MESSENGER and Mariner 10 flyby observations of magnetotail structure and dynamics at Mercury

The first (M1), second (M2), and third (M3) MESSENGER flybys of Mercury traversed the planet's magnetotail from 1.25 to 3.25 RM downstream of the planet, where RM is Mercury's radius (2440 km). The encounters took place under northward, southward, and variable‐polarity interplanetary magnetic field (IMF), respectively. The magnetic field strength B in Mercury's magnetotail follows a power law decrease with increasing antisunward distance ∣X∣, B ∼ ∣X∣G, with G varying from −5.4 for northward to −1.6 for southward IMF. Low‐latitude boundary layers (LLBLs) containing strong northward magnetic field were detected at the tail flanks during two of the flybys. The observed thickness of the LLBL was ∼33% and 16% of the radius of the tail during M1 and M3, respectively, but the boundary layer was completely absent during M2. Clear signatures of tail reconnection are evident in the M2 and M3 magnetic field measurements. Plasmoids and traveling compression regions were observed during M2 and M3 with typical durations of ∼1–3 s, suggesting diameters of ∼500–1500 km. Overall, the response of Mercury's magnetotail to the steady southward IMF during M2 appeared very similar to steady magnetospheric convection events at Earth, which are believed to be driven by quasi‐continuous reconnection. In contrast, the M3 measurements are dominated by tail loading and unloading events that resemble the large‐scale magnetic field reconfigurations observed during magnetospheric substorms at Earth. Key Points Mercury's magnetotail resembles that of Earth, but with thicker boundary layers Mercury exhibits both steady magnetospheric convection and subtorm‐type behavior Reconnection and plasmoid ejection at Mercury reoccurs with a ~30 s periodicity