地球磁場偶極化(dipolarization)是一個讓磁尾的形狀由延伸現象(stretching)回到偶極場的一個過程。一般來說，磁場偶極化在磁副暴發展期啟動(substorm expansion onset)之後開始，接著極光會在對應於偶極場發展區域的電離層出現並且擴張。在這個研究裡我們進行一個有關於地球磁場的延伸現象以及偶極化在不同與磁副暴於地球表面位置(substorm onset location)之間距的研究。磁副暴的事件記錄表是由Polar Ultraviolet (UVI)極光影像圖中判斷得知。Polar UVI極光影像圖位於Lyman-Birge-Hopfield頻帶上(160-180 nm)，用來觀測北半球夜側的極光強度。地球磁場的資料則是由Geostationary Operational Enviromental Satellite (GOES) 8號和9號兩個衛星所得到，並且我們利用此衛星資料計算地球磁場的傾斜角度。 經由資料分析後可以得到以下結論：(1)地球磁場在北向的行星際磁場(IMF)有比較大的偶極化現象，在南向IMF則比較小。不過，在南向IMF時比在北向IMF時有持續時間更長且強度更大的磁副暴復原期(substorm recovery phase)。因此，我們可以推論出地球磁場在磁副暴來臨時的偶極化現象與極光強度(auroral power)之間並沒有很好的關聯性；(2)持續延伸的地球磁場發生在GOES衛星對應在磁副暴於地表延伸位置(substorm onset location)的西邊；持續偶極化的地球磁場則發生在substorm onset location的東邊。由這個結果，我們可以推測磁場傾斜角在東西兩邊有東西不對稱(west-east asymmetry)的現象發生；(3)最大的偶極化發生在onset location東邊一小時當地時間(local time)的位置。 Magnetic dipolarization is a process that the configuration of the magntotail changes from a tail-like to a dipole-like orientation. It is generally believed that the magnetotail starts to dipolarize at the substorm expansion onset; subsequently, the dipolarization region expands and nightside aurora develops. In this study we pursue a study of dependence of magnetic stretching and dipolarization on distance away from the substorm onset location for a list of substorms identified from Polar Ultraviolet (UVI) auroral images. Magnetic field data from Geostationary Operational Enviromental Satellite (GOES) 8 and 9 are used to calculate elevation angles of the Earth’s magnetic field, while Polar UVI images in the Lyman-Birge-Hopfield band (160-180 nm) are used to estimate the Northern Hemispheric nightside auroral power. It is found that: (1) The magnetic field is more dipolarized for northward IMF than that for southward IMF. In addition, events for southward IMF have a longer recovery phase and stronger intensity than those for northward IMF. Therefore, it implies that the dipolarization and auroral power do not have a good relationship during the substorm; (2) The sustained stretching occurred when the GOES footpoint was located at westward of the onset location and the sustained dipolarization occurred when the GOES footpoint was located at eastward of the onset location and out of the SCW region. This result indicates that there is a west-east asymmetry in elevation angle; (3) The largest dipolarization occurred at 1 hour distance eastward of the onset location.
