The modulation of sporadic-E layers by Kelvin-Helmholtz billows in the neutral atmosphere

Modulation of electron densities in ion layers between 90 and 150 km altitude has been observed using a number of ionospheric diagnostic measurements including scatter of VHF radar waves, artificially pumped optical emissions, scintillations of satellite beacon transmissions. Kelvin-Helmholtz (K-H) turbulence driven by a sheared wind profile is a strong candidate for the source of these modulations. A two-dimensional numerical model is used to calculate the nonlinear evolution of ion layers in ionosphere near 100 and 120 km altitude in response to neutral turbulence driven by a wind shear. The amplitude of a K-H billow is allowed to grow as a linear perturbation on the neutral atmosphere to a level that is 10% of the wind shear. The time dependent model of the ionosphere responds to neutral wind perturbation initially by imposing a quasi-sinusoidal modulation near the altitude of the ion layer. This is followed by compression of the initially stratified layer into structures with the wavelength of the K-H instability. These structures are uniform strips in the meridian perpendicular to the direction of the zonal wind. Near 120 km, where the ion gyro frequency (omega(i)) is about equal to the ion collision frequency (vi), the equilibrium solutions are clumps at the altitude of the shear. Near 100 km, two stable, rippled layers are produced with a separation of about 1 km. The amplitudes of the density modulations in the ion layers vary by as much as 500% throughout the simulation. The simulations illustrate the complex evolution of the ion layer structures from small-amplitude, K-H wind turbulence. (C) 2002 Published by Elsevier Science Ltd.