亀田 真吾

Mercury possesses a weak, internal, global magnetic field that supports a small magnetosphere populated by charged particles originating from the solar wind, the planet's exosphere and surface layers. Mercury's exosphere is continuously refilled and eroded through a variety of chemical and physical processes acting in the planet's surface and environment. Using simultaneous two-point measurements from two satellites, ESA's future mission BepiColombo will offer an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with solar radiation and interplanetary dust. The expected data will provide important insights into the evolution of a planet in close proximity of a star. Many payload instruments aboard the two spacecraft making up the mission will be completely, or partially, devoted to studying the close environment of the planet as well as the complex processes that govern it. Coordinated measurements by different onboard instruments will permit a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone. Thus, an important feature of the BepiColombo mission is that simultaneous two-point measurements can be implemented at a location in space other than the Earth. These joint observations are of key importance because many phenomena in Mercury's environment are temporarily and spatially varying. In the present paper, we focus on some of the exciting scientific goals achievable during the BepiColombo mission through making coordinated observations. (C) 2008 Elsevier Ltd. All rights reserved.

The Mercury Sodium Atmosphere Spectral Imager (MSASI) on the Mercury Magnetospheric Orbiter (MMO) of the JAXA/ESA Bepi-Colombo (BC) Mission will address a range of fundamental scientific questions pertaining to Mercury's exosphere. The measurements will provide new information on regolith-exosphere-magnetosphere coupling as well as new understanding of the dynamics governing the exosphere bounded by the planetary surface, the solar wind and interplanetary space. MSASI is a high-dispersion visible spectrometer working in the spectral region near the sodium D(2) emission (589 nm), a major constituent of the Mercury exosphere. A single high-resolution Fabry-Perot etalon is used in combination with a narrow-band interference filter to achieve a compact and efficient instrument design. The etalon and filter are extremely stable with respect to long-term aging and temperature variations. Full-disk images of the planet are obtained by means of a single-axis scanning mirror in combination with the spin of the MMO spacecraft. This paper presents an overview of the MSASI and the design of the Fabry-Perot interferometer used as its spectral analyser. It is concluded that: (1) The MSASI optical design is practical and can be implemented without new or critical technology developments. (2) The thermally stable etalon design is based on concepts, designs and materials that have a good space heritage. (3) The MSASI instrument will achieve a high signal-to-noise ratio (SNR) (> 10) in the range of 2K-10M Rayleigh. (C) 2008 Elsevier Ltd. All rights reserved.

Our understanding of plasmaspheric dynamics has increased in recent years largely due to the information generated during the IMAGE-EUV mission. Even though this successful mission has ended, we have succeeded in imaging the terrestrial helium ions (He(+)) by the Telescope of Extreme Ultraviolet (TEX) aboard the Japanese lunar orbiter KAGUYA by detecting resonantly scattered emission at 30.4 nm. The view afforded by the KAGUYA orbit encompasses the plasma (He(+)) distribution in a single exposure, enabling us to examine for the first time the globally averaged properties of the plasmasphere from the "side" (meridian) perspective. The TEX instrument observed a medium-scale density structure in the dawnside plasmasphere during a quiet period (1-2 June 2008). The meridian shape of the structure clearly agreed with the dipole magnetic field line. The TEX instrument also observed the structure in the plasmasphere co-rotating with a duration of 26 h, which is consistent with results from a number of recent studies derived from the IMAGE-EUV mission. These results confirm that the TEX instrument successfully obtained the spatial distribution and temporal variation of the plasmasphere.

The interplanetary dust (IPD) distribution in the inner solar system is not yet well understood because of lack of direct dust measurements in the inner solar system and so one needs to rely on zodiacal light observations that are difficult to interpret. Mercury has an unstable atmosphere, and the source processes of Na in its atmosphere are unclear. Results of past observations have revealed that the atmospheric Na density has no or low correlation with the solar flux, sunspot number, heliocentric distance, or solar radiation pressure. We show that the variability of Mercury's atmospheric Na density depends strongly on the IPD distribution. That is, Na density is low (high) when Mercury is far away from (close to) the symmetry plane of IPD, and so one can infer the IPD distribution near Mercury orbit from the temporal variability of Na density in Mercury's atmosphere. Citation: Kameda, S., I. Yoshikawa, M. Kagitani, and S. Okano (2009), Interplanetary dust distribution and temporal variability of Mercury's atmospheric Na, Geophys. Res. Lett., 36, L15201, doi:10.1029/2009GL039036.

