GEOTAIL observation of upstream ULF waves associated with lunar wake

Left-handed, circular polarized ULF waves with frequency of 0.3-1.1 Hz were detected by GEOTAIL at 27 lunar radii upstream of the moon when the spacecraft was magnetically connected with the lunar wake. The wave was detected twice at 16:45-17:00 and 18:55-19:02 on October 25, 1994, when the spacecraft and the moon were on the dawn side of the Earth's magnetosphere. The ULF wave was propagating in a direction nearly parallel to the background magnetic field. The observed frequency and polarization are explained by reversal of polarization of right-handed, sunward-propagating electron whistler waves with frequencies above 1.4 Hz in the solar wind frame of reference, which were excited through the interaction with electron beams flowing in anti-sunward direction downstream of the lunar wake. The downstream flow of electron beam is explained by filtering effect of the potential drop at the boundary of the lunar wake. Low-energy components of electrons are reflected back by the potential drop, and the rest components, with energies higher than that of the electric potential penetrate through the wake. The velocity distribution of downstream electrons would be modified to have some bump or shoulder in energy range to form a beam, which is likely to excite whistler mode wave through cyclotron resonance. The lowest energy of the resonant electrons was calculated to be 0.96-2.5 (keV) from the lower boundary of the detected frequency. The variation in the lowest frequency suggests that there are some regions of the lunar wake where potential drop is reduced.