Mercury’s southern inner magnetosphere is an unexplored region as it was not observed by earlier space missions. In October 2021, BepiColombo mission has passed through this region during its first Mercury flyby. Here, we describe the observations of SERENA ion sensors nearby and inside Mercury’s magnetosphere. An intermittent high-energy signal, possibly due to an interplanetary magnetic flux rope, has been observed downstream Mercury, together with low energy solar wind. Low energy ions, possibly due to satellite outgassing, were detected outside the magnetosphere. The dayside magnetopause and bow-shock crossing were much closer to the planet than expected, signature of a highly eroded magnetosphere. Different ion populations have been observed inside the magnetosphere, like low latitude boundary layer at magnetopause inbound and partial ring current at dawn close to the planet. These observations are important for understanding the weak magnetosphere behavior so close to the Sun, revealing details never reached before.

水星的南部内磁层是一个未经探索的区域，因为早期的太空任务没有观测到它。2021 年 10 月，BepiColombo 任务在首次飞越水星期间经过该地区。在这里，我们描述了 SERENA 离子传感器在水星磁层附近和内部的观测结果。在水星下游观察到间歇性高能信号，可能是由于行星际磁通绳，以及低能太阳风。在磁层外检测到低能离子，可能是由于卫星放气所致。白天磁层顶和弓激波穿越距离地球比预期的要近得多，这是磁层高度侵蚀的标志。在磁层内部观察到了不同的离子群，例如磁层顶入站时的低纬度边界层和黎明时靠近地球的部分环流。这些观测对于理解如此接近太阳的弱磁层行为非常重要，揭示了以前从未达到的细节。