土星系統裡的中性分子雲之結構與動力學

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曾瑋玲(Wei-Ling Tseng) 查詢紙本館藏

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論文名稱

土星系統裡的中性分子雲之結構與動力學
(The Structure and Dynamics of the Neutral Cloud in the Saturnian System)

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摘要(中)

從1980年代的航海家太空船觀測，接下來的哈伯太空望遠鏡的觀測，以及最近的卡西尼號太空船的觀測，我們已經知道土星系統沉浸在一大片的中性分子氣體雲裡面，大部分是水分子以及其分解過後的產物，例如氫氧根分子、氧原子與氫原子。絕大多數的中性氣體都是由土衛(二)—恩斯拉達斯南極的冰噴泉機制所噴出的，以及少部分是由其他冰質衛星所貢獻的。另外，土星的環系統也是氧分子與氧原子的重要來源之一，而這些氧分子可以藉由離子—中性分子的碰撞作用產生，進而被彈射到土星磁球層的外圍區域。土衛(六)—泰坦的大氣層也是另一個提供中性分子的重要來源，包含氫分子(原子)、甲烷和氮氣。這些中性分子被游離之後，都是土星磁球層裡電漿成分。在這項研究工作中，我們利用數值模擬方法和卡西尼觀測中最新的電漿環境資訊，來了解這些分子雲氣的結構和成分。
本研究第一部分，是建立模型來了解環系統的氧氣大氣層和離子球層之結構與季節性的變化。模擬中有考慮到離子—中性分子的碰撞作用之電荷交換。因此，環系統的大氣分子可以進入到土星磁球層的外圍區域，而成為氧離子的來源之一。我們的研究結果，顯示如果環系統的氧氣主要是由光分解作用得到的話，則在土星春秋分的季節時，磁球中的氧離子會幾乎消失。
第二部分則是檢驗環系統大氣另一種質量來源，是否會對其結構或是季節性變化有何影響。由恩斯拉達斯而來的中性分子和電漿成分，有可能藉由冰微粒表面化學作用而還原成氧分子。如果這樣來源機制有可能比光分解還有效率的話，則環系統的大氣與離子球層可能不會隨著太陽照射仰角改變而有變化。然而，由其他冰質衛星產生的氧分子，相對於由環系統產生經由散射作用到磁球層外圍區域的比例，可能僅有少許的貢獻。
第三部分則是關於泰坦大氣層與土星磁球層電漿的交互作用。從卡西尼的觀測，我們知道磁場的結構與電漿流場是非常複雜多變的。我們採用合作者的磁流體力學模擬的電漿資料，來研究泰坦的撿拾離子的流量之空間變化，以及關於H2+,CH4+,N2+離子在泰坦外氣層底的能量沉積量之計算。這模擬結果還包含四個不同的泰坦公轉軌道位置。
最後，我們探討有關從泰坦大氣層逃逸出來的氫原子，在土星系統中的分布，隨著土星的季節變化之模擬結果。從前人的研究，已知氫原子的分布，會因太陽輻射壓力而呈現不對稱的形狀。另外，由卡西尼太空船所攜帶的紫外線光譜成像儀(UVIS)之觀測結果，顯示出土星的大氣層也可能是土星系統中的氫原子之重要來源。

摘要(英)

From HST observations, Voyager flyby measurements and the Cassini in-situ measurements, we have learned that the Saturnian system is immersed in a vast neutral gas cloud of oxygen molecules, water molecules and their photodissociative products like OH, O and H. Most of the gas molecules originate from the plumes in the south pole of Enceladus plus some small contribution from other inner icy satellites. In addition, the ring system is an important source of oxygen atoms and molecules which can be injected into the distant Saturnian magnetosphere via scattering processes. Titan’s exosphere is another major source contributing neutral gas like H2 and H, and probably also CH4 and N2. These neutral materials will be fed into the thermal plasma disk in the inner Saturnian magnetosphere. In this work, the model calculations have been performed to simulate the structures and compositions of the neutral gas clouds of different origins making use of an updated photochemical and plasma chemistry model based on the latest plasma measurements from Cassini CAPS instrument.
The present modeling efforts have first led to the picture that an exospheric population of neutral oxygen molecules can be maintained in the vicinity of the main rings by means of photolytic decomposition of ice and other surface reactions. The momentum exchange effect via charge exchange collisions has been taken into consideration in the computation. The ring atmosphere, therefore, serves as a source of O2+ ions throughout Saturn’s magnetosphere. By the same token, our results also show that the magnetopheric O2+ ions should be nearly depleted at Saturn’s equinox if O2 is produced mainly by photolysis of the ring material.
Secondly, we have examined the mass budget of the ring oxygen atmosphere of Saturn taking into account of the possibility of an “exogenic” source i.e. Enceladus’ neutral gas cloud. The maximum O2 source rate from recycling of Enceladus-originated plasma and neutrals might be comparable to the maximum value from photolytic decomposition of the icy ring particles. In this case, the neutral O2 source rate in the Saturnian magnetosphere would be independent of the solar insolation angle. It is also shown that the O2 source from other inner icy satellites is smaller comparable to the scattered O2 component of ring-origin.
The third part of our work is about Titan’s exospheric interaction with the corotating magnetospheric plasma. From the Cassini observations, we know that the magnetic field configuration and plasma flow field are highly variable. We have employed the numerical results of the three dimensional MHD simulation of Kopp and Ip (2001) to study possible spatial and temporal variations in the pickup ion influx. The computation of the ion influx and energy deposit into Titan’s exobase for the H2+, CH4+ and N2+ pickup ions separately are shown. The model results of four different Titan’s orbital locations are also presented.
Finally, we consider the distribution of hydrogen atoms escaping from Titan due to the long-term perturbation effects of the solar radiation pressure and planetary oblateness as Saturn orbits Sun.

