# 3D Magnetic Reconnection with a spatially confined X-line extent -- Implications for Dipolarizing Flux Bundles and the Dawn-Dusk Asymmetry

## TL;DR

This study uses 3D particle-in-cell simulations to investigate how the spatial extent of the X-line affects magnetic reconnection, revealing the formation of inactive regions that influence flux bundle sizes and dawn-dusk asymmetries in planetary magnetotails.

## Contribution

It demonstrates the critical role of the X-line's spatial confinement in magnetic reconnection dynamics and explains observed asymmetries in planetary magnetotails through the Hall effect.

## Key findings

- Reconnection rate decreases when the current sheet extent is less than ~10 ion inertial lengths.
- Inactive regions of ~10 di suppress reconnection when the sheet is short.
- The inactive region explains the minimum size of dipolarizing flux bundles and dawn-dusk asymmetry.

## Abstract

Using 3D particle-in-cell (PIC) 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 (di) scale. The reconnection rate and outflow speed drop significantly when the extent of the thin current sheet in the current direction is < O(10 di). When the thin current sheet extent is long enough, we find it consists of two distinct regions; an inactive region (on the ion-drifting side) exists adjacent to the active region where reconnection proceeds normally as in a 2D case. The extent of this inactive region is ~ O(10 di), 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 inactive region; because electron drifts inside the ion diffusion region transport the reconnected magnetic flux, that drives outflows and furthers the current sheet thinning, away from this region. This is a consequence of the Hall effect in 3D. While this inactive 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, that has a global dawn-dusk extent much shorter than that of Earth.