先前的研究透過地面觀測發現水星的外氣層中含有鈉原子（Potter & Morgan, 1985），而後，信使號在飛掠水星的過程發現水星的鈉層集中在高緯地區。不論地面望遠鏡或是太空船觀測都顯示水星的外氣層其實相當多變，這被歸因於多變的太陽風和行星際磁場的指向——外在環境和水星磁層交互作用，進而影響鈉原子的產生。

我們利用由Amitis程式模擬產生的資料作為部分初始條件，以此模擬水星鈉的外氣層分佈。由於希望得到水星真實環境的分析，我們以太陽神號在水星軌道的資料作為參數，運行Amitis程式得到太陽風質子在水星表面沉降的分佈；在太陽神號的資料中，我們找到了一個高速太陽風事件，並選定了包含事件前、中、後時間點的參數進行模擬。我們的結果顯示，行星際磁場在x方向（太陽與水星連線方向）的分量可能較大程度地影響水星鈉層南北不對稱的分佈，而z方向（平行水星自轉軸方向）的分量則決定了鈉的總體產生量。

未來我們會以同樣的方法模擬其他太陽風事件（例如：日冕物質拋射），並與日後貝皮可倫坡計畫的資料比較，進行進一步的分析。

In 1985, sodium was first detected in Mercury’s exosphere (Potter & Morgan, 1985). Afterwards, the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) spacecraft made in-situ measurements about the Mercury’s exosphere and showed two-peak structures of sodium exosphere during the flybys. Both ground-based and in-situ observations had shown us different profiles of Mercury’s sodium exosphere and it is found to be highly variable. This is thought to be resulted from the variability of solar wind conditions and the orientation of interplanetary magnetic field (IMF), given the interaction between solar wind and Mercury’s magnetosphere and sputtering.

We simulate the Mercury’s sodium exosphere with the solar wind proton precipitation data generated from the Amitis simulation code, which is a three-dimensional time-dependent hybrid model of plasma (Fatemi et al., 2017; Fatemi, Poppe & Barabash, 2020). We focus on the datasets generated based on Helios data to trace the realistic solar wind conditions at Mercury and how the Mercury’s magnetosphere and sodium exosphere change with the environment. Our results show the morphology of sodium during a high-speed stream (HSS) event covering pre-HSS, HSS and post-HSS phases. It may explain the north-south asymmetry on Mercury. The relative flux in northern and southern cusps is correlated with Bx. However, the total production rate is still controlled by Bz.

In the future, the numerical algorithm and methodology generated will be applied to a series of solar wind events including interplanetary coronal mass ejections (ICMEs) with a view to participate in the upcoming BepiColombo observations at Mercury beginning in 2025.

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