關鍵字(中)

★ 土星環
★ 磁球層
★ 電漿
★ 泰坦
★ 外氣層
★ 土星

關鍵字(英)

★ Saturn
★ rings
★ magnetosphere
★ plasma
★ Titan
★ exosphere

論文目次

Table of Contents
Chapter 1: Research Background …………………………………………...1
1.1 Research goals and outline ………………………………………………………...1
Chapter 2: Introduction ……………………………………………………..4
2.1 Saturnian system overview …………………………………………..4
2.1.1 Saturn and its ring system ……………………………………………....4
2.1.2 Enceladus and other inner icy satellites ………………………………...7
2.1.3 Titan …………………………………………………………………….8
2.1.4 Magnetosphere and its plasma interaction with the satellites ………....10
2.2 The neutral cloud environment in the Saturnian system ………………………....13
2.2.1 Previous observations and modeling of the neutral clouds. …..……….13
2.3 Cassini-Huygens Mission Overview ……………………………………………..15
2.3.1 Mission descriptions …..………………………………………………15
2.3.2 Instrument descriptions ………………………………………………..16
Chapter 3: The Structure and Time Variability of the Ring Atmosphere and Ionosphere ……………………………………………………………....17
3.1 Introduction .……………………………………………………………………...17
3.2 Modeling descriptions ……………………………………………………………20
3.2.1 A model of neutral O2 atmosphere ………………………………….....20
3.2.2 Ion production and transport …………………………………………..23
3.2.3 Ion molecule charge exchange collisions …….………………………..24
3.3 Saturn Orbit Insertion conditions ………………………………………….……..28
3.4 Seasonal variations ……………………………………………………….………31
3.5 Discussions ……………………………………………………………….………31
Chapter 4: An Assessment and Test of Enceladus as an Important Source of Saturn’s Ring Atmosphere and Ionosphere …………………………..42
4.1 Introduction ……………………………………………………………………....42
4.2 Model Calculations ………………………………………………………………45
4.2.1 Test particle model of ring O2 atmosphere and O2+ ionosphere ..……….45
4.2.2 Mass transfer from Enceladus’ plume material ……………………….46
4.3 Results …………………………………………………………………………....47
4.3.1 Exploration of the Enceladus-related source of O2 in three cases in SOI and Equinox …………………………………………………………...47
4.4 Discussions ………………………………………………………………………50
Chapter 5: Exospheric Heating by Pickup Ions at Titan …………………55
5.1 Introduction ………………………………………………………………………55
5.2 Model description: MHD model and test particle model ………………………...57
5.3 Results ……………………………………………………………………………59
5.3.1 Energy flux distribution of the H2+, CH4+ and N2+ pickup ions at Titan in four different orbital configurations ………………………………...60
5.4 Discussions ……………………………………………………………………….61
Chapter 6: The Distribution of the Atomic Hydrogen in the Saturnian System …………………………………………………………………...69
6.1 Introduction ………………………………………………………………………69
6.2 Model description: orbital integration and a plasma chemistry network ………...71
6.3 Results ……………………………………………………………………………74
6.4 Discussions ……………………………………………………………………….76
Chapter 7: Summary ………………………………………………………..81
References …………………………………………………………………...84
Appendix A: Published Articles ……………………………………………..92
List of Figures
Figure 2.1: Image of Saturn and its prominent rings taken by Cassini………………………....6
Figure 2.2: The Saturn’s satellites and ring structure…………………………………………..6
Figure 2.3: Image of Enceladus’ plumes taken by Cassini……………………………………..6
Figure 2.4: The appearances of Titan in three wavelengths…………………………………..10
Figure 2.5: Image of the lakes on Titan’s surface taken by Cassini Radar…………………...10
Figure 2.6: The structure of Saturn’s magnetosphere…………………………………………12
Figure 2.7: Illustration of Titan’s interaction with Saturn’s magnetospheric plasma…………12
Figure 2.8: The major sources of neutral clouds in the Saturnian system…………………….13
Figure 3.1a: The flow chart of the Monte Carlo procedure on the trajectory calculation of neutral O2………………………………………………………………………………...26
Figure 3.1b: The flow chart of the Monte Carlo procedure on the trajectory calculation of O2+ ions……………………………………………………………………………………….27
Figure 3.2: The trajectories of the O2+ ion motion in different radial distances from Saturn…35
Figure 3.3a: The neutral O2 number density…………………………………………………..35
Figure 3.3b: The O2 column density with variation of radial distance ……………………….36
Figure 3.3c: The O2 number density along the Z-direction in the different radial distances….36
Figure 3.3d: The neutral O2 scale height in the radial distances in the equatorial plane ...…...37
