The Differences in Onset Time of Conjugate Substorms

The auroral electrojet (AE) index is traditionally calculated from 13 ground magnetometer stations located around the typical northern auroral oval location. Similar coverage in the Southern Hemisphere index (SAE) does not exist, so the AE calculation has only been performed using Northern Hemisphere data. In the present study, we use seven southern auroral region ground magnetometers as well as their conjugate Northern Hemisphere data to calculate conjugate AE indices for 274 days covering all four seasons. With this dataset over 1200 substorm onsets have been identified in the SAE index using the technique of Hsu et al. [2012]. A comparison of the SAE index with the world data center standard AE index shows that the substorm onsets do not always occur at the same time with differences on the order of several minutes. In this study we examine the differences in the onset time and the reason for those differences using our conjugate AE indices and using pairs of conjugate ground magnetometer stations. Specifically, we used the pair of stations at West Antarctica Ice Sheet Divide and Sanikiluaq, Canada and Syowa, Antarctica and Tjörnes, Iceland. The largest differences in onset time appear to be related to the IMF Bz and magnetic field line length. Differences on the order of minutes for the onset time of conjugate substorms have serious implications for substorm theories. The problem is that waves from a current disruption region to the mid tail, or flows from the mid tail to the current disruption region take the same amount of time (~2 minutes), which makes it difficult to decide where the onset disturbance is initiated, particularly when onset indicators have differences on the order of minutes.