On the Ubiquity of Magnetic Reconnection Inside Flux Transfer Event‐Like Structures at the Earth's Magnetopause

Abstract

Flux transfer events (FTEs) are transient phenomena frequently observed at the Earth's magnetopause. Their usual interpretation is a flux rope moving away from the reconnection region. However, the Magnetospheric Multiscale Mission revealed that magnetic reconnection sometimes occurs inside these structures, questioning their flux rope configuration. Here we investigate 229 FTE‐type structures and find reconnection signatures inside 19% of them. We analyze their large‐scale magnetic topology using electron heat flux and find that it is significantly different across the FTE reconnecting current sheets, demonstrating that they are constituted of two magnetically disconnected structures. We also find that the interplanetary magnetic field (IMF) associated with reconnecting FTEs presents a strong By component. We discuss several formation mechanisms to explain these observations. In particular, the maximum magnetic shear model predicts that for large IMF By, two spatially distinct X lines coexist at the magnetopause. They can generate separate magnetic flux tubes that may become interlaced.

Temporal Evolution of Flux Tube Entanglement at the Magnetopause as Observed by the MMS Satellites

“…Peak distribution appears close to the horizontal direction. This agrees with earlier findings that entanglement favors a B y -dominated IMF condition (Fargette et al, 2020;Hesse et al, 1990).…”

“… Note . As labeled in the notes column, some events have been reported in recent publications: 1, Øieroset et al (2019); 2, Hwang, Dokgo, et al (2020); 3, Fargette et al (2020); and 4, this paper. Dates are formatted as YYYY‐MM‐DD.…”

“…Reconnection can happen at multiple locations simultaneously at the magnetopause. Thus, the motion of the flux tubes leaving reconnection sites may become intricate, especially when the equatorial component of the interplanetary magnetic field (IMF) is significant (Fargette et al, 2020;Kan, 1988;Nishida, 1989;Otto, 1991;Zhao, 2019). Two flux tubes, each with one end connected to the Northern/Southern Hemisphere of the Earth and the other end connected to the magnetosheath, that flow away from their original reconnection sites may collide and become entangled.…”

“…Recently, with the improvement in spatial and temporal resolution of instruments, direct observations of flux tube entanglement, that is, interlinked flux tubes (Fargette et al, 2020; Hwang, Dokgo, et al, 2020; Kacem et al, 2018; Kieokaew et al, 2020; Øieroset et al, 2019), were reported. These studies pointed out the differences between a classic flux rope and two entangled/interlinked flux tubes: a significant pressure enhancement which violates the force‐balanced flux rope model (Russell et al, 2017); a sharp rotation of magnetic field at the field strength peak region instead of a smooth bipolar variation in the transverse direction; and disparate plasma on two sides of this thin current indicating the lack of magnetic connectivity.…”

The Location of Magnetic Reconnection at Earth’s Magnetopause

“…Many subsequent studies have used plasma and field observations to examine the topologies and characteristics of FTEs in detail (e.g., Klumpar et al 1990;Owen et al 2001;Pu et al 2013;Roux et al 2015;, including the complex processes occurring between interacting FTEs (e.g., Øieroset et al 2011Fargette et al 2020;Russell and Qi 2020). In the context of this review, the focus herein is on the occurrence of FTEs: both in isolation and in relation to the large-scale expected reconnection line location.…”