Figure 3.4a: The O2+ ion number density in Log10 scale……………………………………...37
Figure 3.4b: The O2+ ion number density at 0.01 RS below and above the ring plane………..38
Figure 3.4c: The O2+ ion column density above the ring plane and below the ring plane…….38
Figure 3.5a: The neutral O2 column density (molecules/cm2) in four situations……………...39
Figure 3.5b: The O2+ ion density for the solar incident angle ~24° south of the ring plane (top-left), ~24° north of the ring plane (top-right), ~14° north of the ring plane (bottom-left), ~4° north of the ring plane (bottom-right)………………………………...40
Figure 3.6: The Saturn atmospheric impact rates and the magnetospheric injection rates with variations of solar incident angles………………………………………………………..41
Figure 4.1: The O2+ ion distributions in SOI phase (QP~1026 s-1). (a) Total source rate = 1.1×1026 s-1 (b) Total source rate = 2.0×1026 s-1 (c) Total source rate = 1.1×1027 s-1…….52
Figure 4.2: The O2+ ion distributions in Equinox phase (QP~1025 s-1). (a) Total source rate = 2.0×1025 s-1 (b) Total source rate = 1.1×1026 s-1 (c) Total source rate = 1.01×1027 s-1…...53
Figure 4.3a: The radial distribution of the neutral O2 column densities in 3 cases in SOI phase: total source rate ~1.1×1026 s-1, total source rate ~2.0×1026 s-1, and total source rate ~1.1×1027 s-1……………………………………………………………………………...54
Figure 4.3b: The radial distribution of the neutral O2 column densities in 3 cases in Equinox phase: total source rate ~2.0×1025 s-1, total source rate ~1.1×1026 s-1, and total source rate ~1.0×1027 s-1……………………………………………………………………………...54
Figure 5.1: The trajectories of the H2+, CH4+ and N2+ pickup ion projected on the x–y plane in the unit of Titan’s radius…………………………………………………………………64
Figure 5.2: The orientations of the dayside ionosphere of Titan with respect to the direction of the corotating plasma flow at four different orbital phases (SL = 0, 6, 12, and 18)……..64
Figure 5.3: The log of the energy influx (eV/m2 s-1) deposit on Titan’s exobase……………..65
Figure 5.4: The log of the energy influx (eV/m2 s-1) of N2+ pickup ions deposit on Titan’s exobase in four different Saturn local time (SL)…………………………………………66
Figure 5.5: The log of the energy influx (eV/m2 s-1) of CH4+ pickup ions deposit on Titan’s exobase in four different Saturn local time (SL)…………………………………………67
Figure 5.6: The log of the energy influx (eV/m2 s-1) of H2+ pickup ions deposit on Titan’s exobase in four different Saturn local time (SL)…………………………………………68
Figure 6.1a: The initial condition for the geometry of the coordinate system: inlination angle=25° (SOI phase) and the orbital phase angle Φ=0°……………………………….73
Figure 6.1b: The initial condition for the geometry of the coordinate system: inlination angle=0° (Equinox phase) and the orbital phase angle Φ=90°…………………………..73
Figure 6.2: The column density of Titan’s atomic hydrogen torus on the equatorial plane in case of SOI (right) and Equinox (left)…………………………………………………...77
Figure 6.3a: The number density of the hydrogen torus in the vertical view (R-Z direction) along the line of Midnight-Noon in case of SOI (upper) and Equinox (bottom)………..77
Figure 6.3b: The number density of the hydrogen torus in the vertical view (R-Z direction) along the line of Dusk-Dawn in case of SOI (upper) and Equinox (bottom)…………....78
Figure 6.4a: Probabiliy distributions of H knetic energy of electron impact rections………...78
Figure 6.4b: The probability distribution of the accumulative energy of the hydrogen atoms suffering the mutual collisions with the ambient H2 at 1 scale height above the exobase.79
Figure 6.5: The column density of Saturn hydrogen plume on the equatorial plane.................80
Figure 6.6: The number density of Saturn hydrogen plume in the vertical view (R-Z direction) along the line of Midnight-Noon………………………………………………………...81
List of Tables
Table 4.1: The atmospheric precipitation rates and magnetospheric injection rates of O2 for all three different cases under SOI phase and Equinox phase………………………………51
Table 5.1: The total ion impact flux (ions/s) into Titan’s exobase……………………………63
Table 5.2: The Total energy deposit rates (eV/s) into Titan’s exobase……………………….63

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指導教授

葉永烜(Wing-Huen Ip)

審核日期

2009-10-19

推文