Structure formation and radiation from ionospheric Langmuir turbulence

We present new results of multi-scale simulations of Langmuir turbulence during heating experiments where a powerful electromagnetic wave injected into the overhead ionosphere excites parametric instabilities and Langmuir turbulence on multiple spatial and temporal scales. The 3-wave parametric decay instability (PDI) and 4-wave oscillating two-stream instability (OTSI) lead to strong Langmuir turbulence with nucleation and burn-out cycles of localized Langmuir wave packets trapped in ion density cavities. The Langmuir turbulence gives rise to electromagnetic radiation both above the turbulent region, in the form of slow X mode waves (or Z mode waves) and below the turbulent region in the form of
O mode waves escaping the ionosphere to be observed on ground as Stimulated Electromagnetic Emissions (SEE). We will discuss spectral features both in frequency (time-domain) and wave vector (space domain) at different altitudes. The full-wave simulations may help to understand the nonlinear turbulent processes that produce the SEE and radar spectra received on ground. The simulations are carried out using a generalized Zakharov model [1, 2, 3] in which the complex phase and amplitude of the high-frequency electromagnetic and electrostatic waves are solved using an implicit method, and are coupled to ion fluctuations via the driven ion-acoustic wave equation.