Observations of Large‐Amplitude Parallel Electric Fields in the Turbulent Magnetotail

In a first‐of‐its‐kind study utilizing the four Magnetospheric Multiscale satellites with accurate 3D electric field measurements, we characterize large‐amplitude (≥25 mV m −1 ) parallel electric fields ( E || ) in Earth's magnetotail during periods of magnetic reconnection‐driven strong turbulence. Twelve turbulent periods covering roughly 3 hours are examined. The results of this study reveal that the largest‐amplitude E || structures reside near the plasma sheet boundary layer and display a wide variety of nonlinear structures. Many of the phase‐coherent E || structures appear in roughly one second wave‐like packets over a broad range of frequencies. One of the most intriguing findings is that, despite the broad range of frequencies, the wavelengths of the E || structures are narrowly confined from ∼1 to ∼4 electron skin depths, which suggests that they are participating in the kinetic range of turbulent dissipation. Many of these wave‐like packets display a particular nonlinearity; the signals have a positive‐to‐negative asymmetry in which one polarity has sharp high‐amplitude peaks, whereas the opposite polarity displays broad, lower‐amplitude peaks. Another impactful finding is that a sub‐class of E || structures can carry net potentials greater than 10,000 V, which exceeds the electron temperature in eV. This study indicates a rich variety of behavior in E || and its potential role in the evolution and dissipation of turbulence.