Statistical study of broadband whistler-mode waves detected by Kaguya near the Moon

Broadband whistler-mode waves in the frequency range from 0.1 to 10 Hz are detected near the Moon by the Lunar Magnetometer (LMAG) on board Kaguya. The generation process and statistical properties of the waves have not been understood yet. We analyze the distributions of their occurrence and reveal that most of the waves are generated by the solar wind interaction with lunar crustal magnetic field. We also clarify that the waves are observed when Kaguya is connected by the ambient magnetic field with the lunar surface. The statistical study indicates that the broadband waves are observed in the vicinity of the region where narrowband whistler-mode waves in the frequency of near 1 Hz are observed, showing the close relationship between them. The analysis of the wave vector directions suggests that these two types of waves are different views of the same waves propagating in the solar wind frame. The narrowband waves are possibly explained by a part of the broadband waves largely Doppler shifted in the spacecraft frame. The present results suggest a possible scenario of the generation process of the two types of waves through the solar wind interaction with the crustal magnetic field.