Abstract
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.