論文内容の要旨

論文題目

Structure and Dynamics of the Lunar Ionized Exosphere（月希薄電離大気の構造とダイナミクスに関する研究）

氏 名

田中 孝明

The Moon is surrounded by extremely tenuous alkali atmosphere. The atmosphere isenough thin to be regarded as exosphere which called surface-bounded exosphere (SBE).
The alkali components such as Na or K have been observed in order to understand thegeneration process and the transport mechanisms. The continuous ground-based
observations and laboratory experiments have confirmed that the alkali exosphericcomponents are produced by ion-induced desorption (sputtering), photon-stimulateddesorption (PSD), meteoriteinduced vaporization and thermal desorption from thesurface. The major loss process of the exospheric particles is photoionization andion-pickup process. The ion-pickup process is quite simple because the Moon has no
global dipole magnetic field. The ionized exospheric particles are transported by the
motional electric field E = −Vsw×Bsw where Vsw is the solar-wind bulk velocity and Bsw
is the interplanetary magnetic field. If we know the electric field, it is possible for us totrace back the ion trajectory and to know where these ions are produced. Though it has
been observed by ground-based technique on many occasions since its discovery, in-situ
observation of the lunar ions by satellite-borne plasma instruments is quite rare. In-situ
observation is another complementary way to study the lunar exosphere because thedetected ions include information of the exosphere.
Japanese lunar orbiter SELENE (KAGUYA), which was launched on September 14,

2007, has the in-situ plasma analyzers named MAP-PACE (MAgnetic field and Plasma
experiment - Plasma energy Angle and Composition Experiment) in order to investigate
the plasma environment at 100km altitude around the Moon. PACE consists of twosensors for low energy electron measurements: ESA (Electron Spectrum Analyzer)-S1,ESA-S2 and two sensors for low energy ion measurements: IEA (Ion Energy Analyzer)
and IMA (Ion Mass Analyzer). SELENE is a three-axis stabilized satellite. The PACE
sensor basically utilizes a method of a top hat electrostatic analyzer with angularscanning deflectors at the entrance and toroidal electrodes inside. The FOV (Field OfView) is electrically scanned between ±45° around the center of the FOV, that is 45°
inclined from the axis of symmetry. With two electron sensors and two ion sensors thatare installed on the +Z (ESA-S1 and IMA: looking down the lunar surface) and -Z(ESA-S2 and IEA: looking against the lunar surface) panels of the spacecraft, thethree-dimensional plasma distribution function is observed. This author has
participated deeply in development, calibration, and data evaluation of the PACEsensors.
IMA, one of the PACE sensors, had observed Moon-originating ions for more than 1 year
until June 11, 2009. We have analyzed the mass-identified ion observation data of IMA
by using the laboratory calibration data and numerical simulation. When the Moon is in
the solar-wind, generated Moon-originating ions are accelerated and transported by aninterplanetary electric field E = −Vsw×Bsw. At 100km altitude, these pick-up ions arepresumably not fully accelerated and have the energy below several hundred electron
volts under typical solar-wind condition. The observed ion energy represents theacceleration path length, and the energy distribution reflects the altitude distribution ofthe lunar exosphere. We analyzed the Moon-originating ion data observed by IMA, andseparated the surface-originated ion with the exospheric ions making use of thesolar-wind motional electric field.
The SELENE ion measurement in Earth's magnetosphere indicated that PSD is themajor generation process of the SBE. In the absence of plasma effects on the source
process, ion species of H+, He++, He+, C+, O+, Na+, K+ and Ar+ are definitively identified.
The ion fluxes were higher when the solar zenith angle was smaller, which is consistent
with the idea that the solar photon driven processes dominates in supplying exosphericcomponents. These Moon-originating ions are transported by the electrostatic potential
difference between the Moon and the satellite.
The lunar atmosphere composition experiment (LACE), a part of the Apollo science
program, set the lunar surface and accomplished firm detection of argon and helium
during the lunar night. The LACE argon data measured at the Apollo 17 showed theasymmetry between the sunset and sunrise. However the ground-based observations of
the lunar alkali exosphere have never showed these dawn-dusk asymmetry. We
calculated the daily variation of the pick-up ion flux, and obtained the SSE(selenocentric solar ecliptic) longitude dependence of the K ion flux. The SSE longitudedependence shows the distinguished dawn-dusk asymmetry. We modeled thisasymmetry by the photon-derived source including the depletion of the surface source.
In this study, we will report the confinement of the atmospheric source based on theresults of several data analysis and discuss the structure and the dynamics of the lunar
SBE.