Electromagnetic Ion Cyclotron Waves Detected by Kaguya and Geotail in the Earth's Magnetotail

Narrowband electromagnetic ion cyclotron waves first discovered by the Apollo 15 and 16 Lunar Surface Magnetometers were surveyed in the magnetic field data obtained by the Kaguya satellite at an altitude of ∼100 km above the Moon in the tail lobe and plasma sheet boundary layer of the Earth's magnetosphere. The frequencies of the waves were typically 0.7 times the local proton cyclotron frequency, and 75% of the waves were left hand polarized with respect to the background magnetic field. They had a significant compressional component and comprised several discrete packets. They were detected on the dayside, nightside, and above the terminator of the Moon, irrespective of the lunar magnetic anomaly, or the magnetic connection to the lunar surface. The waves with the same characteristics were detected by Geotail in the absence of the Moon in the magnetotail. The most likely energy source of the electromagnetic ion cyclotron waves is the ring beam ions in the plasma sheet boundary layer. Plain Language Summary The origin of 10 s waves that were found in the data from Apollo 15 and 16 Lunar Surface Magnetometers were studied by Japanese spacecraft Kaguya and Geotail. The 10 s waves were detected when the Moon was in the Earth's magnetotail, an extended region of the Earth's magnetic field on the antisolar side. They were thought to be generated by ions gyrating around the magnetic field, but the origin of the ions was not understood. Observations form Kaguya and Geotail revealed that the waves were found irrespective of the presence of the Moon, suggesting that the waves were generated by ions intrinsic in the Earth's magnetotail rather than the Moon. Key Points Narrowband electromagnetic ion cyclotron waves around 0.1 Hz were found on the lunar orbit in the Earth's magnetotail The EMIC waves were detected irrespective of the presence of the Moon The EMIC waves were thought to be generated through the cyclotron resonance with anisotropic ions in the plasma sheet boundary layer