Hideo Tsunakawa

First direct detection of ions originating from the Moon by MAP‐PACE IMA onboard SELENE (KAGUYA)

[1] The Moon has no global intrinsic magnetic field and only has a very thin atmosphere. Ion measurements made from lunar orbit provide us with information regarding interactions between the solar wind and planetary surface, the surface composition through secondary ion mass spectrometry and the source and loss mechanisms of planetary tenuous atmosphere. An ion energy mass spectrometer MAP-PACE IMA onboard a lunar orbiter SELENE (KAGUYA) has detected low-energy ions at 100-km altitude. The MAP-PACE measurements have elucidated that the ions originate from the lunar surface and exosphere and that the ions are at least composed of He + , C + , O + , Na + and K + . Following the discovery of the lunar Na and K exospheres by the ground-based observation, MAP-PACE IMA have found the He, C and O exospheres around the Moon.

Simultaneous observation of the electron acceleration and ion deceleration over lunar magnetic anomalies

At ~25 km altitude over magnetic anomalies on the Moon, the deceleration of the solar wind ions, acceleration of the solar wind electrons parallel to the magnetic field, and heating of the ions reflected by magnetic anomalies were simultaneously observed by MAP-PACE on Kaguya. Deceleration of the solar wind ions was observed for two major solar wind ion compositions: protons and alpha particles. Deceleration of the solar wind had the same ΔE /q (ΔE: deceleration energy, q: charge) for both protons and alpha particles. In addition, the acceleration energy of the electrons was almost the same as the deceleration energy of the ions. This indicates the existence of an anti-moonward electric field over the magnetic anomaly above the altitude of Kaguya. The reflected ions were observed in a much larger area than the area where magnetic field enhancement was observed. These reflected ions had a higher temperature and lower bulk velocity than the incident solar wind ions. This suggests the existence of a non-adiabatic dissipative interaction between solar wind ions and lunar magnetic anomalies below Kaguya.

Ground calibration of the high-sensitivity SELENE lunar magnetometer LMAG

Ground calibration experiments of the SELENE high sensitivity fluxgate Lunar Magnetometer (LMAG) have been performed in order to determine the alignment, sensitivity, and offset of the sensors (MGF-S). It is checked out that the sensors are orthogonal to each other within 0.4 degrees, and the linearity of the ambient magnetic field and the output from the sensors are confirmed. Also, the temperature dependences of the offset and sensitivity are examined but no clear signatures of temperature dependencies can be seen. SELENE has an in-flight calibration system in order to determine the direction of the magnetometer routinely. The magnetic fields generated by the sensor alignment monitor coil (SAM-C) system are used for the in-flight calibration. The magnetic field distributions generated by SAM-C are determined and the accuracy of determination of the magnetometer position and direction is estimated. Multiple measurements will allow us to determine the direction of MGF-S with about 0.1-degree accuracy. Appropriate corrections from the results of the ground and in-flight calibrations will allow us to recover the magnetic field near the moon with accuracy about 0.1 nT.

Neocene stress field of the Japanese arcs and its relation to igneous activity

Abstract The Neogene stress field in the southern part of Northeast Japan and the eastern part of Southwest Japan was restored with special reference to its time scale, based mainly on K-Ar ages and strike directions of dike swarms. The present study indicates the changes of stress field at about 22 Ma B.P., 15 Ma B.P., 12 Ma B.P. and 8 Ma B.P. in the southern part of Northeast Japan and at about 15 Ma B.P. in the eastern part of Southwest Japan. The stress changes are also thought to have occurred at about 12 Ma B.P., 9 Ma B.P. and 1 Ma B.P. in the eastern part of Southwest Japan. These results imply that some common causes generated the stress changes in both Northeast and Southwest Japan at about 15 Ma B.P., 12 Ma B.P. and 8–9 Ma B.P. Major igneous activity in the Japanese arcs seems likely to have taken place almost at same times as the stress changes. The history of volcanic activity in the northwest Pacific region during the Miocene also shows the temporal correlation with the change of the stress regime. The tectonic evolution in subduction zones, therefore, appears to be characterized by the synchronism of stress changes and intensive igneous activity.

Room-temperature nanowire ultraviolet lasers: An aqueous pathway for zinc oxide nanowires with low defect density

We report the realization of a low-temperature aqueous pathway for the chemical synthesis of zinc oxide (ZnO) nanowires with low defect density and their room-temperature ultraviolet lasing behavior at low pump fluence. The concentration of solutes determined not only the size of individual nanowires, which influences their optical waveguiding behavior, but also their lattice defect density, which affects the efficiency of ultraviolet emission. The optimal synthesis conditions led to low-temperature growth of ZnO nanowires that showed room-temperature ultraviolet lasing at a low threshold of pump fluence. Based on our experimental results and optical waveguide theory, we report two important factors for realizing high-quality ZnO nanowires that show room-temperature ultraviolet lasing via a low-temperature aqueous approach: control of the density of defects generated in aqueous solutions and the optimal microstructure of the grown nanowires to produce strong optical confinement.

