Nightward ion flow in the Venus ionosphere: Implications of momentum balance

Using global empirical models of Venus ionospheric conditions, we solve for the plasma flow field consistent with the horizontal momentum equation in both viscid and inviscid forms. We find that plasma viscosity is negligible except at low altitudes and that the observed plasma flows are consistent with the inviscid solution above 300 km but are larger than the calculated flows at lower altitudes. This is probably due to downward momentum advection. The strong vertical shear in the calculated and observed flows just beyond the terminator may produce turbulence there, manifested as the observed transterminator waves in plasma density and magnetic field. We discuss the implications of ionospheric superrotation.