The Evolution of Parallel Electric Field Across the Earth's Magnetosheath

Abstract In Earth's collisionless magnetosheath, turbulence evolution and energy transformation across the system, especially at kinetic scales, remain poorly quantified without full‐coverage of high‐resolution observations. Using multi‐traversal observations from the 2023 and 2024 Magnetospheric Multiscale (MMS) Unbiased Magnetosheath Campaign, we construct cross‐sheath profiles of the electric fields () and find that the parallel electric field () peaks on the bow‐shock side and decreases rapidly toward the magnetopause. Enhanced defines an extended “turbulence evolution region” occupying ∼20% of the sheath, corresponding to several tens to order of 100 ion gyro‐radii (). This region is significantly larger than the reflected‐ion shock transition region (∼several to order of 10 ). The signals are primarily ion‐acoustic‐like wave packets, with occasional double layers and phase‐space holes. These observations suggest a region where enhanced corresponds to intensified sub‐kinetic activity and the early stage of magnetosheath turbulence evolution.