MMS Examination of FTEs at the Earth's Subsolar Magnetopause

Abstract

Determining 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.

MMS Study of the Structure of Ion‐Scale Flux Ropes in the Earth's Cross‐Tail Current Sheet

“…These results suggest that the newly formed ion-scale flux ropes have already demonstrated distinct properties in the magnetic structures and plasma features. Previous studies suggested that flux ropes containing less particles and stronger core fields reached a later stage of the evolution (e.g., Akhavan-Tafti et al, 2018 ;Chen et al, 2017 ;Ma et al, 1994), which may suggest that the quasi-2-D flux ropes in this study are in the later stage of evolution than the quasi-1-D flux ropes. The quasi-2-D flux ropes were observed in regions with a larger dawn-dusk field (~0.33) than the quasi-1-D flux ropes (~0.15; Figures 4f and 4g), which suggests that magnetic reconnections under these different magnetic field conditions might have given rise to the two different groups of ion-scale flux ropes.…”

MMS Multi‐Point Analysis of FTE Evolution: Physical Characteristics and Dynamics

“…Our event selection criteria only allow FTEs with (1) small impact parameter (IP < 0.5) and (2) cylindrical symmetry (force‐free model goodness of fit parameter, χ 2 < 0.1; cf. Akhavan‐Tafti et al, ). A complete list of all 63 burst‐mode FTEs used in this study and the force‐free flux rope model results are provided in Table S1 in the supporting information.…”

MESSENGER Observations of Disappearing Dayside Magnetosphere Events at Mercury

“…Further, the fitting method determines the impact parameter (i.e., closest approach -Tafti et al, 2018;DiBraccio et al, 2015;Slavin et al, 2003). They were detected based on their bipolar signature in the maximum variance component of the local magnetic field (B max ) coinciding with a peak in both the intermediate variance component of magnetic field (B int ) and |B|.…”

“…Only structures that satisfy the following criteria are included in the FTE statistics for the DDM events: (i) impact parameter <0.5, (ii) chi-square value <0.1 (see Akhavan-Tafti et al, 2018), and (iii) FTE radius <2,500 km. In addition, the use of the force-free model enables a nonbiased investigation of magnetic structures at the magnetopause.…”