Localized density enhancements in the magnetosheath: Three‐dimensional morphology and possible importance for impulsive penetration

We use Cluster multipoint density measurements, using the spacecraft potential, to identify localized density enhancements (>50%) in the magnetosheath, and estimate their three‐dimensional morphology and orientation. Typically one dimension of the density enhancements is shorter than others, is directed perpendicular to the background magnetic field, and varies from ∼0.1 R E to 10 R E , with the other two dimensions a factor 3–10 greater. The density structures are oriented with the longest sides in the general direction of the bow shock and magnetopause. Examples of density structures both convecting with the same velocity as the background magnetosheath flow (“embedded plasmoids”), and convecting with an excess x GSE velocity component (“fast plasmoids”) are found. Possible importance for the impulsive penetration mechanism for plasma entry in the magnetosphere is analyzed by comparing the results to laboratory results, via a parameter scaling. The estimation of the three‐dimensional topology of the density enhancements will enable a comparison with localized magnetosheath populations inside the magnetosphere, observed earlier, to determine if these originate from penetrated magnetosheath density enhancements.