Slow Electron Holes in the Earth's Magnetosheath

The Earth's magnetosheath is a highly turbulent medium and an ideal natural laboratory for the analysis of plasma turbulence. Spacecraft measurements showed that high-frequency electric field fluctuations in the Earth's magnetosheath are predominantly electrostatic and consist, particularly, of electrostatic solitary waves with bipolar parallel electric field profiles. The properties of these electrostatic fluctuations have been largely unaddressed; moreover, the results of several previous studies were inconsistent. In the present study we carried out statistical analysis of 630 electrostatic solitary waves observed aboard Magnetospheric Multiscale in the Earth's magnetosheath. We revealed that the solitary waves are Debye-scale structures with the electrostatic potential of \blue{positive polarity} and typical amplitudes between 0.01 and 1\% of local electron temperature. We demonstrated that the solitary waves must be electron holes that is purely kinetic structures produced in a nonlinear stage of various electron-streaming instabilities. Even more critical is that these structures are {\it slow}, their plasma frame velocities are well below electron thermal speed but coincide with velocities of the bulk of ions. While the source of electrostatic fluctuations in the Earth's magnetosheath could not be revealed in this study, the finding that these fluctuations can be slow implies they can facilitate efficient energy exchange between ions and electrons.