地球磁層頂的位置會受到南向行星際磁場Bz及太陽風動壓Dp的影響，南向Bz及Dp的增強會使日側磁層頂的位置更接近地球，雖然這已是廣為人知的事實，但對於日側磁層頂在同步軌道附近對南向Bz及Dp反應的研究仍相當有限。為了瞭解宏觀尺度下日側磁層頂與強烈太陽風的交互作用情形，我們針對1986-1992年及1999-2000年間同步軌道衛星GOES有可能進入地球磁鞘的時間區段，詳細比較了每一分鐘衛星的觀測及磁層頂模型的預測結果。藉由以下三種參數：probability of prediction (PoP)、probability of detection (PoD) 及false alarm rate (FAR)，我們分別估計了Petrinec and Russell [1996]、Shue et al. [1998] 及modified Chao et al. [2002] 三個磁層頂模型在強烈太陽風情形下的預測能力，愈高的PoP、PoD值及愈低的FAR值表示該模型具有愈佳的預測能力。至於相對應的太陽風參數則取自於ACE、Wind、IMP 8或 Geotail衛星的觀測結果。 根據2000年4月6日衛星的觀測資料我們發現，即使在南向行星際磁場Bz大於25 nT及太陽風動壓Dp接近8 nPa這樣強烈的太陽風情形下，GOES衛星並未觀測到磁層頂穿越同步軌道的證據，但上述三種磁層頂模型皆預測此太陽風情形足以使日側磁層頂進入同步軌道之內。我們認為此種觀測與預測不一致的現象來自於南向Bz對日側磁層頂位置的影響飽和所致，當太陽風中具有很大的南向行星際磁場分量時，此種飽和效應會使日側磁層頂內移的動作趨於停止。因此，藉由modified Chao et al. [2002] 模型估計磁層頂的位置，分析1999-2000年間可能造成GOES衛星進入地球磁鞘的Bz及Dp，在使FAR最小化及PoP最大化的前提下，我們得到了一個 與Dp的關係式， ，其中 為飽和效應發生時南向行星際磁場Bz的大小，單位為nT，而Dp的單位為nPa。將此關係式應用於modified Chao et al. [2002] 模型，以預測2001年3月31日磁層頂穿越同步軌道的情形，並與LANL MPA儀器的觀測資料相互比較，結果顯示當一磁層頂模型在考慮飽和效應之後，其預測能力的確有顯著的改善。 It is well known that the dayside magnetopause moves closer to the Earth with increasing southward IMF Bz and solar wind dynamic pressure Dp. But the response of magnetopause location to the southward Bz and Dp is unclear when the magnetopause locates around the geosynchronous orbit. To understand the macroscopic interaction between solar wind and dayside magnetopause under such extreme conditions, the predictions of magnetopause models are compared with the magnetosheath encounters observed by GOES spacecraft during 1986-1992 and 1999-2000. This comparison is made for each 1-min data point. Three magnetopause models, Petrinec and Russell [1996], Shue et al. [1998], and modified Chao et al. [2002], are separately examined for their forecasting capability by using the following parameters: probability of prediction (PoP), probability of detection (PoD), and false alarm rate (FAR). Higher PoP and PoD with a lower FAR imply a better forecasting model. The corresponding solar wind conditions are obtained from the upstream monitors of ACE, Wind, IMP 8 or Geotail spacecraft. On 6 April 2000, the predicted magnetopause is inside the geosynchronous orbit for corresponding Dp ~ 8 nPa and southward Bz > 25 nT, while the magnetopause is still outside 6.6 RE for such strong solar wind condition based on the magnetic field observations of GOES spacecraft. We propose this phenomenon comes from the saturated Bz-influence on magnetopause locations when southward Bz is large such that the earthward motion of dayside magnetopause stagnates. By means of the calculations of the modified Chao et al. [2002] model, possible magnetosheath encounters at geosynchronous orbit during 1999-2000 are used to obtain a relationship between and Dp, , such that the FAR is minimized and PoP is maximized, where is the threshold of IMF Bz for saturation occurring in nT and Dp is in unit of nPa. This obtained dependence is applied to the modified Chao et al. [2002] model to compare against the magnetosheath encounters observed by the LANL MPA instruments on 31 March 2001. Our results indicate that the model’s prediction capability is indeed improved when the saturation effect is considered.
