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NASA's Magnetospheric Multiscale (MMS) mission is a four-spacecraft formation of Earth orbiting satellites that have been providing unparalleled measurements of the local kinetic-scale plasma dynamics in near-Earth space for the past 8 years. The spacecraft carry a full complement of space plasma instrumentation capable of measuring the 3D electromagnetic fields and particle distribution functions at cadences up to 100 times faster than previous missions and with interspacecraft separations as small as ∼ 5 km, approaching the characteristic electron scales in many of the plasmas that MMS samples. In this Special Topic, we bring together 26 papers covering a broad range of topics—from magnetic reconnection, shocks, and turbulence to some of the basic nuances of collisionless dynamics—highlighting the many ways in which MMS is helping us to better understand both the dynamics of Earth's magnetosphere and the fundamental physics of plasmas. [ABSTRACT FROM AUTHOR]

During magnetic reconnection, field lines interconnect in electron diffusion regions (EDRs). In some EDRs, the reconnection and energy conversion rates are controlled by a steady out-of-plane electric field. In other EDRs, the energy conversion rate J → · E → ′ is "patchy," with electron-scale large-amplitude positive and negative peaks. We investigate 22 EDRs observed by NASA's Magnetospheric Multiscale mission in a wide range of conditions to determine the cause of patchy J → · E → ′. The patchiness of the energy conversion is quantified and correlated with seven parameters describing various aspects of the asymptotic inflow regions that affect the structure, stability, and efficiency of reconnection. We find that (1) neither the guide field strength nor the asymmetries in the inflow ion pressure, electron pressure, nor number density are well correlated with the patchiness of the EDR energy conversion; (2) the out-of-plane axes of the 22 EDRs are typically fairly well aligned with the "preferred" axes, which bisect the time-averaged inflow magnetic fields and maximize the reconnection rate; and (3) the time-variability in the upstream magnetic field direction is best correlated with the patchiness of the EDR J → · E → ′. A 3D fully kinetic simulation of reconnection with a non-uniform inflow magnetic field is analyzed; the variation in the magnetic field generates secondary X-lines, which develop to maximize the reconnection rate for the time-varying inflow magnetic field. The results suggest that magnetopause reconnection, for which the inflow magnetic field direction is often highly variable, may commonly be patchy in space, at least at the electron scale. [ABSTRACT FROM AUTHOR]

NASA’s Magnetospheric Multiscale (MMS) mission has observed a number of electrondiffusion/dissipation regions (EDRs) of both symmetric and asymmetric magneticreconnection. In this talk, we compare and contrast the features of the non-ideal energyconversion rate J⃗ ⋅⃗E′ for these two regimes. We show that in the asymmetric EDR,J⃗⋅⃗E′ > 0 is typically much larger and patchier than predicted and can be distributed across ahighly-structured current sheet surrounding the X-line, whereas in the symmetric EDR,J⃗ ⋅⃗E′ > 0 is typically confined to the region of strongest current very near the X-line.J⃗⋅⃗E′ < 0 also commonly appears in the central EDR of asymmetric reconnection, whereasin symmetric reconnection, ⃗J ⋅⃗E′ < 0 is limited to the outer EDR and separatrices. [ABSTRACT FROM AUTHOR]