MESSENGER observations of flux ropes in Mercury’s magnetotail

Abstract

We report an investigation of magnetic reconnection in Mercury’s magnetotail conducted with MESSENGER Magnetometer and Fast Imaging Plasma Spectrometer measurements during seven “hot seasons” when the periapsis of the spacecraft orbit is on Mercury’s dayside. Flux ropes are formed in the cross-tail current sheet by reconnection. We have analyzed 49 flux ropes observed between 1.7 RM and 2.8 RM (where RM is Mercury’s radius, or 2440km) down the tail from the center of the planet, for which minimum variance analysis indicates that the spacecraft passed near the central axis of the structure. An average Alfvén speed of 465kms−1 is measured in the plasma sheet surrounding these flux ropes. Under the assumption that the flux ropes moved at the local Alfvén speed, the mean duration of 0.74±0.15s determined for these structures implies a typical diameter of ~345km, or ~0.14 RM, which is comparable to a proton gyroradius in the plasma sheet of ~380km. We successfully fit the magnetic signatures of 16 flux ropes to a force-free model. The mean radius and core field determined in this manner were ~450km, or ~0.18 RM, and ~40nT, respectively. A superposed epoch analysis of the magnetic field during these events shows variations similar to those observed at Earth, including the presence of a post-plasmoid plasma sheet, filled with disconnected magnetic flux, but the timescales are 40 times shorter at Mercury. The results of this flux rope survey indicate that intense magnetic reconnection occurs frequently in the cross-tail current layer of this small but extremely dynamic magnetosphere.