Formation of sporadic E (Es) layer by homogeneous and inhomogeneous horizontal winds

The possibility of sporadic E (Es) layer formation in the mid-latitude nighttime lower thermosphere by horizontal homogeneous and inhomogeneous (vertically changing) winds is investigated by analytical and numerical solutions of continuity equation for dominant heavy metallic ions Fe+. The study firstly showed that when ions maximal vertical convergence rate exceeds the diffusive displacement of their initial layer, then their convergence into Es type thin dense layer is possible. The ions maximal convergence rate corresponds to the minimal negative values in their vertical drift velocity divergence. Es layers are localizing either at the ion vertical drift velocity nodes or at the regions where the drift velocity and convergence rates vanish. Ions ambipolar diffusion influences their convergence processes and controls Es layer densities. The wind direction, value and vertical shear, together with geomagnetic field and vertically changing ion-neutral collision frequency, determine the maximal ion/electron convergence rate, so formation and localization of Es layer is predictable. Es layers formation is possible by diversely directed horizontal wind (even it is homogeneous) at any height between 95 and 150 km, where they are observable. By the suggested theory it is shown, for the first time, that the horizontal wind with vertically changing meridional and zonal components determined by tidal wind (or atmospheric waves), as well as those taken from the horizontal wind model (e.g., HWM14) and observed data can cause the formation of a single or double peak sporadic E. In the regions with the maximal positive divergence of the ion vertical drift velocity, their density is depleting.