太陽風和地球磁層交互作用的過程中，磁層頂日下點對峙距離是一項重要的參數，其值主要由行星際磁場的Z方向分量 (Bz)和太陽風動壓 (Dp)所控制，但也與行星際磁場錐角有關，先前的研究發現行星際磁場錐角值小時，亦即行星際磁場幾乎平行日地連線時，日下點對峙距離值會較大。日下點對峙距離和行星際磁場錐角值在不同的太陽風條件下的關係至今並未被系統地討論。從西蜜斯(Time History of Events and their Macroscopic Interactions during Substorms, THEMIS)的衛星資料庫判斷日下點附近的磁層頂的磁層頂穿越事件，再由OMNI資料庫選取相對應的太陽風條件資料，然後利用這些事件，區分成南向行星際磁場與北向行星際磁場和太陽風高動壓與低動壓情況。我們發現日下點對峙距離和行星際磁場錐角值的關係會隨著行星際磁場的Z方向分量和太陽風動壓改變，在南向行星際磁場和高動壓時，發現行星際磁場錐角接近90度時有最小的日下點對峙距離值。 The subsolar standoff distance of the Earth’s magnetopause is an important parameter in studying interactions between the solar wind and the magnetosphere. Its value is mainly controlled by the north-south component (Bz) of the interplanetary magnetic field (IMF) and solar wind dynamic pressure (Dp). Recent studies have found that the subsolar standoff distance is large for small cone angle of the IMF, that is, the orientation of the IMF is almost parallel to the Sun-Earth line. However, relationship between the subsolar standoff distance and cone angle under different IMF Bz and Dp conditions have not been comprehensively studied. With a large set of subsolar magnetopause crossings identified from the THEMIS data and their corresponding solar wind conditions obtained from the OMNI data, we are able to bin the magnetopause crossings in four categories by the southward and northward polarities of the IMF, and large and small Dp. Here we find that the relationship between the subsolar standoff distance and cone angle changes with IMF Bz and Dp. The smallest subsolar standoff distance occurs when the cone angle is near 90 degrees in the category of southward IMF and high Dp.