The first successful observations of resonant scattering emission from the lunar sodium exosphere were made from the lunar orbiter SELENE (Kaguya) using TVIS instruments during the period 17-19 December, 2008. The emission intensity of the NaD-line decreased by 12 +/- 6%, with an average value of 5.4 kR (kilorayleighs) in this period, which was preceded, by I day, by enhancement of the solar proton flux associated with a corotating interaction region. The results suggest that solar wind particles foster the diffusion of sodium atoms or ions in the lunar regolith up to the surface and that the time scale of the diffusion is a few tens of hours. The declining activity of the Geminid meteor shower is also one possible explanation for the decreasing sodium exosphere.

The Upper Atmosphere and Plasma Imager (UPI) was placed in a lunar orbit in order to Study both the Moon and Earth. The UPI consists of two telescopes: a Telescope of Extreme Ultraviolet (TEX) and a Telescope of Visible Light (TVIS), which are both mounted on a two-axis gimbals system. The TVIS is equipped with fast catadioptric optics and a high-sensitivity CCD to image swift aurora and dark airglow ill the terrestrial upper atmosphere. TVIS has a field-of-view equivalent to the Earth's disk as seen from the Moon. The spatial resolution is about 30 km x 70 km on the Earth's surface at auroral latitudes. The observation wavelengths can be changed by selecting different bandpass filters. Using the images of the northern and Southern auroral ovals taken by TVIS, the intensities and shapes of the conjugate auroras will be quantitatively compared. Using the an-low imaging, medium- and large-scale ionospheric disturbances will be Studied. In this paper, the instrumental design and performance of TVIS are presented.

A rare, but normal, astronomical event occurred on November 9th 2006 (JST) as Mercury passed in front of the Sun from the perspective of the Earth. The abundance of the sodium vapor above the planet limb was observed by detecting an excess absorption in the solar sodium line D, during this event. The observation was performed with a 10-m spectrograph of Czerny-Turnar system at Domeless Solar Tower Telescope at the Hida Observatory in Japan. The excess absorption was red-shifted by 10 pm relative to the solar line, and was measured at the dawnside (eastside) and duskside (westside) of Mercury. Between the dawn and dusksides, an asymmetry of total sodium abundance was clearly identified. At the dawnside, the total sodium column density was 6.1 x 10(10) Na atoms/cm(2), while it was 4.1 x 10(10) Na atoms/cm(2) at the duskside. The investigation of dawn-dusk asymmetry of the sodium exosphere of Mercury is a clue to understand the release mechanism of sodium from the surface rock. Our result suggests that a thermal desorption is a main source process for sodium vapor in the vicinity of Mercury. (C) 2008 Elsevier Ltd. All rights reserved.

The Upper Atmosphere and Plasma Imager (UPI) is to be launched in 2007 and sent to the Moon. From the lunar orbit, two telescopes are to be directed towards the Earth. The Moon has no atmosphere, which results in there being no active emission near the spacecraft; consequently, we will have a high-quality image of the near-Earth environment. As the Moon orbits the Earth once a month, the Earth will also be observed from many different directions. This is called a "science from the Moon". The two telescopes are mounted on a two-axis gimbal system, the Telescope of Extreme ultraviolet (TEX) and Telescope of Visible light (TVIS). TEX detects the O II (83.4 nm) and He II (30.4 nm) emissions scattered by ionized oxygen and helium, respectively. The targets of extreme-ultraviolet (EUV) imaging are the polar ionosphere, the polar wind, and the plasmasphere and inner magnetosphere. The maximum spatial and time resolutions are 0.09 Re and 1 min, respectively.

