| 摘要: | 二胞型極光(two-cell aurora)產生的機制通常被認為與地球高緯度電離層因南向行星際磁場的二胞型電漿對流有關,它的明顯的特徵是:在靠近地球極區,晨曦以及黃昏的地區有著往方位角延伸至少四個小時local time的細長型極光型態。根據之前的研究顯示,在發生二胞型極光的所有事件之前,皆有一個對應的由北轉南向行星際磁場,然而在每次行星際磁場轉南向,是否皆可看到二胞型極光尚未清楚了解。在這篇研究論文中,我們使用Polar衛星的Ultraviolet Imager (UVI),Wind衛星的行星際磁場與電場資料以及Polar-cap index(pcn)的資料,分析行星際磁場與二胞型極光之間的關係。另外,計算在晨曦與黃昏處的極光power最大值來觀察二胞型極光的強度變化。我們發現二胞型極光的發生與太陽風磁場轉南向有關,當太陽風磁場轉南向時,二胞型極光的發生的機率為32%。太陽風電場強度可能是二胞型極光發生的指標,當有二胞型極光發生時,86%的事件其太陽風電場強度超過-2.5 mV。在某些磁副爆的例子中,由太陽風帶到磁尾的能量可以儲存3個小時以上後才反映在高緯度電離層上,而某些例子中,當太陽風由南轉北向後極光活動仍可持續3個小時以上。此外,代表高緯度電離層對流電場強度的pcn值與二胞型極光的發生與否並無特定關係。 The main feature of the two-cell aurora is azimuthally elongated over at least four hours of local time on both the duskside and dawnside. The two-cell aurora is believed to be associated with a convection enhancement caused by southward interplanetary magnetic field (IMF). According to previous studies on the occurrence of the two-cell aurora, a period of southward IMF always occurs before the two-cell aurora appears. But whether the two-cell aurora will occur over a period of southward IMF is still unknown. In this study we analyze Polar Ultraviolet Imager (UVI) auroral images, Wind interplanetary magnetic and electric field data and the polar-cap index (pcn) data in order to determine the relationships between the IMF and the two-cell aurora. In addition, we calculate the maximum auroral power on both the duskside and dawnside as a representation of change in the intensity change of the two-cell aurora. We find that the percentage of the occurrence of the two-cell aurora is 32% when the IMF turns southward. The intensity of the electric field in the solar wind could be an important parameter in determing the existence of the two-cell aurora. When the two-cell aurora occurred, there was a high probability (86%) that the electric field exceeded a critical value (<-2.5 mV). Some events show that the high-latitude ionosphere was quiet for three hours before a substorm aurora occurred while some other events show that the aurora activities could last for three hours after the IMF turns northward. Also the pcn index, which is served as a proxy of the ionospheric convection, does not have a good correlation with the occurrence of the two-cell aurora. |
