Generation of Electron Cyclotron Harmonic Waves around the Moon

We study plasma wave generations around the moon based on the plasma wave data observed by the KAGUYA spacecraft which is the Japanese mission to the moon. The Wave-Form Capture receiver revealed that various plasma waves are excited due to moon-space plasma interactions. In the present paper, we focus on the Electron Cyclotron Harmonics (ECH) among the plasma wave phenomena taking place around the moon. The ECH waves have been widely studied in the relation to the electron precipitation in the terrestrial polar region due to the loss cone instability. However, that does not directly link to the observation of the ECH around the moon orbit at the distance of 60RE from the Earth. KAGUYA observes the ECH around its orbit very frequently. That is unlikely to occur without the moon. By examining the plasma wave data, we found the close relation of the ECH waves to the magnetic anomaly of the moon. The ambient magnetic fields at the ECH observation points are connected to magnetic anomalies on the surface of the moon. Lunar Prospector revealed the existence of electron loss cone over the magnetic anomaly. By the connection of the ambient magnetic field and the magnetic anomaly, the resultant mirror force causes the reflection of electrons with their velocity distributions above the loss cone angle. In addition to the loss cone distribution, Lunar Prospector found the existence of low energy electron beams that are reflected by the negative potential of the moon surface on the nightside. The KAGUYA plasma data show the consistent results with those by Lunar Prospector. Moreover we found the good correlation of the ECH waves to the loss cone distribution with low energy electron beams. It is well known that the loss cone velocity distribution of electrons destabilizes the ECH waves. However, the ECH waves cannot be seen over the magnetic anomaly in the dayside region of the moon when the moon stays inside the lobe region of the terrestrial magnetosphere. In order to clear the generation of the ECH waves, we calculated the linear growth rate by solving the kinetic plasma dispersion relation using the realistic plasma parameters based on the KAGUYA observation. The results show that the loss cone distribution is not enough to excite the ECH waves under the environments around the moon. We need the low energy electron beams accelerated away from the moon. The coexistence of the loss cone and low energy beam leads to the generation of the ECH waves. This is the reason why we do not see the ECH waves on the dayside of the moon when the moon is inside the lobe region. The low energy electron beam is missing in this region, because the potential of the moon surface is positively charged. In the present paper, we examine the observation of the ECH waves around the moon using the KGAUYA plasma wave data and discuss their generation model by using the linear dispersion analysis results.