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  1. Abstract

    We examine the 11 July 2017 electron diffusion region (EDR)observed by the MagnetosphericMultiscale (MMS) mission using Poynting's theorem. The terms in Poynting's theorem are determined using a linear gradient approximation to obtain barycentric averages within the MMS tetrahedron. We find that Poynting's theorem is approximately balanced in the EDR and the balance is improved if the calculation ofis restricted to the LN plane. The work rate per unit volumeis mostly balanced by the divergence of the electromagnetic energy flux, indicating that the electromagnetic energy density remains relatively constant within the EDR during the encounter. We also use particle‐in‐cell (PIC) simulations to examine Poynting's theorem near an x line evolving in time. The central EDR in the simulation is characterized by approximate time independent balance in Poynting's theorem during reconnection growth, while the outer EDR exhibits time‐dependent fluctuations indicative of more chaotic behavior.

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  2. Magnetic reconnection is an energy conversion process that occurs in many astrophysical contexts including Earth’s magnetosphere, where the process can be investigated in situ by spacecraft. On 11 July 2017, the four Magnetospheric Multiscale spacecraft encountered a reconnection site in Earth’s magnetotail, where reconnection involves symmetric inflow conditions. The electron-scale plasma measurements revealed (i) super-Alfvénic electron jets reaching 15,000 kilometers per second; (ii) electron meandering motion and acceleration by the electric field, producing multiple crescent-shaped structures in the velocity distributions; and (iii) the spatial dimensions of the electron diffusion region with an aspect ratio of 0.1 to 0.2, consistent with fast reconnection. The well-structured multiple layers of electron populations indicate that the dominant electron dynamics are mostly laminar, despite the presence of turbulence near the reconnection site. 
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  3. Abstract

    Electron inflow and outflow velocities during magnetic reconnection at and near the dayside magnetopause are measured using satellites from NASA's Magnetospheric Multiscale (MMS) mission. A case study is examined in detail, and three other events with similar behavior are shown, with one of them being a recently published electron‐only reconnection event in the magnetosheath. The measured inflow speeds of 200–400 km/s imply dimensionless reconnection rates of 0.05–0.25 when normalized to the relevant electron Alfvén speed, which are within the range of expectations. The outflow speeds are about 1.5–3 times the inflow speeds, which is consistent with theoretical predictions of the aspect ratio of the inner electron diffusion region. A reconnection rate of 0.04 ± 25% was obtained for the case study event using the reconnection electric field as compared to the 0.12 ± 20% rate determined from the inflow velocity.

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