KAGUYA is a lunar orbiting satellite launched from Tanegashima Space Center (TNSC) in September 2007. The satellite conducted observation of the Moon at around 100km above the lunar surface for more than one and a half years until it impacted onto the Moon on June 11, 2009. KAGUYA carried 14 science instruments. In this article, I will introduce the observation result of the low energy charged particle instrument MAP-PACE (MAgnetic field and Plasma experiment-Plasma-energy Angle and Composition Experiment).
The Moon is the closest object to the earth and mankind landed on its surface back in the 1960s. Nonetheless, the plasma environment around the Moon remains surprisingly little known. The plasma environmentEreferred to in this article denotes the electric fields, magnetic fields and the distribution of charged particles such as electrons and ions. In the 1960s and 1970s, satellites carrying low energy charged particle instruments observed the Moon. Because the performance of those instruments was poor compared to current ones, we were unable to obtain sufficient observational data. Satellite missions that visited the Moon in the intervening decades focused on lunar surface imaging. Thus, new data on the lunar plasma environment were not retrieved. The Lunar Prospector, a lunar orbiting satellite launched by the U.S. in 1998, had an instrument to observe electrons and acquired a great deal of information on electron distribution around the Moon. Data about ion distribution, however, were not available until the launch of KAGUYA.
Plasma called solar windEflows continually from the Sun at a very high speed of 500km/sec. The solar wind contains charged particles such as electrons and ions at a density of about a few particles per 1cm3. Ions in the solar wind are mainly hydrogen nuclei, followed by helium nuclei, oxygen ions, etc. Our earth has an intrinsic magnetic field. We can consider the earth as a large magnet. If a magnetic field exists, charged particles perform rotational motion (called the gyro motion), i.e. rotating around the field, and thus free movement of the particles becomes impossible. For this reason, electrons and ions in the solar wind cannot come near the earth freely. Instead, a region called the magnetosphere surrounding the earth is formed, whose plasma is distinguished from the solar winds plasma. On the other hand, there is no strong intrinsic magnetic field on the Moon. Moreover, since the Moon, unlike the earth, has no dense atmosphere, electrons and ions in the solar wind collide directly with the lunar surface. Observations by KAGUYA revealed that these collisions considerably affect the plasma environment around the Moon.
MAP-PACE consists of four sensors: two Electron Spectrum Analyzers (ESA), ESA-S1 and ESA-S2, to measure energy and the amount of electrons of less than 15keV: an Ion Energy Analyzer (IEA) to measure energy and the amount of ions of less than 28keV/q: and an IMA (Ion Mass Analyzer) to measure the mass of ions in addition to the functions of IEA. KAGUYA orbits the Moon in a fixed attitude in order to always orient the same side to the lunar surface. To measure electrons and ions coming from all directions, ESA-S1 and IMA were mounted on the satellite to enable observation of the hemisphere on the lunar-surface side, while ESA-S2 and IEA were mounted to enable observation of the hemisphere in the opposite direction of the Moon. Fig. 1 shows the observational views of IEA and IMA.