Structure of the Magnetic Reconnection Diffusion Region from Four-Spacecraft Observations

TLDR

Magnetic reconnection converts energy over large volumes but begins in tiny diffusion regions that are rarely sampled by spacecraft. The study reports a four‑spacecraft observation of a magnetopause diffusion region to distinguish spatial from temporal features. Using a four‑spacecraft configuration at the magnetopause, the authors captured a diffusion region encounter to separate spatial from temporal variations. The diffusion region is stable on ion scales, dominated by Hall physics, exhibits a fast reconnection rate of ~0.1, and hosts strong parallel currents along separatrices linked to high‑frequency Langmuir/upper‑hybrid waves.

Abstract

Magnetic reconnection leads to energy conversion in large volumes in space but is initiated in small diffusion regions. Because of the small sizes of the diffusion regions, their crossings by spacecraft are rare. We report four-spacecraft observations of a diffusion region encounter at the Earth's magnetopause that allow us to reliably distinguish spatial from temporal features. We find that the diffusion region is stable on ion time and length scales in agreement with numerical simulations. The electric field normal to the current sheet is balanced by the Hall term in the generalized Ohm's law, E(n) approximately jxB/ne.n, thus establishing that Hall physics is dominating inside the diffusion region. The reconnection rate is fast, approximately 0.1. We show that strong parallel currents flow along the separatrices; they are correlated with observations of high-frequency Langmuir/upper hybrid waves.