Meta-equilibrium state of multi-species ambipolar diffusion and its relevance to polar mesopheric summer echoes

► Mesospheric electron spatial profile quickly relaxes to a long-lived meta-equilibrium which: ► is simple algebraic solution of coupled electron-ion-aerosol coupled diffusion equations; ► gives electron spatial modulation responsible for Polar Mesosphere Summer Echoes; ► depends on aerosol initial profile, but not on electron initial condition.