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（Hässig 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。

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.

Altwegg, K., Balsiger, H., & Geiss, J. (1999). Composition of the volatile material in halley’s coma from in situ measurements. In Composition and origin of cometary materials (pp. 3-18). Springer
Ajello, J. M.. (1971a). Emission cross sections of CO2 by electron impact in.the interval 1260-4500 Å. J.Chem.Phys.,55,3169, doi:10.1063/1.676564
Ajello, J. M.. (1971b). Emission cross section of CO by electron impact in.the interval 1260-5000 Å. J.Chem.Phys.,55,3158, doi:10.1063/1.676563
Balsiger, H., Altwegg, K., Buhler, F., Geiss, J., Ghielmetti, A. G., et al. (1986). Ion composition and dynamics at comet Halley. Nature, 321, 330-334
Baumjohann, W., R. A. Treumann (1996). Basic Space Plasma Physics, Imperial Coll. Press, London
Bieler, A., Altwegg, K., Balsiger, H., et al. (2015). Abundant molecular oxygen in the coma of comet 67P/churyumov-gerasimenko. Nature, 526(7575), 678-81. doi:10.1038/nature1570
Bird, G.A. (1994). Molecular Gas Dynamics and the Direct Simulation of Gas Flows (Clarendon Press, Oxford,)
Blyth, R.C.G.; Powis, I.; Danby, C.J. (1981). Competing pre-dissociations of O2+(B 2Σg-). Chem. Phys. Lett., 84, 272.
Bodewits, D., Lara, L. M., A’Hearn, et al. (2016). Changes in the physical environment of the inner coma of 67P/Churyumov-Gerasimenko with decreasing heliocentric distance. The Astronomical Journal, 152(5), 130. doi:10.3847/0004-6256/152/5/13
Broiles, T. W., Livadiotis, G., Burch, J. L., Chae, K., et al. (2016). Characterizing cometary electrons with kappa distributions. Journal of Geophysical Research: Space Physics. 121(8), 7407-7422. doi:10.1002/2016ja02297
Burch, J. L., Goldstein, R., Cravens, T. E., et al. (2006). RPC-IES: The ion and electron sensor of the rosetta plasma consortium. Space Science Reviews, 128(1-4), 697-712. doi:10.1007/s11214-006-9002-
Bussieres, N. & Marmet, P. (1977). Ionization and dissociative ionization of CO2 by electron impact. Can. J. Phys., 55, 1889, doi:10.1139/p77-230
Chamberlain, J. W. (1962), Upper atmospheres of the planets, Astrophys. J., 136, 582–593, doi:10.1086/147409.
Crowe, A. & McConkey, J.W. (1974). Dissociative ionization by electron impact. III. O+, CO+ and C+ from CO2. J. Phys. B:,7, 349, doi: 10.1088/0022-3700/7/3/005
Churyumov, K. I., & Gerasimenko, S. I. (1972). Physical observations of the short-period comet 1969 IV. In Symposium-International astronomical union (Vol. 45, pp. 27-34)
Eland, J.H.D.& Berkowitz, J. (1977). Formation and predissociation of CO2+(C2Σ+g). J. Chem. Phys., 1977, 67, 2782
Feldman, P. D., A’Hearn, M. F., Bertaux, J.,et al. (2015). Measurements of the near-nucleus coma of comet 67P/churyumov-gerasimenko with the alice far-ultraviolet spectrograph on rosetta. Astronomy and Astrophysics, 583, A8. doi:10.1051/0004-6361/20152592
Fuselier, S. A., Altwegg, K., Balsiger, H., et al. (2015). ROSINA/DFMS and IES observations of 67P: Ion-neutral chemistry in the coma of a weakly outgassing comet. Astronomy and Astrophysics, 583, A2. doi:10.1051/0004-6361/20152621
Fuselier, S. A., Altwegg, K., Balsiger, H., et al. (2016). Ion chemistry in the coma of comet 67P near perihelion. Monthly Notices of the Royal Astronomical Society, 462(Suppl 1), S67-S77. doi:10.1093/mnras/stw214
Goetz, C., Koenders, C., Richter, I., Altwegg, K., et al. (2016). First detection of a diamagnetic cavity at comet 67P/churyumov-gerasimenko. Astronomy and Astrophysics, 588, A24. doi:10.1051/0004-6361/20152772
Gombosi, T. (2015), Physics of Cometary Magnetospheres, in Magnetotails in the Solar System, eds. A. Keiling, C. M. Jackman, & P. A. Delamere, Geophysical Monograph 207 (American Geophysical Union), 169
Grade, M., Wienecke, J., Rosinger, W., Hirschwald, W. (1983). Electron impact investigation of the molecules SeS(g) and TeSe(g) under high-temperature equilibrium conditions. Ber. Bunsen-Ges. Phys. Chem., 87, 355. doi: 10.1002/bbpc.19830870418
Hansen, K. C., Altwegg, K., Berthelier, J. -J.,et al. (2016). Evolution of water production of 67P/churyumov-gerasimenko: An empirical model and a multi-instrument study. Monthly Notices of the Royal Astronomical Society, stw2413.
