One Sentence Summary: NASA's Magnetospheric Multiscale mission detected fast magnetic reconnection and high-speed electron jets in the Earth's magnetotail.Abstract: Magnetic reconnection is an energy conversion process important in many astrophysical contexts including the Earth's magnetosphere, where the process can be investigated in-situ. Here we present the first encounter of a reconnection site by NASA's Magnetospheric Multiscale (MMS)

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.

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 (δB) 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 |δE|/|δB| 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.