The Lunar Wake as a Unique Plasma Physics Labortory

Introduction: The Moon is an obstruction in the outward flowing solar wind, leaving trailing solar wind plasma void in the anti-sunward direction. This trailing disturbance is called the lunar wake. Solar wind plasma incident on the lunar dayside is absorbed, and the region anti-sunward of the Moon contains a near vacuum, with anomalously low plasma densities. The solar wind eventually fills-in the void, but the exact process by which the near vacuum is replenished is still being studied and understood. As such, the lunar wake is an excellent basic plasma physics laboratory for understanding the general process of plasma expansion into a vacuum. Originally, the wake was considered primarily a magnetosonic disturbance, filling in the plasma void via a large-scale fluid structure that closes on itself near 5 lunar radii [1]. However, more recent observations of the wake with Wind and Lunar Prospector (LP) suggest a very different view of the wake as a kinetic (i.e., electron-scale) ion sonic disturbance [2]. The evidence for the change in view includes the following: 1) counter streaming, cross-tail ions beams observed within the wake tail consistent with the presence of an ambipolar E-field near the wake flanks [3], 2) a magnetic disturbance that is consistent with a diamagnetic current along the wake boundary [4], and 3) intense electrostatic wave activity detected in the central region of the lunar wake [5]. The ion beams, ambipolar E-fields, and electrostatic waves are not included in the older magnetosonic/hydrodynamic viewpoint of the wake [1] and thus some other framework is required to incorporate these important electron-scale phenomena. The ion sonic-driven/selfsimilar plasma expansion appears to best explain the Wind and LP observations most completely. Figure 1 shows the plasma expansion/lunar wake from a particle-in-cell code [6] that incorporates electron scale phenomena including the ambipolar Efields and wave activity. Note that the ion density wake extends over 30 lunar radii in the anti-sunward direction consistent with Wind observations of the disturbance at > 20 lunar radii. Also note the formation of the ambipolar E-field (strongest near the Moon) and downstream plasma wave activity. A New Description of Plasma Expansion into the Wake: The new view of the plasma expansion into the wake void is illustrated in Figure 2. The streaming solar wind is absorbed on the lunar frontside, leaving a trailing void antisunward of the Moon. Along the “sidewall” or flank of the wake, solar wind electrons with thermal velocities greater than the more massive ions migrate into the void ahead of the ions, breaking plasma quasi-neutrality [3]. In doing so, they form an inward-propagating electron “cloud”. However, an electrostatic field (ambipolar E-field) quickly forms quasi-normal to the wake flank to retard the electron migration and to accelerate ions. The accelerated ions are in the form of ion beams directed toward