MESSENGER Observations of Giant Plasmoids in Mercury’s Magnetotail

Small-scale flux ropes, with estimated diameters of ∼500 km and that pass over the MESSENGER spacecraft on timescales of seconds or less, are a common feature in Mercury’s magnetosphere. These magnetic structures, sometimes referred to as plasmoids, are believed to form as a result of rapid transient reconnection in the cross-tail current sheet at Mercury and the other planets. Here we report the occurrence of unusually large plasmoids in Mercury’s magnetotail observed, by the MESSENGER spacecraft. These plasmoids are remarkable for several reasons. Their spatial scales in the north–south direction exceed Mercury’s radius of 2440 km, and their time durations are comparable to or longer than the average Dungey cycle time of ∼200 s. They also have a more loop-like magnetic structure than the more common helical-like flux rope topology. These new MESSENGER observations suggest that Mercury’s magnetosphere can dissipate large quantities of magnetic flux and energy not only through the formation of a large number of small flux ropes, but also occasionally by the formation and release of a single giant plasmoid.