Futoshi Takahashi

Knowledge of shock remanent magnetization (SRM) property is crucial for interpreting the spatial change in a magnetic anomaly observed over an impact crater. This study conducted two series of impact-induced SRM acquisition experiments by varying the applied field and impact conditions, and the remanences of cube-shaped subsamples cut from shocked basalt containing single-domain titanomagnetite were measured to investigate the pressure and temperature dependence of the SRM intensity. The peak pressure and peak temperature distributions in the shocked samples were estimated using shock-physics modeling. SRM intensity was proportional to the apple field intensity up to 400 µT. The SRM intensities under different projectile conditions were consistent at the same pressure values. An empirical equation of SRM intensity is proposed to be the power function of pressure and a linear function of temperature, which can express the experimental SRM intensity values in a range of pressures up to 10 GPa and temperatures up to the Curie temperature. The magnetic anomaly estimation over an impact crater was demonstrated using the empirical equation, and the anomaly distribution shows a distinct feature approximated as a combination of two dipoles located at the basement of the crater and a deeper part.

The solar wind particles reflected by the lunar magnetic field are the major energy source of electromagnetic wave activities, such as the 100 s magnetohydrodynamic waves and the 1 Hz whistler‐mode waves generated by protons and the non‐monochromatic whistler‐mode waves generated by mirror‐reflected electrons. Kaguya found a new type of whistler‐mode waves at 100 km altitude above the polar regions of the Moon with a broad frequency range of 1–16 Hz. The waves appear diffuse in both the time and frequency domains, and their occurrence is less sensitive to the magnetic connection to the lunar surface. The polarization is right‐handed with respect to the background magnetic field, and the wave number vector is nearly parallel to the magnetic field perpendicular to the solar wind flow. The diffuse waves are thought to be generated by the solar wind ions reflected by the lunar magnetic field through cyclotron resonance. The resonant ions are expected to have a velocity component parallel to the magnetic field larger than the solar wind bulk speed; however, such ions were not always simultaneously detected by Kaguya. The waves may have been generated above the dayside of the Moon and then propagated along the magnetic field being convected by the solar wind to reach the polar regions to be detected by Kaguya.

The solar wind particles reflected by the lunar magnetic field are the major energy source of electromagnetic wave activities, such as the 100 s magnetohydrodynamic waves and the 1 Hz whistler-mode waves generated by protons and the non-monochromatic whistler-mode waves generated by mirror-reflected electrons. Kaguya found a new type of whistler-mode waves at 100 km altitude above the polar regions of the moon with a broad frequency range of 1–16 Hz. The waves appear diffuse in both the time and frequency domains, and their occurrence is less sensitive to the magnetic connection to the lunar surface. The polarization is right-handed with respect to the background magnetic field, and the wave number vector is nearly parallel to the magnetic field perpendicular to the solar wind flow. The diffuse waves are thought to be generated by the solar wind ions reflected by the lunar magnetic field through cyclotron resonance. The resonant ions are expected to have a velocity component parallel to the magnetic field larger than the solar wind bulk speed; however, such ions were not always simultaneously detected by Kaguya. The waves may have been generated above the dayside of the moon and then propagated along the magnetic field being convected by the solar wind to reach the polar regions to be detected by Kaguya.

Solar wind particles reflected by the lunar magnetic field are major energy source of electromagnetic wave activities around the moon. In addition to known waves such as 100 s magnetohydrodynamic waves and 1 Hz whistler mode waves generated by protons, or non-monochromatic whistler mode waves generated by mirror-reflected electron beams, Kaguya found a new type of whistler mode waves at an altitude of 100 km above the polar regions of the moon, not above intense magnetic anomalies. The frequency range 1-16 Hz was broad like the non-monochromatic waves generated by electron beams, but their occurrence was less sensitive to the magnetic connection like the waves generated by reflected protons. They appear diffuse both in time and frequency domains, and the polarization was right-handed with respect to the background magnetic field. The wave number vector was nearly parallel to the background magnetic field which was perpendicular to the solar wind flow. The diffuse waves are thought to be generated by solar wind ions reflected by the lunar magnetic field through cyclotron resonance. The resonant ions were expected to have velocity component parallel to the magnetic field larger than the solar wind bulk speed, but such ions were not always detected simultaneously. There is a possibility that the waves were generated above the dayside moon and then propagated along the magnetic field convected by the solar wind to reach polar regions to be detected by Kaguya.

A structure of polarization of magnetic field variation in the frequency range from 0.01 to 0.3 Hz was found by Kaguya in the lunar wake. In a dawnward‐directed magnetic field, the polarization was left‐handed in the southern hemisphere and right‐handed in the northern hemisphere of the lunar wake. The sense of rotation was consistent with vortices behind an obstacle in a fast stream, although no such vortices were detected by Kaguya. The direction of magnetic field variation was predominantly perpendicular to the background magnetic field, suggesting that the waves were of shear Alfvén mode. The same polarization structure was found in rotated magnetic fields. In a selenocentric coordinate system with x axis antiparallel to the solar wind flow and the magnetic field in x‐z plane, the polarization was dependent only on y position of the spacecraft. The polarization structure was stable and unchanged in various solar wind conditions showing no preferences. It was observed in the magnetosheath as well as in the solar wind, but not in the terrestrial magnetosphere. The power density showed preferences to high‐temperature, high‐speed solar wind. The power density was higher at the wake boundary than in the center of the wake, suggesting that the magnetic fluctuations were generated at the wake boundary by ions refilling the wake, and propagate into the deep wake as shear Alfvén waves.

Reversal of unified polarization of low-frequency magnetic field variation was found by Kaguya at equator crossings in the lunar wake under the condition of dawn-dusk directed background magnetic field. The polarization was left-handed over a frequency range from 0.01 to 0.3 Hz in the southern hemisphere of the lunar wake in the dawnward-directed magnetic field. It turned to be right-handed when the spacecraft entered the northern hemisphere of the wake. In the duskward-directed magnetic field the polarization reversed. The same configuration by 90 degrees rotated was observed in northward-directed magnetic field, too. The sense of rotation was alike that of surface waves generated by Kelvin-Helmhlotz instability, but it was not likely because the waves of unified polarization were not always accompanied by velocity shear. They penetrated into the very center of the wake, suggesting that they were not propagating along a surface of wake boundary. The polarization was highly elliptic except for high frequency component. The magnetic field variation was predominantly perpendicular to the background magnetic field, suggesting that they were shear Alfven waves. The mechanism of unifying the polarization is not yet understood.

Carbon is a volatile element that has a considerable influence on the formation and evolution of planetary bodies, although it was previously believed to be depleted in the Moon. We present observations by the lunar orbiter KAGUYA of carbon ions emitted from the Moon. These emissions were distributed over almost the total lunar surface, but amounts were differed with respect to lunar geographical areas. The estimated emission fluxes to space were ~5.0 × 10⁴ per square centimeter per second, which is greater than possible ongoing supplies from the solar wind and micrometeoroids. Our estimates demonstrate that indigenous carbon exists over the entire Moon, supporting the hypothesis of a carbon-containing Moon, where the carbon was embedded at its formation and/or was transported billions of years ago.