Midlatitude E region plasma accumulation driven by planetary wave horizontal wind shears

Recent findings suggested the possibility that planetary waves play a role in the occurrence of midlatitude sporadic E layers. To account for this, we propose here a new mechanism for large-scale accumulation of metallic ions in the midlatitude E region ionosphere driven by planetary waves in the lo wer thermosphere. In this process, the plasma is forced to converge horizon tally and accumulate inside areas of positive vorticity set up by cyclonic neutral. wind shears within a planetary wave. In its simplest form, the pro posed model is similar to the well-known vertical wind shear mechanism of E -s formation, but with the geometry "turned on its side." Because of the lo ng times required for ambipolar diffusion, the new mechanism can lead to si gnificant plasma accumulation, acting as complementary to the vertical wind shear process so that dense E-s can form more efficiently and frequently. The present model provides a physical base for understanding the long-term periodicities in occurrence and also the seasonal dependence of strong spor adic E layers at midlatitude.