Recent observations have shown that, prior to the onset of equatorial spread-F, there exists an altitude modulation in bottomside F-region electron density contours. It has generally been accepted that such a modulation is caused by internal-gravity waves propagating upward from below. In this brief note we suggest that gravity waves by generated in situ, via the wind shear mechanism. Theoretical calculations and some observations indicate that large wind shears can occur during early evening hours in the low latitude thermosphere. We show that the wind shear may possibly generate gravity waves with horizontal wavelengths on the order of a few hundred kilometers, which is the scale of the observed electron density modulation.