Anomalous deformation of the Earth's bow shock in the lunar wake: Joint observations by Chang'E-1 and SELENE

Along its orbital motion around the Earth, the Moon crosses the bow shock (BS) at the dusk and dawn flanks. Meanwhile, behind of the Moon along the solar wind (SW) flow forms a tenuous region called lunar wake, because the SW plasma is obstructed by the Moon. Here we report, with simultaneous observations by Chang'E-1 and SELENE, that the Earth's BS surface is locally deformed in the lunar wake. Despite the quasi-perpendicular shock configuration encountered at dusk-flank under the Parker-spiral magnetic field, no clear shock surface can be found in the lunar wake, while instead gradual transition of the magnetic field from the upstream to the downstream value was found for a several-minute interval. This finding suggests that the 'magnetic ramp' is highly broadened in the wake where the flow can be sub-Alfvenic due to the lowest density and that a fast-mode shock is no longer maintained there.