Protons Reflected by Lunar Magnetic Anomalies Persistently Alter the Solar Wind Flow

Abstract Near‐terminator Chandrayaan‐1 observations reveal that protons reflected by the South Pole–Aitken (SPA) lunar magnetic anomaly frequently alter the solar wind flow above 100 km altitude. These disturbances occurred under a wide range of solar wind conditions and resembled earlier compressional events reported in equatorial regions. They were observed at ∼100 km altitude and extended well beyond the crustal fields of the SPA magnetic cluster (>1,000 km). The disturbances were characterized by plasma heating, bulk flow deflections of up to 15° for protons and 7° for alpha particles, and 100 eV/q ion decelerations. Their spatial distribution and characteristics indicate that they resulted from interactions between protons reflected by the SPA cluster and the solar wind, rather than direct low‐altitude interactions between the primary solar wind and lunar crustal magnetic fields. We interpret these observations as evidence that protons reflected by the SPA magnetic cluster create a persistent proto‐magnetosheath region that extends well above 100 km altitude and disturbs global solar wind precipitation at the surface.