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

Abstract

Previous studies have indicated that flux transfer events (FTEs) grow as they convect away from the reconnection site along the magnetopause. This increase in FTE diameter may occur via adiabatic expansion in response to decreasing external pressure away from the subsolar region or due to a continuous supply of magnetic flux and plasma to the FTEs' outer layers by magnetic reconnection. Here we investigate an ensemble of 55 FTEs at the subsolar magnetopause using Magnetospheric Multiscale (MMS) multi‐point measurements. The FTEs are initially modeled as quasi‐force‐free flux ropes in order to infer their geometry and the spacecraft trajectory relative to their central axis. The MMS observations reveal a radially‐inward net force at the outer layers of FTEs which can accelerate plasmas and fields toward the FTE's core region. Inside the FTEs, near the central axis, plasma density is found to decrease as the axial net force increases. It is interpreted that the axial net force accelerates plasmas along the axis in the region of compressing field lines. Statistical analysis of the MMS observations of the 55 FTEs indicates that plasma pressure, Pth, decreases with increasing FTE diameter, λ, as Pth,obsv ∝ λ−0.24. Assuming that all 55 FTEs started out with similar diameters, this rate of plasma pressure decrease with increasing FTE diameter is at least an order of magnitude slower than the theoretical rate for adiabatic expansion (i.e., Pth,adiab. ∝ λ−3.3), suggesting the presence of efficient plasma heating mechanisms, such as magnetic reconnection, to facilitate FTE growth.

MMS Observations of Plasma Heating Associated With FTE Growth

“…Outside FTEs include field lines in the magnetosheath, which, in many cases, are in the near reconnection exhaust regions of X-lines that generated the FTEs (e.g., Lee & Fu, 1985) or contributed to their growth (e.g., Akhavan-Tafti et al, 2019). As shown by Akhavan-Tafti et al (2019), the electrons and ions remained frozen-in inside the 55 FTEs further validating the guiding center limit. Figure 2 compares averaged cross-sectional profiles of the energization mechanisms, including parallel electric fields and betatron and first-order Fermi processes, inside and outside FTEs.…”

“…Fifty‐five FTE‐type flux ropes are selected based on the following criteria: 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, , ).…”

“…Here, the term "outside" refers to the FTEs' outer perimeters in the draping field region surrounding each FTE within 10 local ion inertial lengths, d i (Akhavan-Tafti et al, 2019;Farrugia et al, 1988). Outside FTEs include field lines in the magnetosheath, which, in many cases, are in the near reconnection exhaust regions of X-lines that generated the FTEs (e.g., Lee & Fu, 1985) or contributed to their growth (e.g., Akhavan-Tafti et al, 2019). As shown by Akhavan-Tafti et al (2019), the electrons and ions remained frozen-in inside the 55 FTEs further validating the guiding center limit.…”

Comparative Analysis of the Vlasiator Simulations and MMS Observations of Multiple X‐Line Reconnection and Flux Transfer Events

“…Akhavan-Tafti, Slavin, Eastwood, et al (2019) classified flux transfer event (FTE) growth mechanisms into two main categories: (1) FTE growth via adiabatic expansion due to decreasing external pressure away from the reconnection region and (2) magnetic reconnection. In the latter category, FTE growth occurs via continuous supply of magnetic flux and plasma to the outer layers of FTEs by reconnection at adjacent X-lines and/or coalescence with the neighboring FTEs.…”

“…Figure 1 shows a magnetic island which is a 2-D projection of a flux rope generated due to primary, multiple X-line reconnection. The magnetic island can grow via continuous reconnection (Akhavan-Tafti, Slavin, Eastwood, et al, 2019) at adjacent X-lines. The X-lines at the two ends of the magnetic island are represented as ion diffusion regions (IDRs).…”