A shear instability seeding mechanism for quasiperiodic radar echoes

Abstract

The quasiperiodic echoes with periods in the range of ∼ 1–10 minutes originally discovered at the Middle and Upper Atmosphere (MU) radar in Japan in the early 1990s are now known to be a common feature of midlatitude sporadic E. A possible role of shear instabilities and the associated Kelvin‐Helmholtz structure in seeding the plasma instabilities that produce the radar echoes is suggested in this paper. The proposed mechanism has the advantage that the seed structure is generated in situ as part of the same neutral wind structure that serves to initiate the generation of the sporadic E layer. In addition, the range of observed periods can easily be explained. Since the Kelvin‐Helmholtz structure is embedded in the shear flow and moves with the mean flow in the shear layer, the period observed in the ground‐based frame is simply a product of the horizontal wavelength and the mean flow velocity at the center of the disturbance. Finally, a typical feature of the radar observations has been that the quasiperiodic structure often ebbs and wanes in intensity over a period of 30 to perhaps 60 min. Recent simulations of unstable neutral shears show that the timescale for the build up and eventual restabilization of the shear flow is 30–40 min, comparable to the timescale found in the observations.