| 摘要: | 摘要： Chapman-Ferraro (C-F) 電流是一個造成地磁變化的關鍵結構。這是一個因太陽風等離子體流與磁層磁場相互作用而產生的屏蔽電流。C-F電流產生的的磁場在磁層邊界(磁層頂)之外終止了地磁場的存在。由於太陽風是超音速及超Alfven速，它在磁層頂的上游形成了弓激波和磁層頂。這個交互作用區域的複雜結構導致了C-F電流對於上游的太陽風的複雜反應。此外，地球磁層電流的整體系統均受到太陽風狀態的影響。因此，不僅是C-F電流的強度，它的形狀和方向都可能有所變化。一般公認太陽風的壓力控制C-F電流的強度與地磁場的變化。另一個廣為人知的效應是南向行星際磁場（IMF）和其導致的黃昏方向電場 [VxB] 對磁層電流的整體反應。後者 (電場) 會強化橫跨磁尾的電流和朝著太陽方向的對流，增加C-F電流的強度以及磁尾的大小。因此，C-F電流的曲率半徑會擴大。然而，日側C-F電流的扭曲仍然很缺乏調查。一些磁流體 (MHD) 的電腦模擬預測到在南向IMF很強時，日側C-F電流會消失。此外，在日側的磁層遭到激烈侵蝕時可能形成一個削弱的C-F電流。低頻磁場振盪通常被認為與Kelvin-Helmholtz (K-H) 的不穩定性有關，從而導致C-F電流在磁層頂側翼的振盪。最近，一些研究報告發現了與K-H不穩定性無關的日側磁層頂和地磁場的起伏。磁鞘的擾動對日側磁層頂動態所扮演的重要角色，已得到先進太空實驗的支持。各式各樣的瞬態事件，例如噴射，不連續性等，已被發現為C-F電流中局部的強大扭曲的原因。在太陽風與IMF平行的特殊磁鞘狀況下，C-F電流也展現不尋常的強度和方向。我們在此提出一研究計劃，使用Geotail, CLUSTER, DSP, THEMIS太空船，地球同步軌道衛星，以及地面測磁計的觀測數據，來探討C-F電流的扭曲和相關地磁場變化。磁場的扭曲將由觀測數據與IGRF地磁場模型和Tsyganenko [2005] 模型的比較來確定。當有可能時，C-F電流的大規模形狀改變可從多衛星觀測得出。通過這種方式，我們將能夠找到一些磁層邊界扭曲和地磁變化事件，以及它們與太陽風條件的關係。這一信息可提供對日側磁層動力更深入了解。在實用上，它將使我們能夠估計磁層頂位置在不同條件下的內部的不確定性。 Abstract: Chapman-Ferraro (C-F) current is a key structure responsible for geomagnetic variations. That is a shielding current generated in interaction of the solar wind plasma flow with the magnetospheric magnetic field. The magnetic field produced by the C-F current yields termination of the geomagnetic field outside the magnetospheric boundary, magnetopause. Because the solar wind is supersonic and superalfvenic, the bow shock and magnetosheath are formed upstream of the magnetopause. Complex configuration of the interaction region leads to complicated response of the C-F current to the upstream solar wind. In addition, geomagnetic current system as whole depends on the solar wind conditions. As a result, not only the strength but also the shape and direction of C-F current can vary. It is well established that solar wind pressure controls the strength of C-F current and changes the geomagnetic field. Another well-known effect is a global response of magnetospheric currents to southward interplanetary magnetic field (IMF) and induced dawn-dusk electric field [VxB]. The latter one intensifies the cross-tail current and sunward convection that increase the C-F current and size of the magnetotail. Hence, the radius of C-F current curvature enlarges. However, the distortion of dayside C-F current is still poorly investigated. Some MHD simulations predict disappearing of the dayside C-F current for very strong southward IMF. In addition, intense erosion at the dayside magnetopause might form a blunted C-F current. Low-frequency geomagnetic field oscillations are commonly related to Kelvin-Helmgoltz (K-H) instability, which leads to oscillations of the C-F current at the flank magnetopause. Recently, some authors report undulations of the dayside magnetopause and geomagnetic field, which are not related to the K-H instability. Driving role of the magnetosheath perturbations in the dayside magnetopause dynamics is supported by modern space experiments. Various transient events, such as jets, discontinuities, etc. have been found to be responsible for strong local distortions of the C-F current. A specific magnetosheath regime under solar-wind-aligned IMF also demonstrates features of unusual C-F current strength and orientation. Here we propose to study C-F current distortions and related geomagnetic variations using experimental data acquired from space missions Geotail, CLUSTER, DSP, THEMIS as well as from geosynchronous and ground-based magnetometers. The magnetic distortions will be determined on the base of comparison with the IGRF and Tsyganenko-04 models. When possible, large-scale changes of the shape will be studies from multi-satellite observations. By this way we will be able finding a number of events of distorted magnetospheric boundary and related geomagnetic variations in connection with various solar wind conditions. This information provides deeper understanding of the dayside magnetosphere dynamics. In practice, it will make possible estimation of the internal uncertainties of the magnetopause location under various conditions. 研究期間：9908 ~ 10007 |
