations (> 70) were available for the computation of the SME indices. We demonstrate that the SME indices exhibit grossly similar UT and seasonal variations as observed in the AE indices. However, there are subtle differences which arise due to difference in number of stations. Our study suggests that most of the UT and seasonal variations of the AE indices, reported earlier, were mainly not due the sparse distribution of stations, but rather to the actual physical processes that control them.