Three‐Dimensional Magnetic Reconnection With a Spatially Confined X‐Line Extent: Implications for Dipolarizing Flux Bundles and the Dawn‐Dusk Asymmetry

Abstract

Using 3-D particle-in-cell simulations, we study magnetic reconnection with the X-line being spatially confined in the current direction. We include thick current layers to prevent reconnection at two ends of a thin current sheet that has a thickness on an ion inertial (d i ) scale. The reconnection rate and outflow speed drop significantly when the extent of the thin current sheet in the current direction is ≲ O(10d i ). When the thin current sheet extent is long enough, we find that it consists of two distinct regions; a suppressed reconnecting region (on the ion-drifting side) exists adjacent to the active region where reconnection proceeds normally as in a 2-D case with a typical fast rate value ≃ 0.1. The extent of this suppression region is ≃ O(10d i ), and it suppresses reconnection when the thin current sheet extent is comparable or shorter. The time scale of current sheet thinning toward fast reconnection can be translated into the spatial scale of this suppression region, because electron drifts inside the ion diffusion region transport the reconnected magnetic flux, which drives outflows and furthers the current sheet thinning, away from this region. This is a consequence of the Hall effect in 3-D. While the existence of this suppression region may explain the shortest possible azimuthal extent of dipolarizing flux bundles at Earth, it may also explain the dawn-dusk asymmetry observed at the magnetotail of Mercury, which has a global dawn-dusk extent much shorter than that of Earth.

Studying Dawn‐Dusk Asymmetries of Mercury's Magnetotail Using MHD‐EPIC Simulations

“…These simulations suggest that Mercury's magnetotail reconnection sites slightly prefer the duskside (Figure ) when the dawnside current sheet is significantly thicker than the duskside (Figures a and b) under a moderate IMF driver (MHD‐EPIC‐A), and the reconnection sites prefer the dawnside (Figure ) when the dawnside current sheet is almost as thin as the duskside (Figures c and d) under a strong driver (MHD‐EPIC‐B). The results of MHD‐EPIC‐B simulation are consistent with what Liu et al () found in 3‐D box PIC simulations. They found that there is a reconnection “suppression region” on the ion drifting side (the duskside in our simulations) of a thin current sheet, so that the magnetic reconnection prefers the electron‐drifting side.…”

“…The X-lines estimated from the tailward jets (Figurs 11 and 8) are more than 1 R M wide in the cross-tail direction initially. As soon as the MHD-EPIC simulation starts, the duskside X-lines start to shrink (solid red lines in Figures 11 and 12), so that almost all the reconnection sites are in the dawn sector at t = 30 s. The shrinkage of the X lines may be related to the reconnection suppression region discussed by Liu et al (2019).…”

Three‐Dimensional X‐line Spreading in Asymmetric Magnetic Reconnection

“…When the CS thickness is twice , the X‐line even appears to be simply drifting, without clearly extending (Shay et al, ). Recent work (Liu et al, ) indicates that a localized X‐line can be well confined between a thick ambient CS of 8 . The above mentioned studies use a reduced mass ratio of 100.…”

“…It is a primary driver of space weather surrounding the Earth. While two‐dimensional (2‐D) models have been widely used to describe the essential aspects of reconnection (Birn et al, ), the three‐dimensional (3‐D) dynamics represents a frontier of current reconnection research (e.g., Daughton et al, ; Janvier, ; Liu et al, , ; Nakamura et al, ; Price et al, ). In this work, we focus on the evolution of the extension of a localized reconnection X‐line in the out‐of‐plane direction.…”

Large‐Amplitude Oscillatory Motion of Mercury's Cross‐Tail Current Sheet

“…Although the structure and processes occurring in Mercury's magnetotail are known to be qualitatively similar to that of Earth's, they are different in spatial and temporal scale (e.g., Gershman et al, 2014;Poh et al, 2017b;Raines et al, 2011;Sun et al, 2015). Recent simulation studies (Chen et al, 2019;Liu et al, 2019) suggest that kinetic-scale dynamics and instabilities dominate in Mercury's small magnetotail (~10 d i wide, where d i is the ion inertial length), thereby explaining the observed asymmetric structure and occurrence of processes in the tail.…”