Multi-scale behavior of Langmuir turbulence during ionospheric heating experiments

We present new results of multi-scale simulations of Langmuir turbulence during heating experiments where a powerful electromagnetic wave is injected into the overhead ionosphere where it excites parametric instabilities leading to turbulence on multiple spatial and temporal scales. The simulations are carried out using a generalized Zakharov model [1,2] 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 ponderomotive force acting on the electrons, which pull the ions via the space charge electric field. 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.

We present new results of multi-scale simulations of Langmuir turbulence during heating experiments where a powerful electromagnetic wave is injected into the overhead ionosphere where it excites parametric instabilities leading to turbulence on multiple spatial and temporal scales. The simulations are carried out using a generalized Zakharov model [1,2] 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 ponderomotive force acting on the electrons, which pull the ions via the space charge electric field. 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.


Figure 1. Development of Langmuir turbulence with time at different altitudes.

The turbulence shown in Fig. 1 leads to mode conversion of the electromagnetic wave to Langmuir waves on the turbulent ion fluctuations which work as an anomalously resistive layer. 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.