| 摘要: | 磁層頂是由限制地磁場的Chapman-Ferraro電流系統所形成的邊界層，其大小和形狀取決於太陽風狀態和磁層電流。磁層頂大小主要受太陽風壓力的影響，而其形狀受行星際磁場（IMF）方向影響極大。當IMF某一分量與地磁場反平行時（也稱南向IMF），磁層電流增強並重新分佈，從而使磁層頂大小和形狀都發生變化。此外，南向IMF通過諸如磁重聯這樣的過渡過程還會侵蝕磁層頂。磁層頂形狀具有多種不對稱性。首先是由地球公轉產生的變形，即整個磁層頂在黃道面上的旋轉。第二個不對稱性與磁傾角的變化有關，它導致磁層頂斜向南極或北極。磁暴期間，當部分環電流增強時，磁層頂昏側就偏離地球徑向。由於南向IMF常常侵蝕磁層頂不同部分，可能形成局部不對稱性。不對稱性是造成磁層頂模型不準確的一個重要因素。到目前爲止，由於數據的局限，該因素在模型中未能充分考慮。過去十年有了許多關於磁層頂位置和相應太陽風以及地磁狀態的觀測數據。我們已經積累了比以前十倍多的磁層頂穿越觀測數據，為深入全面地研究寧靜和擾動狀態下的磁層頂提供新的機遇。基於新的數據，我們將分析與不同現象相關的磁層頂不對稱性。第一步將研究兩個或多個衛星（如GEOTAIL, CLUSTER, Double Star, THEMIS以及地球同步衛星GOES and LANL）緊鄰磁層頂時的觀測事例，這樣可以直接發現晨－昏側不對稱性。另一個方法是應用標準軸對稱模型來分析模型預期結果對觀測的系統偏差（如Shue et al. [1998]），使我們能對晨－昏側和南－北向不對稱性進行統計研究，並分析不對稱性對太陽風和地磁參數的依賴關係，最後為建立不對稱磁層頂模型提供新的基礎。表 C011 Magnetopause is a magnetospheric boundary layer formed by a so-called Chapman-Ferraro current system, which confines the geomagnetic field. The magnetopause size and shape depend on both solar wind conditions and magnetospheric currents. The size is mainly controlled by the solar wind pressure and the shape is mostly affected by the orientation of interplanetary magnetic field (IMF). In a case when the IMF has a component anti-parallel to the geomagnetic field (so-called southward IMF), the magnetospheric currents are intensified and redistributed that in turn also causes changes of the magnetopause size and shape. In addition, the southward IMF causes erosion of the magnetopause due to transitional process such as magnetic reconnection. As a result the shape of magnetopause is characterized by various kinds of asymmetry. The first one is an aberration due to the Earth’s orbital rotation, which causes a turning of whole magnetopause in the ecliptic plane. Another asymmetry is associated with a varying tilt of geo-dipole that leads to consequent skewing of the magnetopause toward southern or northern poles. Magnetopause duskward skewing of about earth radius occurs during geomagnetic storms, when the partial ring current is intensified. Under the southward IMF, local asymmetries might be developed due to the erosion prevailing at different sites of the magnetopause. The asymmetry is an important factor contributing to uncertainty of the magnetopause modeling. Up to now this factor is only partially accounted in models because of limitation of data sets. Last decade provides a lot of experimental information about magnetopause location and corresponding solar wind and geomagnetic conditions. We have accumulated a set of magnetopause crossings containing 10-times more data. That data set provides new opportunities for comprehensive studies of the magnetopause under quiet and disturbed conditions. Based on the new data, we propose to analyze the magnetopause asymmetries associated with different phenomena. As a first approach, we will study case events when two or more satellites, such as GEOTAIL, CLUSTER, Double Star, THEMIS and/or geosynchronous GOES and LANL, are located in close vicinity of the magnetopause. This way allows direct finding the magnetopause asymmetry. Another approach is based on application of a standard axially-symmetrical models, such as Shue et al. [1998], for analysis of systematical deviations of the model predictions from observations. That enables us to study statistically both dawn-dusk and north-south asymmetries. A dependence of asymmetries on the solar wind and geomagnetic parameters will be analyzed. As a result we plan to develop a new basis for modeling the asymmetrical magnetopause. 研究期間 : 9808 ~ 9907 |
