On the origin of magnetosheath plasmoids and their relation to magnetosheath jets

Abstract

We investigate localized magnetosheath and solar wind density enhancements, associated with clear magnetic field changes, and therefore referred to as magnetosheath/solar wind plasmoids, respectively. Using Cluster data, we show that there are two distinct populations of magnetosheath plasmoids, one associated with a decrease of magnetic field strength (diamagnetic plasmoids), and one with an increased magnetic field strength (paramagnetic plasmoids). The diamagnetic magnetosheath plasmoids have scale sizes of the order of 1–10 RE, while the paramagnetic ones are an order of magnitude smaller. The diamagnetic plasmoids are not associated with any change in the magnetosheath plasma flow velocity, and they are classified as embedded plasmoids in the terminology of Karlsson et al. (2012). The paramagnetic plasmoids may either be embedded or associated with increases in flow velocity (fast plasmoids). A search for plasmoids in the pristine solar wind resulted in identification of 62 diamagnetic plasmoids with very similar properties to the magnetosheath diamagnetic plasmoids, making it probable that the solar wind is the source of these structures. No paramagnetic plasmoids are found in the pristine solar wind, indicating that these are instead created at the bow shock or in the magnetosheath. We discuss the relation of the plasmoids to the phenomenon of magnetosheath jets, with which they have many properties in common, and suggest that the paramagnetic plasmoids can be regarded as a subset of these or a closely related phenomenon. We also discuss how the results from this study relate to theories addressing the formation of magnetosheath jets.

Jets Downstream of Collisionless Shocks

“…6), since the magnetopause's location and motion are largely controlled by the balance of pressure on either side of the boundary. However, a number of studies have investigated enhancements in the density (Karlsson et al , 2015bGutynska et al 2015) or in the Fig. 4 Example "high-speed jet", an enhancement in the anti-sunward velocity and dynamic pressure based on the x component of the ion velocity (in GSE).…”

“…Finally, the name "plasmoid" has also been applied citing its use in describing either a "plasma-magnetic entity" (Bostick 1956) or "a coherent mass of plasma" (Simpson and Weiner 1989). This has typically been ascribed to enhancements in magnetosheath density (Karlsson et al , 2015b, though curiously Gunell et al (2014) used the term to describe velocity structures. Gunell et al (2014) discussed how the nomenclature of the transient structures may implicitly evoke their spatial structure.…”

“…It is not explicitly stated what this means, though perhaps it is related to their required minimum 30 s duration. Finally, Karlsson et al ( , 2015b subdivide their plasmoids into "fast" and "embedded" categories based on the velocity within the density enhancements compared with the background value.…”

“…In terms of the transients in discussion, two general approaches have been used based on either the upstream conditions (e.g., Amata et al 2011; in the pristine solar wind or the local background magnetosheath conditions (e.g., Karlsson et al , 2015b. We use angular brackets to denote local background values, which are often determined by a running average whereby the timescale is displayed in subscript.…”

“…These are shown as the dashed lines. The vertical green lines correspond to the Karlsson et al ( , 2015b "plasmoid" criterion, where we have separated the "fast" and "embedded" events. Gutynska et al (2015) later used the same criterion but without this latter distinction.…”