Metallic ion layers are commonly observed in the ionosphere. In particular, those which occur at E-region heights show high plasma concentrations, and they can cause anomalous high frequency (HF) radio propagation, known as Es. The characterization of the phenomenon and the formation mechanism for Es layers have been a subject of study for many decades (see, e.g., reviews by Haldoupis, 2011;Mathews, 1998;Whitehead, 1989). Observations of Es have been made using various instruments and techniques, including ionosondes (e.g., Taguchi, 1961), sounding rockets (e.g., Smith, 1966), incoherent scatter radars (e.g., Miller & Smith, 1978), lidars (e.g., Gardner et al., 1993, and radio occultation measurements (e.g., Hocke et al., 2001). It is now well established that Es layers occur most frequently at mid latitudes (20°-50° latitude) in the summer hemisphere at altitudes of 90-120 km.The formation of an Es layer requires the convergence of ion flux. The wind shear theory (Axford, 1963;Whitehead, 1961) suggests that the required ion flux convergence can be realized by the vertical shear of horizontal neutral winds. The following simple expression can be obtained for the vertical component of the ion velocity, under the assumption of balance between the Lorentz force due to ion motions across the ambient geomagnetic field and the frictional force due to ion-neutral collisions (e.g., Haldoupis, 2011):
