要約のみTohoku University寺田直樹課

Masunaga  Kei

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Tohoku University博士(理学)博士学位論文 (Thesis(doctor))要約のみthesi

Masunaga, Kei

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Oxygen Ion Outflow from Venus and theMechanism of the Ion Acceleration著者 Masunaga  Kei学位授与機関 Tohoku University学位授与番号 11301甲第15576号URL http://hdl.handle.net/10097/58819AbstractAtmospheric escape from planets is one of the important issues to understandthe planetary atmospheric evolution. The lack of an intrinsic magnetic eld ofVenus causes a direct interaction between its upper atmosphere and the solarwind. The solar wind interaction makes ionospheric or exospheric ions energized,and thus drives ion losses from the Venusian upper atmosphere. The mechanismsof ion acceleration and the amount of ion losses from Venus are still uncertainthough many studies have been conducted to date. For dispelling this uncertainty,giving a better understanding of the mechanism for the ion outow is needed.This thesis describes the dependence of the oxygen ion outow from Venus ona certain solar wind condition, namely, on the direction of the interplanetary mag-netic eld (IMF), and the mechanism of the oxygen ion acceleration at the upperatmosphere of Venus. All works in the thesis use the data obtained from the Ana-lyzer of Space Plasma and Energetic Atoms (ASPERA-4) and the magnetometer(MAG) onboard Venus Express which has been orbiting Venus since April, 2006.To understand the eects of the IMF direction on the oxygen ion outow, theshape of the induced magnetosphere and O+ escape uxes in the nightside forthe perpendicular IMF case and the parallel IMF case, where the IMF directsperpendicular and parallel to the Venus-Sun line, respectively, are investigated.The comparison between the two IMF cases reveals that the formation of theplasma sheet, which is an oxygen ion outow channel, is controlled by the IMFdirection. For the perpendicular IMF case the IMF drapes simply around Venus,iresulting in the formation of a single plasma sheet, and thus oxygen ions escapefrom the single outow channel. For the parallel IMF case, on the other hand,the IMF drapes more complicatedly and this results in forming multiple plasmasheets, namely, forming multiple ion escape channels. Statistical analysis showsthat there is no signicant dierence in the oxygen escape rates between the twoIMF cases. In addition, it shows that temporal variations of the IMF direction donot aect the escape rate. Conclusion of this work is that the IMF direction doesnot change oxygen ion escape rate from Venus even though the morphology of theinduced magnetosphere largely depends on it.The velocity distribution functions of oxygen ions which can be obtained bythe Ion Mass Analyzer, a part of the ASPERA-4 instrument, provides importantinformation for identifying the ion acceleration processes. Statistical analysis ofthe O+ velocity distribution functions indicates that the O+ velocity distributionfunctions have broader distribution in the local convection electric eld directionthan other directions. Dierent characteristics in velocity distribution functionsare found in dierent magnetospheric regions. In the magnetosheath, the O+ ve-locity shifts from the initial phase of the ring distribution calculated from localproton velocity and magnetic eld to the bulk velocity of the local proton. Suchions are mostly observed in the +EL hemisphere where the local convection elec-tric eld points away from the surface. This indicates that ion pickup occurs inthe magnetosheath but the ions are immediately incorporated with the local pro-ton bulk ow with breaking their E x B drift motion. In the dayside inducedmagnetosphere in the +EL hemisphere, measurements show a scattered velocitydistribution function of O+. This velocity distribution function has two ion com-ponents depending on whether their gyro radius is larger or not than the scale ofthe induced magnetosphere. For O+ ions with small gyro radius (< 500 km), theO+ velocity distribution function appears on the middle phase of the ring distri-bution. On the other hand for the O+ ions with a large gyro radius (> 500 km),iithe O+ velocity distribution function is similar to the one in the magnetosheath.This means that two components of O+ ions exist in the induced magnetosphere:pickup ions subject to the E x B drift and ions moving with the local proton bulkvelocity. Since both ion components ow tailward, they are convected toward thenightside to escape. In the nightside of the induced magnetosphere, velocity dis-tribution function shows initial and last phase of the ring distributions togetherwith a ring in the plane where the local magnetic eld and the local convectionelectric eld lie. The possible location where such velocity distribution function isrealized could be the center of the plasma sheet. In summary, oxygen ion escapechannel at Venus are the magnetosheath in the +EL hemisphere and the plasmasheet in the induced magnetosphere.The studies shown in this thesis indicate that the ion escape at Venus is quiteconstant against the solar wind condition. Rather, it is possible that the escaperate of oxygen ions are determined by the amount of the ion source. Hisaki, theJapanese small spacecraft project, observes Venus upper atmosphere by using theEUV spectrometer aboard itself. The optical observation aimed to the ion escapeof Venus have never conducted to date. Using the EUV spectrometer, we will havechances to observe variations of dayside exospheric and ionospheric components,and the nightside escaping ions from Venus. Simultaneous observations of VenusExpress with Hisaki observations are planed. The observations may allow us tond relationships between the ion source and the escape rate.In the studies described in the thesis, cold ion escape, which is believed to havea big impact on the atmospheric escape from Venus, is missing due to the instru-mental restrictions. So, study of the dependence of the cold ion escape on the solarwind condition is needed in the future. NASA's mission MAVEN (Mars Atmo-sphere and Volatile EvolutioN mission), which was successfully launched November2013, will arrive at Mars in September 2014 and observe Martian atmospheric es-cape processes. MAVEN carries excellent instruments to observe escaping particlesiiifrom Mars. Mars has a similar plasma environment to Venus therefore there will begood opportunities to study the solar wind interaction with the Venus-like planetin the near future.iv

Oxygen Ion Outflow from Venus and the Mechanism of the Ion Acceleration

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金星超高層における酸素イオン流出とイオン加速メカニズムの研究

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