Numerical simulations of the ionospheric interchange instability

The nonlinear dynamics and spectral characteristics of the ionospheric interchange instability are studied numerically and analytically. A two-dimensional model is derived. The model is valid for both the collision-dominated regime and the inertia-dominated regime. Numerical solutions for the evolution of rising plasma bubbles, unstable plasma strata, and driven plasma irregularities are presented and discussed for both regimes. In the collisional regime, the principal result is that the evolution of the instability tends to an anisotropic state consisting of nearly sinusoidal (quasi-periodic) variation along the horizontal direction, and steepened (shock-like) structures propagating in the vertical direction along the extreme of the quasi-periodic structures. In the inertial regime, if the coupling between the density and the vorticity equations is weak, the instability leads to an isotropic turbulent state which has the same characteristics as the two-dimensional Navier-Stokes turbulence with a passive scalar. For strongly driven interchange instability, enstrophy invariance is violated and only an energy-cascade inertial subrange is observed. Results are compared with in-situ rocket and satellite measurements of the intermediate-wavelength equatorial F-region irregularities