MMS Examination of FTEs at the Earth's Subsolar Magnetopause

M Akhavan‐Tafti,R Nakamura,C T Russell,G Le,R B Torbert,J A Slavin,J P Eastwood,D J Gershman,W Baumjohann,J L Burch,B L Giles,R J Strangeway

doi:10.1002/2017ja024681

Abstract

AbstractDetermining the magnetic field structure, electric currents, and plasma distributions within flux transfer event (FTE)‐type flux ropes is critical to the understanding of their origin, evolution, and dynamics. Here the Magnetospheric Multiscale mission's high‐resolution magnetic field and plasma measurements are used to identify FTEs in the vicinity of the subsolar magnetopause. The constant‐α flux rope model is used to identify quasi‐force free flux ropes and to infer the size, the core magnetic field strength, the magnetic flux content, and the spacecraft trajectories through these structures. Our statistical analysis determines a mean diameter of 1,700 ± 400 km (~30 ± 9 di) and an average magnetic flux content of 100 ± 30 kWb for the quasi‐force free FTEs at the Earth's subsolar magnetopause which are smaller than values reported by Cluster at high latitudes. These observed nonlinear size and magnetic flux content distributions of FTEs appear consistent with the plasmoid instability theory, which relies on the merging of neighboring, small‐scale FTEs to generate larger structures. The ratio of the perpendicular to parallel components of current density, RJ, indicates that our FTEs are magnetically force‐free, defined as RJ < 1, in their core regions (<0.6 Rflux rope). Plasma density is shown to be larger in smaller, newly formed FTEs and dropping with increasing FTE size. It is also shown that parallel ion velocity dominates inside FTEs with largest plasma density. Field‐aligned flow facilitates the evacuation of plasma inside newly formed FTEs, while their core magnetic field strengthens with increasing FTE size.

Highlights

Magnetic reconnection is the underlying physical process responsible for the release of energy stored in magnetospheric magnetic fields that powers plasma heating and acceleration (e.g., Yamada et al, 2010; Gonzalez and Parker, 2016)
These observed non-linear size and magnetic flux content distributions of flux transfer event (FTE) appear consistent with the plasmoid instability theory which relies on the merging of neighboring, small-scale FTEs to generate larger structures
Our results indicate that 1) small-scale FTEs are observed at the magnetopause more frequently than previously reported, 2) electric currents inside growing FTEs help reduce magnetic forces over time, and 3) plasma density and core magnetic field are inversely related inside evolving FTEs

Summary

Introduction

Magnetic reconnection is the underlying physical process responsible for the release of energy stored in magnetospheric magnetic fields that powers plasma heating and acceleration (e.g., Yamada et al, 2010; Gonzalez and Parker, 2016). Flux Transfer Events (FTEs) are transient signatures of magnetic reconnection. FTEs are twisted open flux tubes associated with the transfer of plasma from the magnetosheath into the magnetosphere. FTEs are identified by a bipolar signature in the component of magnetic field normal to the magnetopause together with a maximum in the axial component of magnetic field [Russell and Elphic, 1979; Rijnbeek et al, 1984]. According to the magnetohydrodynamic (MHD) model of magnetic reconnection, low reconnection rates are expected in highly elongated current sheets [Parker, 1957]. 2D kinetic simulations have argued that laminar Sweet-Parker layers such as the magnetopause become unstable to the formation of plasmoids with increasing Lundquist number, S ∝ VA LSP η-1, where VA ∝ B ρ-1/2 is the Alfven speed, Lsp is the Sweet-Parker current sheet thickness, and η is the resistivity (Daughton et al, 2009; Huang and Bhattacharjee, 2010, 2013)

Methods

Results

Conclusion