Topside Ionospheric Ambipolar Diffusion Derived From ICON Observations

Abstract The scarcity of ambipolar diffusion observations has constrained our understanding of ionospheric physical processes. We present the first comprehensive analysis of topside ionospheric ambipolar diffusion using the difference between the field‐aligned ion drifts and neutral winds observed by the Ionospheric Connection Explorer (ICON) satellite. The ambipolar diffusion exhibits a distinct evening enhancement within ±10° MLAT. The reversal of its direction near ±10° MLAT demarcates the boundary between the trans‐equatorial diffusion‐dominated region (within ±10° MLAT) and the area governed by the vertical gradient‐driven diffusion‐dominated region (out of ±10° MLAT). For the field‐aligned drift, the neutral wind contribution is also significant, but less influential than diffusion. The equatorial fountain effect further modulates diffusion, enhancing winter‐hemisphere transport and creating its longitudinal structures. These findings provide crucial observational evidence to strengthen the understanding of ionospheric diffusion mechanisms, offering reliable and critical inputs for ionospheric theoretical models.