A Persistent Proto-Magnetosheath formed by Protons Reflected above the South Pole-Aitken Magnetic Anomaly Cluster

Abstract:

Despite their small vertical scale, lunar magnetic anomalies can produce macroscopic, high-altitude compressional disturbances in the solar wind flow [1–5]. Most of these disturbances have been attributed to interactions between the solar wind and counter streaming protons reflected from magnetic structures at low altitudes (<30 km) above the South Pole–Aitken (SPA) magnetic anomaly cluster [4–6]. While previous studies suggest that such disturbances are rare in equatorial regions, hybrid plasma simulations [6] and remote energetic neutral atom observations [7] indicate that they may form frequently above the SPA cluster between 30°S and 90°S. However, in these higher-latitude regions, their occurrence rate and spatial morphology have not yet been characterized using in situ plasma instrumentation.

Using near-terminator ion observations from the Sub-keV Atom Reflecting Analyzer onboard Chandrayaan-1 [8], we find that strong solar wind disturbances, similar to those observed in equatorial regions [4,5], occur frequently between 30°S and 90°S above the SPA magnetic anomaly cluster. These disturbances are observed at 100 km altitude and extend well beyond the crustal fields of the SPA cluster (>1,000 km). Their spatial distribution and properties support the interpretation that they result from interactions between protons reflected by the SPA cluster and the solar wind, rather than from direct low-altitude interactions between the primary solar wind and lunar crustal magnetic fields.

These results suggest that protons reflected by the SPA cluster create a persistent proto-magnetosheath within which solar wind dynamics are governed by the same processes as those in early planetary magnetosheath formation. This proto-magnetosheath region forms above the mini-magnetosphere (altitude <~30 km), and can extend to 500–1,000 km altitude. These findings indicate that the SPA magnetic anomaly cluster dynamically influences solar wind precipitation over a much larger area (>1,000 km) than previously recognized, potentially causing near-surface plasma conditions to differ from those inferred using upstream measurements.

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