Diffuse Whistler-Mode Waves Detected by Kaguya in the Lunar Polar Region

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.