In-orbit calibration of the lunar magnetometer onboard SELENE (KAGUYA)

The high-sensitivity fluxgate Lunar MAGnetometer (LMAG) is mounted on SELENE (KAGUYA) to investigate the near-surface electromagnetic environment and the evolution of the Moon through magnetic field observation. To avoid possible electromagnetic interferences, a triaxial fluxgate sensor (MGF-S) is installed at the far end of a 12-m-long mast. It is critical for the accurate observation to monitor MGF-S alignment in orbit, and thus we have calibrated the sensor alignment by measuring the known magnetic fields generated by the sensor alignment monitor coil (SAM-C) wound onto the mast canister. In-orbit calibration of the MGF-S alignment was performed twice each revolution during the initial check-out phase of the satellite. It is concluded that there is no systematic difference in the sensor alignment between the day-side and night-side. Applying a new technique based on the Davis-Smith method to the observed magnetic field data when KAGUYA was exposed to the solar wind, a zero offset of each axis was quickly and stably determined every month. As a result, LMAG has been calibrated with an accuracy that is sufficient for detection of the lunar magnetic anomaly at an altitude of 100 km and for high-resolution electron reflectometry.

Surface vector mapping of magnetic anomalies over the Moon using Kaguya and Lunar Prospector observations

We have provided preliminary global maps of three components of the lunar magnetic anomaly on the surface applying the surface vector mapping (SVM) method. The data used in the present study consist of about 5 million observations of the lunar magnetic field at 10–45 km altitudes by Kaguya and Lunar Prospector. The lunar magnetic anomalies were mapped at 0.2° equi-distance points on the surface by the SVM method, showing the highest intensity of 718 nT in the Crisium antipodal region. Overall features on the SVM maps indicate that elongating magnetic anomalies are likely to be dominant on the Moon except for the young large basins with the impact demagnetization. Remarkable demagnetization features suggested by previous studies are also recognized at Hertzsprung and Kolorev craters on the farside. These features indicate that demagnetized areas extend to about 1–2 radii of the basins/craters. There are well-isolated central magnetic anomalies at four craters: Leibnitz, Aitken, Jules Verne, and Grimaldi craters. Their magnetic poles through the dipole source approximation suggest occurrence of the polar wander prior to 3.3–3.5 Ga. When compared with high-albedo markings at several magnetic anomalies such as the Reiner Gamma anomalies, three-dimensional structures of the magnetic field on/near the surface are well correlated with high-albedo areas. These results indicate that the global SVM maps are useful for the study of the lunar magnetic anomalies in comparison with various geological and geophysical data.

K-Ar dating on volcanic rocks in the Bonin Islands and its tectonic implication

Abstract The K-Ar dating method was applied to the volcanic rocks, the boninite and related rocks, from the Bonin Islands. Four fresh samples gave K-Ar ages of about 40 Ma. These ages suggest that the main volcanism of the Bonin Islands occurred about 40 Ma ago, as a result of the initiation of subduction of the Pacific plate at its western margin. Results of the K-Ar dating on other rocks suggest that some volcanism or thermal disturbance may have occurred 5 ~ 10 Ma ago, at least, in Chichijima.

Paleomagnetism of late Mesozoic rocks from northeastern China: the role of the Tan-Lu fault in the North China Block

Paleomagnetic studies were performed on Jurassic, Cretaceous and Tertiary rocks sampled from the Qitaihe area in Heilongjiang and Benxi area in Liaoning Provinces, northeast China. Both locations are near the Tancheng-Lujiang (Tan-Lu) fault system; Benxi is close to but on the eastern side of the fault while Qitahe lies between two major branches of the northwestern extension of this fault. In Mesozoic rocks, secondary magnetization in the present field direction was observed, but it was possible to retrieve the primary components by taking the high-temperature portion of the demagnetizing spectra. The Mesozoic poles thus obtained, especially those for the Cretaceous, deviate from the paleomagnetic poles of similar ages from the central part of the North China Block (NCB), Siberian Block or South China Block (SCB). Although the distances to the poles (flattening) are quite similar, the Benxi pole suggests a small clockwise rotation with respect to the central NCB poles, while the the Qitaihe poles indicate a much larger rotation in the opposite direction. It is shown that the deviation of the Benxi pole is similar to that observed for the Korean Peninsula and Shangdong Province, which all lie to the east of the Tan-Lu fault in the NCB. The Qitaihe pole position is quite different from the poles either west or east of the Tan-Lu fault. From these observations, it is concluded that a left-lateral strike-slip movement at the Tan-Lu fault system since the Cretaceous is the cause of systematic deviation in the position of the poles obtained from east of the fault including the Benxi area, while anomalous direction of Qitaihe rocks may represent a small scale rotation within the Tan-Lu fault system. The estimation of the movement along the Tan-Lu fault depends on which branch of the fault system is considered most active. If the main branch is assumed to be the place of slip, the movement can be represented by an Euler pole which lies to the south of Honshu Island (20°N, 150°E), with an estimated total displacement of 800 km since the Cretaceous.

Geomagnetic paleosecular variation for the past 5 Ma in the Society Islands, French Polynesia

We report a paleomagnetic secular variation in the Society Islands, French Polynesia for the past 5 Ma. Paleomagnetic measurements were performed on volcanic rocks applying thermal and alternating field demagnetizations and principal-component analysis. As a result, 82 normal, 48 reversed, and 10 intermediate directions were observed after the correction of the site localities for the absolute motion of the Pacific plate. A mean VGP of the combined data set of normal and reversed polarities is located at 87.5°N and 4.0°W (A95 = 2.3°), which is close to the geographic pole. The angular standard deviation (ASD) around the mean VGP resulted in 14.6° (+1.3°, −1.2°). There is no significant inclination anomaly for the time-averaged field. However, if the plate motion is not taken into account, a significant anomaly of −3.4° would be observed at a 95% confidence level. This suggests that correction for the absolute plate motion is needed for the precise paleodirectional analysis of the Pacific region. These features are also supported by an analysis combining the previous data set with our data.