Abstract We apply a hybrid model (kinetic ions, fluid electrons) to provide context for MErcury Surface Space ENvironment, GEochemistry, and Ranging (MESSENGER) observations of Disappearing Dayside Magnetosphere (DDM) events at Mercury. Such events have been observed on four occasions and are caused by Coronal Mass Ejections completely removing the dayside magnetopause (MP) through erosion and compression. We investigate how different Interplanetary Magnetic Field (IMF) orientations, resulting in different reconnection rates, influence Mercury's magnetosphere under these extreme conditions. The model uses constant upstream conditions and the highest ram pressure reported for any DDM event to date. Our results are: (a) Under DDM conditions, a bow shock forms above Mercury's low‐latitude surface for northward or duskward IMF orientation. (b) However, when the IMF points southward, the solar wind impacts unhindered onto the equatorial dayside surface, and a shock develops only at high latitudes. In this case, the IMF is antiparallel to the dayside planetary field, and any closed field lines in the upstream hemisphere are eroded through reconnection. Hence, despite the favorable IMF orientation, no return plasma flow forms: protons largely travel along the outer flanks of the MP toward downstream. (c) The wakeside draping pattern seen during a DDM event in 2013 is consistent with a drop in upstream pressure by at least while MESSENGER passed through the magnetotail. The observed neutral sheet position suggests that the IMF changed from duskward to dawnward orientation during the spacecraft's passage through the magnetosphere. (d) Magnetotail twisting may have caused the southward displacement of the neutral sheet observed during the 2013 event.
Glebe et al. (Sun,) studied this question.