Evolutionary Structures of Kelvin–Helmholtz Instability in the Ionosphere Ca⁺ Layer Observed by Lidar

Kelvin-Helmholtz (KH) instability driven by neutral wind shear is an important mechanism for the generation of sporadic-E (Es) layer irregularities. However, direct observational evidence describing the morphological evolution of these instabilities across different height regimes in the mesosphere and lower thermosphere (MLT) region, from collision-dominated to magnetized, remains rare. Here we present high-resolution lidar observations of the Ca⁺ layer at Beijing (40.5°N, 116.0°E), revealing structural morphology at different heights. In the lower E region (~110 km), we identify a cat's eye characteristic of KH turbulence, indicating that ions are effectively dragged by neutral motion due to high ion-neutral collision frequency. In addition to the cat's-eye features, the Ca⁺ ion layer also exhibits quasi-sinusoidal structures and streak-like features, demonstrating a pronounced periodicity. In contrast, at higher altitudes (>120 km) extending to 180 km, these layers evolve into isolated patches and streaks. Using numerical simulations with a coupled neutral ion fluid model, we successfully reproduce these height-dependent features. The model shows that although neutral wind waves at ~110 km altitude induce quasi sinusoidal modulation, the dominant role of the Lorentz force at high altitudes (~180 km) constrains ion motion along magnetic field lines, causing plasma to aggregate into dense clumps rather than overturning waves. These results provide observational verification of neutral turbulence modulating ionospheric plasma.