AU指數主要用來測量當磁副暴發生時，極光區東向電噴流對地面所造成的磁場擾動(向北為正)。一般來說AU指數會大於零，然而我們發現在2005年8月24日這天卻有連續約一小時的時間AU都小於零，於是開始尋找類似的事件。我們蒐集了1998年2月5日到2013年10月21日的資料，經過篩選後，我們得到了超過五分之四的極端負AU事件發生於行星際磁場(IMF)同時有晨向分量與南向分量的時候。我們以類似作法分析了AU很大的事件，發現了超過五分之四的極端正AU事件發生於IMF有黃昏向分量的時候。並且我們也討論與分析了SymH對AU造成的影響，發現當AU愈負，SymH對AU的貢獻度愈低。同時我們也反過來探討在行星際磁場南向分量夠強的事件中，IMF By的正負對AU指數造成的影響。我們發現IMF By愈負，極端負AU事件發生機率愈高。最後我們將SuperMAG的資料加入分析後，找到了極端負AU事件可能與東向電噴流位置往赤道方向移動有關的證據。;AU index is designed to measure the substorm magnetic disturbance on the ground due to eastward auroral electrojet. AU index is usually greater than zero, but we find an event with AU much less than zero on 24 August 2005. So we start looking for other similar events. We analyze data from 5 February 1998 to 21 October 2013 and find 80% of the extreme negative-AU events occurred when the interplanetary magnetic field (IMF) has dawnward and southward components. We apply the similar analysis to large AU events and find 80% of the extreme positive-AU events occurred when IMF is duskward. We also analyze the effect of SymH and find that the contribution of SymH decreases with decreasing AU in the negative-AU events. We also examine the sufficient conditions for the occurrence of the extreme negative-AU events. We find that the occurrence of the extreme negative-AU events increases with increasing dawnward component of the IMF when the IMF has a clear southward component. Finally, we analyze the SuperMAG data, and find the possible evidence of equatorward movement of the eastward electrojet during the extreme negative-AU events.
