A survey of the reconnection layer around Mercury’s magnetopause

An anomalous reconnection layer (ARL) forms in the Earth’s magnetosheath near the magnetopause when the solar wind has a low Alfvén Mach number. This ARL structure is similar to a magnetic reconnection outflow region and is characterized by a weaker total magnetic field and a higher flux of energetic ions than those outside the ARL region. ARLs around Earth are rare because the solar wind at 1 AU mostly has high Alfvén Mach numbers. Based on the past literature, the solar wind around Mercury is typically in a low-Alfvén-Mach-number regime. Hypothetically, the ARL structures should be a common feature on Mercury’s magnetosphere. To test this, we identified 116 ARL-like crossing events using the magnetic field and ion flux data from the magnetometer and the fast imaging plasma spectrometer instrument onboard the MESSENGER satellite, respectively. We adopt the term ‘ARL-like’ because, while these structures represent plausible reconnection exhausts under Mercury’s prevalent low-Alfvén-Mach-number solar wind, MESSENGER’s lack of plasma bulk-velocity measurements precludes a definitive identification. Thus, they should be regarded as candidate ARLs rather than confirmed detections. Notably, the normalized thickness of ARL-like events statistically follows a log-normal distribution with a mean value of 0.12 (~ one-tenth of the local magnetosheath thickness, or ~132 km). Furthermore, the latitudinal distribution of this thickness exhibits a minimum near 30° N, suggesting that stable magnetic reconnection sites are likely located around this latitude. These results indicate that ARL-like structures are a recurring feature near Mercury’s magnetopause, offering new insights into solar wind–magnetosphere interactions in low-Alfvén-Mach-number environment, providing a testable hypothesis for the upcoming BepiColombo mission.