Instabilities in the Midlatitude Ionosphere in Terms of E–F Coupling

Abstract

There are two intriguing phenomena in the midlatitude ionosphere: quasi-periodic (QP) radar echoes associated with a sporadic-E (E s) layer in the E region and medium-scale traveling ionospheric disturbances (MSTIDs) in the F region, both of which often show northwest–southeast alignment and propagate southwestward in the Northern Hemisphere. The Perkins instability in the F region and the E s-layer instability in the E region can produce the tilted structure, but the growth rate of the Perkins instability alone cannot account for the observed MSTIDs. The electrodynamical coupling between these instabilities is now believed to be an important factor for the rapid growth of MSTIDs. From recent numerical studies, it is found that the polarization process in the E region is essential for the full development of MSTIDs as well as for the seeding of NW-SE perturbation in the F region. Further observations to identify the cause and effect of the coupling process are required in the future.