Where should MMS look for electron diffusion regions?

TL;DR

This paper explores the potential for MMS to detect electron diffusion regions not only at primary reconnection sites but also at secondary sites formed downstream due to cascading reconnection events in 3D.

Contribution

It introduces the idea that secondary reconnection sites can form downstream of primary sites in 3D, expanding the search area for MMS beyond traditional primary reconnection regions.

Findings

Secondary reconnection sites form downstream of primary sites.

Cascading reconnection can occur even far from initial reconnection points.

MMS has increased chances to detect reconnection events at secondary sites.

Abstract

A great possible achievement for the MMS mission would be crossing electron diffusion regions (EDR). EDR are regions in proximity of reconnection sites where electrons decouple from field lines, breaking the frozen in condition. Decades of research on reconnection have produced a widely shared map of where EDRs are. We expect reconnection to take place around a so called x-point formed by the intersection of the separatrices dividing inflowing from outflowing plasma. The EDR forms around this x-point as a small electron scale box nested inside a larger ion diffusion region. But this point of view is based on a 2D mentality. We have recently proposed that once the problem is considered in full 3D, secondary reconnection events can form [Lapenta et al., Nature Physics, 11, 690, 2015] in the outflow regions even far downstream from the primary reconnection site. We revisit here this new idea confirming that even using additional indicators of reconnection and even considering longer periods and wider distances the conclusion remains true: secondary reconnection sites form downstream of a reconnection outflow causing a sort of chain reaction of cascading reconnection sites. If we are right, MMS will have an interesting journey even when not crossing necessarily the primary site. The chances are greatly increased that even if missing a primary site during an orbit, MMS could stumble instead on one of these secondary sites.