Inner Coma Ionospheric Chemical Model of Comet 67P/ Churyumov-Gerasimenko

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姓名

魏辰恩(Chen-En Wei) 查詢紙本館藏

畢業系所

天文研究所

論文名稱

(Inner Coma Ionospheric Chemical Model of Comet 67P/ Churyumov-Gerasimenko)

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摘要(中)

羅賽塔任務（Rosetta Mission）是歐洲太空總署（European Space Agency）所主導的近距離彗星觀測任務。羅賽塔任務於2004年發射，經過了十年的旅程，於2014年開始進行彗星67P/C-G的觀測任務，觀測從2014年8月一直到了2016年9月底任務結束，期間彗星由3.5 AU逐漸靠近太陽，一路抵達近日點（1.2AU），再逐漸遠離太陽；羅賽塔任務是人類史上第一次繞行並觀測一顆彗星，也是首次登陸彗星表面（Lander: Philae）。
羅賽塔任務的觀測發現了一些原子及分子的發射譜線（emission），像是 O1, H1, OH 等，可能是因為electron impact 的化學反應而來（Feldman et al. 2015, Bodewits et al. 2016）。於此次研究之中，我們利用觀測到的資料作為初始條件，藉由連續方程式建立起彗星內層大氣的化學模型，希望能進一步了解electron impact 於彗星內層大氣化學模型中的重要性，並利用H3O+/H2O+的數量密度比與Fuselier et al. (2015) 及Fuselier et al. (2015) 的化學模型結果互相比較。
初始條件的設定，我們設定了兩種不同彗星－太陽距離：第一個是彗星距離太陽3 AU，時間上約是2014年10月底左右；第二個設定為彗星到達近日點1.2 AU，約是2015年8月中。除了兩種與太陽不同的距離外，我們將彗星分成南北半球，其原因是由於觀測上發現，化學成份在彗星的南北半球呈現相異的分佈特性（Hassig et al. 2016）。此外，為了進一步了解electron impact 的重要性，我們設定兩種條件，一是化學模型中沒有加入electron impact的效應，二是將electron impact的效應加入彗星內層大氣的化學模型之中。為了將electron impact加入化學模型中，我們考慮三種不同的電子能量分佈狀態（Maxwellian distribution/source spectra due to the photoionization/ observation from Rosetta ）。綜合以上的條件，我們可以深入了解彗星內部大氣的化學成分結構。

摘要(英)

Thanks to the Rosetta Mission, we have an opportunity to study comet 67P/C-G in the long journey from Aug. 2014 to the end of Sep. 2016. This is the first time to orbit and land on the comet.
According to Feldman et al. (2015) and Bodewits et al. (2016), some of the atomic and molecular emissions such as O1, H1, OH, etc. might be attributed to electron impact dissociation. As a result, we want to understand the effect of the electron impact mechanism on the cometary coma. An ion-neutral chemical model in used to estimate the number density ratio of H3O+/H2O+ under different physical conditions.
For the model setting, to account for the seasonal effect, we consider two heliocentric distances: 3 AU and 1.2 AU, i.e., the perihelion, respectively. The surface composition of the comet in further divided into two parts: the Northern and Southern hemisphere. We also use three different electron energy spectra (Maxwellian distribution/ source spectra due to the photoionization/ observation from Rosetta ) to simulate the resultant coma structure.

關鍵字(中)

★ 彗星
★ 內層大氣
★ 光化學模型

關鍵字(英)

★ comet
★ inner coma

論文目次

摘要 ……………….……………….……………….……………….……………….…… i
Abstract ……………….……………….……………….……………….……………….…… ii
Acknowledgement ……………….……………….……………….……………….……………….…… iii
Contents ……………….……………….……………….……………….……………….…… iv
List of Figures ……………….……………….……………….……………….……………….…… vi
List of Tables ……………….……………….……………….……………….……………….…… xi
Chapter 1 Introduction ….……………….…… 1
1-1 Comets ….……………….…… 1
1-2 Cometary structure ….……………….…… 2
1-2-1 The Nucleus ….……………….…… 2
1-2-2 Cometary coma ….……………….…… 2
1-3 The composition of cometary coma ….……………….…… 4
1-4 The chemistry in gas coma ….……………….…… 4
1-5 Cometary plasma environment ….……………….…… 6
Chapter 2 The Rosetta Mission ….……………….…… 9
2-1 Comet 67P/ Churyumov-Gerasimenko ….……………….…… 10
2-2 The Composition of Comet 67P/C-G ….……………….…… 11
Chapter 3 Methodology ….……………….…… 12
3-1 The photoelectron source spectra ….……………….…… 12
3-1-1 Input parameters ….……………….…… 13
3-2 The observed electron spectra ….……………….…… 23
3-2-1 The distribution function ….……………….…… 23
3-2-2 The Kappa distribution function ….……………….…… 24
3-2-3 Differential electron flux ….……………….…… 28
3-3 The ionospheric structure of Comet 67P/C-G ….……………….…… 30
3-3-1 Collision zone ….……………….…… 30
3-2-2 Contact surface (The diamagnetic cavity boundary) ….……………….…… 31
3-4 The ionospheric chemical model ….……………….…… 33
3-4-1 The 4th Runge-Kutta method ….……………….…… 34
3-4-2 The chemical network ….……………….…… 35
3-4-3 Input parameters ….……………….…… 36
Chapter 4 Results and Discussion ….……………….…… 44
4-1 Chemical models without electron impact mechanism ….……………….…… 44
4-1-1 Results without electron impact mechanism at 3 AU ….……………….…… 45
4-1-2 Results without electron impact mechanism at 1.2 AU ….……………….…… 49
4-2 Chemical models with electron impact mechanism ….……………….…… 53
4-2-1 Results with electron impact mechanism at 3 AU ….……………….…… 53
4-2-2 Results with electron impact mechanism at 1.2 AU ….……………….…… 71
4-3 The ratio of H3O+/H2O+ ….……………….…… 90
4-3-1 Results of the H3O+/H2O+ ion number density ratio ….……………….…… 91
4-3-2 The comparison between different models ….……………….…… 93
4-4 The ionospheric structure of Comet 67P/C-G ….……………….…… 97
Chapter 5 Conclusion ….……………….…… 99
Bibliography ….……………….…… 100
Appendix I Species list ….……………….…… 104
Appendix II Rate coefficients of chemical reactions ….……………….…… 105
Appendix III Electron impact rate coefficients ….……………….…… 108

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指導教授

葉永烜(Wing-Huen Ip)

審核日期

2017-1-5

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