ARTEMIS Observations of Well-structured Lunar Wake in Subsonic Plasma Flow

Typical lunar wakes are usually formed in the supersonic background plasma in the solar wind or distant magnetosheath because the plasmas do not have enough time to refill into the wake with thermal velocities. Here we present ARTEMIS observations of a well-structured lunar wake under subsonic plasma near the Earth's magnetopause, where the background plasma flows are nearly field-aligned and have relatively low plasma beta. This lunar wake is quite different from the typical lunar wakes in the supersonic solar wind. The boundaries of this lunar wake are clearly outside the boundaries of the optical shadow, indicating that there is no gradual refilling of ambient plasma into the wake. Meanwhile, no enhancement of magnetic strength in the lunar wake is evident for the absence of diamagnetic currents at the boundaries of the lunar wake. The significantly strong gradient of plasma pressure at the wake boundary cannot be maintained under the subsonic condition because the rarefaction wake can propagate upstream and thus the Mach cone outside the wake will disappear so that the upstream plasma pressure supplement will decrease. Our results also show that the very low plasma beta plays a crucial role in the formation of the lunar wake in subsonic plasma.