Saturn’s dynamic magnetotail: A comprehensive magnetic field and plasma survey of plasmoids and travelling compression regions, and their role in global magnetospheric dynamics
We present a comprehensive study of the magnetic field and plasma signatures of reconnection events observed with the Cassini spacecraft during the tail orbits of 2006. We examine their “local” properties in terms of magnetic field reconfiguration and changing plasma flows. We also describe the “global” impact of reconnection in terms of the contribution to mass loss, flux closure, and large scale tail structure. The signatures of 69 plasmoids, 17 travelling compression regions (TCRs), and 13 planetward-moving structures have been found. The direction of motion is inferred from the sign of the change in the Bθ component of the magnetic field in the first instance, and confirmed through plasma flow data where available. The plasmoids are interpreted as detached structures, observed by the spacecraft tailward of the reconnection site and the TCRs are interpreted as the effects of the draping and compression of lobe magnetic field lines around passing plasmoids. We focus on the analysis and interpretation of the tailward-moving (south-to-north field change) plasmoids and TCRs in this work, considering the planetward-moving signatures only from the point of view of understanding the reconnection x-line position and recurrence rates. We discuss the location spread of the observations, showing that where spacecraft coverage is symmetric about midnight, reconnection signatures are observed more frequently on the dawn flank than on the dusk flank. We show an example of a chain of two plasmoids and two TCRs over three hours, and suggest that such a scenario is associated with a single reconnection event ejecting multiple successive plasmoids. Plasma data reveal that one of these plasmoids contains H + at lower energy and W + at higher energy, consistent with an inner magnetospheric source, and the total flow speed inside the plasmoid is estimated with an upper limit of 170 km/s. We probe the interior structure of plasmoids and find that the vast majority of examples at Saturn show a localized decrease in field magnitude as the spacecraft passes through the structure. We take the trajectory of Cassini into account, as, during 2006, the spacecraft's largely equatorial position beneath the hinged current sheet meant that it rarely traversed the centre of plasmoids. We present an innovative method of optimizing the window size for minimum variance analysis (MVA) and apply this MVA across several plasmoids to explore their interior morphology in more detail, finding that Saturn's tail contains both loop-like and flux rope-like plasmoids. We estimate the mass lost downtail through reconnection and suggest that the apparent imbalance between mass input and observed plasmoid ejection may mean that alternative mass loss methods contribute to balancing Saturn's mass budget. We also estimate the rate of magnetic flux closure in the tail and find that, where open field line closure is active, it plays a very significant role in flux cycling at Saturn.
Authors: Jackman, C.M., Slavin, J. A, Kivelson, M.G., Southwood, D. J., Achilleos, N., Thomsen, M.F., DiBraccio, G. A., Eastwood, J.P., Freeman, M.P. ORCID record for M.P. Freeman, Dougherty, M.K., Vogt, M.F.