MESSENGER observations of Mercury's dayside magnetosphere under extreme solar wind conditions

The structure of Mercury’s dayside magnetosphere is investigated during three extreme solar wind dynamic pressure events. Two were the result of coronal mass ejections (CMEs), and one was from a high-speed stream (HSS). The inferred pressures for these events are ~ 45 to 65 nPa. The CME events produced thick, low-β (where β is the ratio of plasma thermal to magnetic pressure) plasma depletion layers and high reconnection rates of 0.1–0.2, despite small magnetic shear angles across the magnetopause of only 27 to 60°. For one of the CME events, brief, ~ 1–2 s long diamagnetic decreases, which we term cusp plasma filaments, were observed within and adjacent to the cusp. These filaments may map magnetically to flux transfer events at the magnetopause. The HSS event produced a high-β magnetosheath with no plasma depletion layer and large magnetic shear angles of 148 to 166°, but low reconnection rates of 0.03 to 0.1. These results confirm that magnetic reconnection at Mercury is very intense, and its rate is primarily controlled by plasma β in the adjacent magnetosheath. The distance to the subsolar magnetopause is reduced during these events from its mean of 1.45 Mercury radii (RM) from the planetary magnetic dipole to between 1.03 and 1.12 RM. The shielding provided by induction currents in Mercury’s interior, which temporarily increase Mercury’s magnetic moment, was negated by reconnection-driven magnetic flux erosion.