“Ionospheric Drizzle” Observed in the Pre‐Dawn E‐F Valley Over Sanya

A new incoherent scattering radar established at Sanya (18.3°N, 109.6°E; SYISR) has made it possible to directly measure the fine structure of the ionospheric E‐F valley. An intriguing phenomenon termed “ionospheric drizzle” was revealed by the month‐long continuous observation of complete ionospheric profiles with a temporal and vertical resolution of 0.8 s and 4.5 km. It manifests as weak plasma “streaks” in the strength of ∼103–104 cm−3 that detach from the F‐layer base and extends downward to the sporadic E (Es) layer during pre‐dawn hours, with an apparent descent speed of ∼10–20 m/s. The frequently observed “ionospheric drizzle” provides additional plasma to enhance the Es layer. Subsequent testing particle simulations were performed, revealing the critical role of the neutral winds in the plasma descent. This downward coupling process enhancing the Es layer is thus supposed to be important in communication and navigation. Key Points Using the Sanya ISR, “ionospheric drizzle”, a phenomenon linking the F‐layer and sporadic E layer at pre‐dawn hours, was revealed Streak‐like plasma detaches from the F‐layer base with a descent speed of ∼10–20 m/s and significantly enhances the Es layer at ∼120 km Test particle simulations are performed to explain the critical role of neutral winds in the “ionospheric drizzle” formation