Multisatellite Observations of Ion Holes in the Earth's Plasma Sheet

We present the first observations of electrostatic solitary waves with electrostatic potential of negative polarity around a fast plasma flow in the Earth's plasma sheet. The solitary waves are observed aboard four Magnetospheric Multiscale spacecraft, which allowed accurately estimating solitary wave properties. Based on a data set of 153 solitary waves, we show that they are locally one‐dimensional Debye‐scale structures with amplitudes up to 20% of local electron temperature and they propagate at plasma frame speeds ranging from a tenth to a few ion‐acoustic speeds at arbitrary angles to the local magnetic field. The solitary waves are associated with multi‐component proton distributions and their velocities are around those of a beam‐like proton population. We argue that the solitary waves are ion holes, nonlinear structures produced by ion‐streaming instabilities, and conclude that once ions are not magnetized, ion holes can propagate oblique to local magnetic field. Plain Language Summary Electrostatic solitary waves or, equivalently, localized perturbations of the electrostatic potential, are frequently observed in various regions of the near‐Earth space and can contribute to particle heating and thermalization. Those with positive polarity of the electrostatic potential, named electron holes, are well‐studied, while ion holes, which are solitary waves of negative polarity, have been observed and studied only in the auroral region and Earth's bow shock using a single spacecraft. We present over 150 ion holes observed on all four Magnetospheric Multiscale spacecraft around a fast plasma flow in the Earth's plasma sheet. Using four‐spacecraft interferometry, we obtained statistical distributions of ion hole parameters. Most notably, the ion holes propagate obliquely to local magnetic field similar to those in the Earth's bow shock, as opposed to parallel propagation in the auroral region. The analysis of ion velocity distribution functions showed that the ion holes are associated with proton beams and, hence, very likely produced by ion‐streaming instabilities. The major conclusion is that once ions are not magnetized, ion holes can propagate oblique to local magnetic field. The presented results should advance our understanding of the effects of electrostatic solitary waves in various regions of the near‐Earth space. Key Points Observations of ion holes in the plasma sheet are presented for the first time Ion holes propagate at arbitrary angles to lo