Plasmoids in Saturn's magnetotail



[1] Plasmoids in Saturn's magnetotail are identified by a reversal (northward turning) of the normally southward component of the magnetic field across the tail current sheet. Three large plasmoids have been identified by the Cassini magnetometer, one near 0300 local time at a planet-centered distance of 44 RS and two near midnight at 48–49 RS. (RS ≈ 60,300 km is Saturn's equatorial radius.) Two of these events, including in particular the 0300 event, coincided with current-sheet crossings by the spacecraft and thus provided sufficient plasma fluxes to determine ion composition and velocity moments from Cassini Plasma Spectrometer data. The composition was largely dominated by water-group ions, indicating an inner-magnetosphere source. The flow was subcorotational and strongly tailward, as expected for a plasmoid. Just before the in situ detection of the 0300 plasmoid, the Magnetospheric Imaging Instrument observed an outburst of energetic neutral atoms emanating from a location midway between Saturn and Cassini, probably a signature of the reconnection event that spawned the plasmoid.