Electrostatic solitary wave modeling in lunar wake plasma.

This investigation is inspired by the first flyby of NASA's ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) mission, which observed the signatures of electrostatics waves in the lunar wake region. We have developed a lunar plasma model consisting of protons, α-particles, an electron beam originating from the solar wind and suprathermal electrons. A pseudopotential technique has been employed to investigate the existence of electrostatic solitary waves from first principles. Due to the presence of the beam, three harmonic modes may be excited, namely an ion-acoustic mode and two distinct beam-driven electron-acoustic modes, with different phase speed (to be referred to as the fast and slow mode). The coexistence of positive and negative polarity structures associated with the ion-acoustic mode has been examined. Only negative polarity structures may occur in relation with the fast (supersonic) or the slow (subsonic) electron-acoustic modes. The combined effects of the beam and electron superthermality have been analyzed parametrically. The results of this investigation are in good agreement with observations of electrostatic waves reported in the lunar wake region. Our findings should help unfold the (mostly unexplored) dynamical characteristics of nonlinear waves observed in the lunar wake region.