Reconnection in Earth's magnetotail: Magnetic fields in plasmas can rapidly rearrange themselves in a process known as magnetic reconnection, which releases energy and accelerates particles. Torbert et al. used the Magnetospheric Multiscale (MMS) mission to probe a reconnection event in Earth's magnetotail—the region of plasma downstream from the planet as it moves through the solar wind. MMS has previously studied reconnection in the upstream magnetopause, but a different orbit was used to study the magnetotail, where the symmetry of the process is different. The authors measured plasma properties on scales of the electron dynamics, leading to insights that will apply in other regions where magnetic reconnection occurs. Science , this issue p. 1391

The four Magnetospheric Multiscale (MMS) spacecraft recorded the first direct evidence of reconnection exhausts associated with Kelvin‐Helmholtz (KH) waves at the duskside magnetopause on 8 September 2015 which allows for local mass and energy transport across the flank magnetopause. Pressure anisotropy‐weighted Walén analyses confirmed in‐plane exhausts across 22 of 42 KH‐related trailing magnetopause current sheets (CSs). Twenty‐one jets were observed by all spacecraft, with small variations in ion velocity, along the same sunward or antisunward direction with nearly equal probability. One exhaust was only observed by the MMS‐1,2 pair, while MMS‐3,4 traversed a narrow CS (1.5 ion inertial length) in the vicinity of an electron diffusion region. The exhausts were locally 2‐D planar in nature as MMS‐1,2 observed almost identical signatures separated along the guide‐field. Asymmetric magnetic and electric Hall fields are reported in agreement with a strong guide‐field and a weak plasma density asymmetry across the magnetopause CS.

Turbulent plasmas generate intense current structures, which have long been suggested as magnetic reconnection sites. Recent Magnetospheric Multiscale observations in Earth’s magnetosheath revealed a novel form of reconnection where the dynamics only couple to electrons, without ion involvement. It was suggested that such dynamics were driven by magnetosheath turbulence. In this study, the fluctuations are examined to determine the properties of the turbulence and if a signature of reconnection is present in the turbulence statistics. The study reveals statistical properties consistent with plasma turbulence with a correlation length of ∼10 ion inertial lengths. When reconnection is more prevalent, a steepening of the magnetic spectrum occurs at the length scale of the reconnecting current sheets. The statistics of intense currents suggest the prevalence of electron-scale current sheets favorable for electron reconnection. The results support the hypothesis that electron reconnection is driven by turbulence and highlight diagnostics that may provide insight into reconnection in other turbulent plasmas.

We report observations of turbulent dissipation and particle acceleration from large‐amplitude electric fields (E) associated with strong magnetic field (B) fluctuations in the Earth's plasma sheet. The turbulence occurs in a region of depleted density with anti‐earthward flows followed by earthward flows suggesting ongoing magnetic reconnection. In the turbulent region, ions and electrons have a significant increase in energy, occasionally >100 keV, and strong variation. There are numerous occurrences of |E| >100 mV/m including occurrences of large potentials (>1 kV) parallel to B and occurrences with extraordinarily large J · E (J is current density). In this event, we find that the perpendicular contribution of J · E with frequencies near or below the ion cyclotron frequency (f_ci) provide the majority net positive J · E. Large‐amplitude parallel E events with frequencies above f_ci to several times the lower hybrid frequency provide significant dissipation and can result in energetic electron acceleration.

We report observations of large‐amplitude (>50 mV/m) electric fields primarily associated with bursty bulk flow events. These electric fields reach ~500 mV/m, which are some of the largest electric fields ( E ) observed in the magnetotail. E not only has a larger than expected component perpendicular to the magnetic field but often has an intense parallel component. High time resolution waveforms reveal nonlinear structures such as electron phase‐space holes and double layers, which suggest strong field‐aligned currents or electron beams. Further examination shows that these large‐amplitude electric fields are almost always accompanied by enhanced magnetic field fluctuations. The electric fields are enhanced both above and below the ion cyclotron frequency, whereas the magnetic field fluctuations ( δ B ) are mostly below the ion cyclotron frequency. Analysis of the wave spectra and the Poynting flux suggest that shear Alfvén waves are participating in these events. The Alfvén waves are revealed through the | δ E |/| δ B | ratio and strong field‐aligned Poynting flux, sometimes reaching nearly 1 mW/m ² . This value, when mapped to the low‐altitude auroral region, exceeds 1 W/m ² , which is an extreme value for that region. This Alfvénic activity is accompanied by evidence of compressional modes. These observations support a hypothesis whereby intense currents or electron beams, generated by kinetic Alfvénic waves that result from a turbulent cascade in bursty bulk flow (BBF) braking region, may be an energy source for large‐amplitude electric fields. The large‐amplitude electric fields may act as a dissipation mechanism and relax the highly tangled magnetic fields that result from the turbulence. Furthermore, these observations offer strong support that Alfvénic Poynting flux from the BBF braking region can be the energy source for Alfvénic aurora.

Spatial and high‐time‐resolution properties of the velocities, magnetic field, and 3‐D electric field within plasma turbulence are examined observationally using data from the Magnetospheric Multiscale mission. Observations from a Kelvin‐Helmholtz instability (KHI) on the Earth's magnetopause are examined, which both provides a series of repeatable intervals to analyze, giving better statistics, and provides a first look at the properties of turbulence in the KHI. For the first time direct observations of both the high‐frequency ion and electron velocity spectra are examined, showing differing ion and electron behavior at kinetic scales. Temporal spectra exhibit power law behavior with changes in slope near the ion gyrofrequency and lower hybrid frequency. The work provides the first observational evidence for turbulent intermittency and anisotropy consistent with quasi two‐dimensional turbulence in association with the KHI. The behavior of kinetic‐scale intermittency is found to have differences from previous studies of solar wind turbulence, leading to novel insights on the turbulent dynamics in the KHI.