The Mercury's sodium atmosphere spectral imager (MSASI) on BepiColombo (BC) will address a range of fundamental scientific questions pertaining to Mercury's exosphere. The measurements will provide new information on regolith-exosphere-magnetosphere coupling as well as new understanding of the dynamics governing the exosphere bounded by the planetary surface, the solar wind and interplanetary space. MSASI is a high-dispersion visible spectrometer working in the spectral range around sodium D2 emission (589 nm). A tandem Fabry-Perot etalon is used to achieve a compact design. A one-degree-of-freedom scanning mirror is employed to obtain full-disk images of the planet. This paper presents an overview of the MSASI and the design of its spectral analyzer, which uses a Fabry-Perot interferometer. We conclude that: (1) The MSASI optical design is practical and can be implemented without new or critical technology developments. (2) The thermally tuned etalon design is based on concepts, designs and materials that have good space heritage. (3) The MSASI instrument achieves a high SNR (> 10) in the range of 2k-10 MRayleigh. (C) 2007 Elsevier Ltd. All rights reserved.

We conducted continuous spectroscopic observations of the Mercury's sodium exosphere with a 188 cm telescope and a high dispersion echelle spectrograph, for 1-6h in the daytime on December 4, 13, 14, and 15, 2005. To correct the images of the sodium emission blurred by Earth's atmosphere, the observed distribution was deconvolved with the point spread function which was obtained using Hapke's surface reflection model and the observed surface reflection. The average column density of sodium atoms was 1-2 x 10(11) atoms/cm(2) and significant diurnal changes were not observed. However, the sodium densities at low latitudes and high latitude changed during the observation and the rate of change in density at low latitude was higher than that at high latitude on December 14 and 15. Although the rates of suggested release processes are higher than the observed rate, the suggested release processes cannot explain the rapid change in density at low latitude. This may suggest the effect of transport of neutral atoms and the recycling of ions to the surface dominates the time variation in the spatial distribution of exospheric sodium atoms on Mercury. (C) 2007 Elsevier Ltd. All rights reserved.

The Mercury Sodium Atmosphere Spectral Imager (MSASI) in the BepiColombo mission will address a wealth of fundamental scientific questions pertaining to the Mercury's exosphere. Together, our measurements on the overall scale will provide ample new information on regolith-exo sphere-magneto sphere coupling as well as new understanding of the dynamics governing the 'surface-bounded exosphere'. It arises quite clearly from ground-based observations that the regolith of Mercury releases a fraction of its content to the exosphere. Some processes are identified up to now as leading to this ejection, e.g., photon-stimulated desorption. These processes are associated with different energies of ejection and behaviors in different regions of Mercury's surface. Therefore, different types of population are born from the surface, depending on the processes. The MSASI measurements definitely can identify the release processes, how exospheric sodium is born from the regolith. MSASI/BepiColombo is the first and unique opportunity to study the formation, circulation, and maintenance of the surface-bounded exosphere. (c) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

我々はBepiColombo水星探査計画の磁気圏探査機(MMO)の搭載機器として、ナトリウム大気の運動を可視化するカメラ(MSASI)を開発している。これはファブリペロー干渉計を用いてナトリウムD2線を分光観測する装置である。 水星のナトリウム大気は地上からの観測により、その放出機構として太陽光による光脱離、微小隕石の衝突による気化、太陽風イオンによるスパッタリングが主な候補として考えられている。今回この三つの場合でそれぞれ数値計算を行いナトリウムのコラム密度を求めた結果、放出機構によりナトリウム大気の分布に特徴が表れることが確認された。さらにMSASIによる撮像を想定し、MMOの軌道・太陽活動度・ナトリウム大気の明るさなどを考慮することでD2線の強度分布を計算した結果、コラム密度の場合と同様に放出機構によってそれぞれ特徴的な大気分布がMSASIで撮像できることを確認した。今回の計算結果より水星探査機に搭載する大気カメラ(MSASI)での観測が実現することで、発見以来謎とされていた水星大気の生成メカニズムを解明することが可能であると結論付けた。

