Cluster Observations of Magnetosheath Plasmoids

We use Cluster multi-point measurements to identify magnetosheath plasmoids, which we define as localized plasma blobs with a higher density than the background plasma, and some associated variation in the magnetic field. Several criteria are used to discriminate between blobs convecting together with the plasma flow from signatures associated with motion of the magnetopause or bow shock. Having identified such plasmoids, we determine their geometry along and perpendicular to the plasma flow and magnetic field, and their orientation relative to the bow shock and magnetopause. We also determine the dynamic beta value (the ratio of the kinetic and magnetic energy densities) and ratio of the scale size perpendicular to the flow to the ion gyro radius (evaluated using the drift velocity). These are critical parameters for determining the penetration of such plasmoids into the magnetosphere during impulsive penetration events. Determination of these parameters enables a comparison with theoretical and laboratory results, showing if typical plasmoids can penetrate into the magnetosphere, and what type of penetration mechanism will be likely.