Hässig, M., Altwegg, K., Balsiger, H., et al. (2015), Time variability and heterogeneity in the coma of 67P/Churyumov-Gerasimenko. Science, 347, 6220. doi: 10.1126/science.aaa0276
Hierl, P.M.& Franklin, J.L. (1967). Appearance potentials and kinetic energies of ions from N2, CO, and NO. J. Chem. Phys., 47, 3154. doi: 10.1063/1.1712367
Huebner, W.F., Mukherjee, J., (2015). Photoionization and Photodissociation rates in solar and blackbody radiation field. Planetary and Space Science. doi:10.1016/j.pss.2014.11.022
Huebner, W.F., Carpenter, C.W., (1979). Solar Photo Rates Coefficient. Los Alamos Scientific Laboratory report LA-8085-MS.
Huebner, W.F., Keady, J.J., Lyon, S.P., (1992). Solar Photo Rates for Planeatry Atmosphere and Atmospheric Pollutants. Astrophys Space Sci, 195: 1. doi:10.1007/BF00644558
Ip W. H., Axford W. I., (1982), in Wilkening L. L., ed., Proc. IAU Colloq. 61, Vol. 1, Comet Discoveries, Statistics, and Observational Selection. Univ. Arizona Press, Tucson, AZ, p. 588
Ip, W. -H., & Axford, W. I. (1987). The formation of a magnetic-field-free cavity at comet Halley. Nature, 325, 418.
Ip, W. -H. (2015). Estimates of the size of the ionosphere of comet 67P/churyumov--gerasimenko during its perihelion passage in 2014/2015. In Planetary exploration and science: Recent results and advances (pp. 271-278). Springer.