The Mercury Sodium Atmosphere Spectral Imager (MSASI) in the BepiColombo mission will address a wealth of fundamental scientific questions pertaining to the Mercury's exosphere. Together, our measurements on the overall scale will provide ample new information on regolith-exosphere-magnetosphere coupling as well as new understanding of the dynamics governing the 'surface-bounded exosphere'. It arises quite clearly from ground-based observations that the regolith of Mercury releases a fraction of its content to the exosphere. Some processes are identified up to now as leading to this ejection, e.g., photon-stimulated desorption. These processes are associated with different energies of ejection and behaviors in different regions of Mercury's surface. Therefore, different types of population are born from the surface, depending on the processes. The MSASI measurements definitely can identify the release processes, how exospheric sodium is born from the regolith. MSASI/BepiColombo is the first and unique opportunity to study the formation, circulation, and maintenance of the surface-bounded exosphere. (c) 2006 COSPAR. Published by Elsevier Ltd. All rights reserved.

The Mercury Sodium Atmosphere Spectral Imager (MSASI) on the BepiColombo/Mercury Magnetospheric Orbiter (MMO) is a high-dispersion Fabry-Perot imager. MSASI will address a wealth of fundamental scientific questions pertaining to the Mercury's exosphere. Together, our measurement on the overall scale will provide ample new information on regolith-exospheremagnetosphere coupling as well as new understanding of the dynamics governing the 'surfacebounded exosphere'. It arises quite clearly from continuous ground-based observations that the regolith of Mercury releases a fraction of its content to Mercury's exosphere. Some processes are identified up to now as leading to this ejection. These processes are associated with different energies of ejection, behavior in different regions of Mercury's surface and eject different types of population from the surface. The responsible processes are (1) Chemical sputtering, (2) Thermal desorption, (3) Photon-stimulated desorption, (4) Ion sputtering, and (5) Micro-meteoroid impact/vaporization. Each candidate seems to be fairly operative, but any cannot completely explain phenomena observed from the Earth. Also, the fate of ejecta from the regolith is still unknown. Some are expected to return to the lithosphere, the other are lost to interplanetary space. Circulation of lithospheric sodium atoms via exosphere-magnetosphere might bring a significant change in the composition of surface layer on Mercury. The MSASI measurements clearly and definitely can identify the release mechanism, how exospheric sodium is born from the regolith, and bring comprehensive picture of global circulation of regolith materials. Also, BepiColombo is the first and unique opportunity to study the formation, circulation, maintenance of this surface-bounded exosphere, which is a different type of terrestrial atmosphere. Below we describe in somewhat more detail the primary scientific objectives of MSASI.

Extreme and far ultraviolet imaging spectrometers are proposed for the low-altitude orbiter of the BepiColombo mission. The UV instrument, consisting of the two spectrometers with common electronics, aims at measuring (1) emission lines from molecules, atoms and ions present in the Mercury's tenuous atmosphere and (2) the reflectance spectrum of Mercury's surface. The instrument pursues a complete coverage in UV spectroscopy. The extreme UV spectrometer covers the spectral range of 30-150 nm with the field of view of 5.0degrees, and the spectrum from 130 to 430 nm is obtained by the far UV spectrometer. The extreme UV spectrometer employs multi-layer coating technology to enhance its sensitivity at particular emission lines. This technology enables us to identify small ionospheric signatures such as He II (30.4 nm) and Na II (37.2 nm), which could not be detected with conventional optics. (C) 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

"我々は,極端紫外領域にある水星大気の共鳴散乱光を分光する極端紫外分光撮像器(EUIS:Extreme Ultraviolet Imaging Spectrometer)を水星探査計画に向け開発している.本稿では,本観測器開発の鍵となる多層膜回折格子の性能試験について報告する.結論は以下の通りである.(a)機械刻線回折格子とホログラフィック回折格子のどちらが適しているか調査し,その結果迷光の少なさという点からホログラフィック回折格子が優れていると結論した.(b)ホログラフィック平面回折格子にMoとSiのペアからなる多層膜コーティングと金コーティングを施し,回折効率を比較した.多層膜回折格子が金単層膜回折格子よりも1桁ほど効率が高いことを確認した."