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

• Published in: Journal of geophysical research. Space physics 2019-07, Vol.124 (7), p.5376-5395
• Main Authors: Akhavan‐Tafti, M., Slavin, J. A., Eastwood, J. P., Cassak, P. A., Gershman, D. J.
• Format: Article
• Language: English
• Subjects: Adiabatic flow; Evolution; External pressure; Fluctuations; Flux transfer events; force‐free flux rope model; FTE coalescence; FTE evolution; FTE growth mechanisms; Magnetic flux; Magnetic reconnection; Magnetopause; Magnetospheres; Magnetospheric Multiscale mission; Physical properties; Plasma; Plasma density; Plasma heating; Plasma pressure; Plasmas (physics); Spacecraft; Spacecraft trajectories; Statistical analysis
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1029/2018JA026311

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. Key Points The forces inside FTEs observed by MMS suggest plasma acceleration toward and along the FTE's central axis causing plasma to escape The roles of adiabatic expansion and reconnection in FTE growth are explored using MMS observations The observed sub‐adiabatic decrease of plasma pressure as FTE size increases requires plasma heating mechanisms such as reconnection