Itikawa, Y., & Mason, N. (2005). Cross sections for electron collisions with water molecules. Journal of Physical and Chemical Reference Data, 34, 1. doi: 10.1063/1.1799251
Kanik, I. (2003). Electron impact dissociative excitation of O 2 : 2. Absolute emission cross sections of the OI(130.4 nm) and OI(135.6 nm) lines . J. Geophys. Res., 108(E11). doi:10.1029/2000je001423
Kenneth R. Lang. (2011). The Cambridge Guide to the Solar System. Cambridge University Press,
Kimura, K., Katsumata, S., Achiba, et al. (1981). Ionization energies, Ab initio assignments, and valence electronic structure for 200 molecules. Handbook of HeI Photoelectron Spectra of Fundamental Organic Compounds, Japan Scientific Soc. Press, Tokyo,
Lee, S., von Allmen, P., Allen, M., et al. (2015). Spatial and diurnal variation of water outgassing on comet 67P/churyumov-gerasimenko observed from rosetta/MIRO in august 2014. Astronomy and Astrophysics, 583, A5. doi:10.1051/0004-6361/20152615
Le Roy, L., Altwegg, K., Balsiger, H., et al. (2015). Inventory of the volatiles on comet 67P/churyumov-gerasimenko from rosetta/ROSINA. Astronomy and Astrophysics, 583, A1. doi:10.1051/0004-6361/201526450
Lefaivre, D.; Marmet, P. (1978). Electroionization of D2O and H2O and study of fragments H+ and OH+. Can. J. Phys., 56, 1549. doi: 10.1139/p78-208
Madanian, H., Cravens, T. E., Rahmati, A., et al. (2016). Suprathermal electrons near the nucleus of comet 67P/churyumov-gerasimenko at 3 AU: Model comparisons with rosetta data. Journal of Geophysical Research: Space Physics, 121(6), 5815-5836. doi:10.1002/2016ja02261
Mandt, K. E., Eriksson, A., Edberg, N. J. T., et al. (2016). RPC observation of the development and evolution of plasma interaction boundaries at 67P/churyumov-gerasimenko. Monthly Notices of the Royal Astronomical Society, 462(Suppl 1), S9-S22. doi:10.1093/mnras/stw1736
McCulloh, K.E. (1976). Energetics and mechanisms of fragment ion formation in the photoionization of normal and deuterated water and ammonia, Int. J. Mass Spectrom. Ion Phys., 21, 333. doi:10.1016/0020-7381(76)80131-3
Morrison, J.D.; Traeger, J.C. (1973). Ionization and dissociation by electron impact. I. H2O and H2S, Int. J. Mass Spectrom. Ion Phys., 11, 77. doi: 10.1016/0020-7381(73)80001-4
Mumma, M. J., Stone, E. J., Borst, W. L., & Zipf, E. C. (1972). Dissociative excitation of vacuum ultraviolet emission features by electron impact on molecular gases. III. CO2. The Journal of Chemical Physics, 57(1), 68-75.. doi: 10.1063/1.1678019
Neubauer, F. M., Glassmeier, K. H., Pohl, M., et al. (1986). First results from the giotto magnetometer experiment at comet halley. Nature 321, 352 - 355, doi:10.1038/321352a0
Neubauer F. M., 1988, in Grewing M., Praderie F., Reinhard R., eds, Exploration of Halley’s Comet. Springer, Berlin, Heidelberg, p. 73
Potts, A.W. & Price, W.C. (1972). Photoelectron spectra and valence shell orbital structures of groups V VI hydrides, Proc. R. Soc. London A:, 326, 181
Vigren, E., & Galand, M. (2013). Predictions of ion production rates and ion number densities within the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko at perihelion. ApJ, 772(1), 33. doi:10.1088/0004-637x/772/1/3
Vigren, E., Galand, M., Eriksson, A. I., et al. (2015). On the electron-to-neutral number density ratio in the coma of comet 67P/Churyumov-Gerasimenko: Guiding expression and sources for deviations. ApJ, 812(1), 54. doi:10.1088/0004-637x/812/1/5
Schmidt, H U, and Rudolf Wegmann. Plasma Flow and Magnetic Fields in Comets. In Scientific and Experimental Aspects of the Giotto Mission. 1981.
Schmidt, H. U., Wegmann, R., Huebner, W. F., et al. (1988). Cometary gas and plasma flow with detailed chemistry. Computer Physics Communications, 49(1), 17-59. doi:10.1016/0010-4655(88)90214-7
Schwenn R., Ip W. H., Rosenbauer H., et al. (1988), in Grewing M., Praderie F., Reinhard R., eds, Exploration of Halley’s Comet. Springer, Berlin, p. 160
Rauer, H. (2007), Trans-Neptunian Objects and Comets, Saas-Fee Advanced Courses, Vol. 35 (Berlin: Springer), 165
Rabalais, J.W., Debies, T.P., Berkosky, J.L., et al. (1974). Calculated photoionization cross sections relative experimental photoionization intensities for a selection of small molecules, J. Chem. Phys., 61, 516. doi: 10.1063/1.1681926