Magnetic structure of the reconnection layer and core field generation in plasmoids

Abstract

Plasmoids/flux ropes have been observed both at Earth's magnetopause as well as in the magnetotail. Magnetic field measurements of such structures often reveal that rather than a minimum in field strength at their centers as expected from a simple O‐type neutral line picture, they exhibit a strong core field. To address this issue, two‐dimensional (2‐D) and 3‐D hybrid simulations are used to investigate the magnetic structure of reconnection layer in general and the formation of the core field within plasmoids in particular. The reconnection layer in the magnetotail is found to be unstable to the fire hose instability. As a result, the region between the lobe and the central plasma sheet is nearly at the marginal fire hose condition. The magnetic signatures of single and multiple X line geometries are contrasted, and it is shown that the interaction of outflowing jets from neighboring X lines leads in general to a highly complex magnetic structure within a plasmoid. The large observed core fields are explained in terms of Hall‐generated currents which can naturally lead to core field strengths that even exceed the ambient lobe field in magnitude. Ion beta and the presence of a preexisting guide field are two important factors controlling the Hall‐generated fields. In particular, it is shown that the presence of the small ubiquitous cross‐tail field component in the magnetotail can under certain conditions lead to a strong unipolar plasmoid core field. There exist significant differences between core fields associated with plasmoids at the magnetopause and those in the tail. This is due to (1) high plasma beta in the magnetosheath and (2) the asymmetry in plasma density across the magnetopause. The former leads to smaller core fields at the magnetopause, whereas the latter leads to differences in the polarity and structure of core fields within magnetopause and magnetotail plasmoids. Such differences are illustrated through examples.

Kinetic simulations of plasmoid chain dynamics

“…This is in agreement with the idea that plasmoid core field originates from a compression of a pre-existing seed guide field [50,51].…”

Quantitative Study of Guide-Field Effects on Hall Reconnection in a Laboratory Plasma

“…[15][16][17][18]) routinely show that the two-fluid reconnection rate is reduced by the presence of guide field. This reduction is physically attributed to a nonlinear interaction between the in-plane Hall currents (which produce the quadrupole field) and the applied guide field [12,18]. The electron flow is deflected, and the modified current patterns result in an additional J × B force which opposes the reconnection flow.…”

“…Guide field, the component of magnetic field which is perpendicular to the reconnection plane (see Figure 1), plays an important role in the dynamics of reconnection. Most instances of reconnection in nature [3][4][5] and the laboratory [6][7][8][9][10] contain a significant guide field (B g ) in comparison with the reconnecting field strength (B rec ), prompting the study of this type of reconnection both theoretically and numerically [11][12][13][14][15][16][17][18]. In magnetosphere reconnection [3,4], for example, guide fields often reach the level of the reconnecting field (B g ∼ B rec ), while reconnection in fusion experiments (such as during tokamak [19] or reversed-field pinch [20] sawteeth) can have guide fields exceeding 20B rec .…”

Role of electron inertia and reconnection dynamics in a stressedX-point collapse with a guide-field

“…Similarly to the 2D case, the locations of elevated current density correspond to the centre of the flux-tubes, which have been shown to be the 3D equivalent of magnetic islands (Karimabadi et al 1999) where magnetic fields are compressed and currents are increased (Huang et al 2013). Interestingly, while the initial fragmentation in panel c) appears to be random, the final isosurface shows a distinct tubular structure.…”