New directions in theory and experiment for HF excitation of Langmuir turbulence in the ionosphere

Don DuBois

Lodestar Research Corp*., Boulder, CO, and Los Alamos National Laboratory

In the last few years observations at Tromsoe and Arecibo ,using very low duty cycle hf heating, have verified, in semi quantitative detail, all the theoretical predictions for the plasma line and ion line incoherent scatter radar power spectra. These predictions where made for a smooth ionosphere using the reduced plasma modeling based on the extended Zakharov model. Low duty cycle heating allowed the experimental realization of a smooth ionosphere.

The challenge for the future is to build on this success to explore new regimes of hf heating. In higher duty cycle regimes of hf heating the ionosphere is not smooth and new problems arise. These include understanding the Langmuir turbulence in the presence of field aligned ducts or striations (see paper by Mjoelhus et all, this meeting), the formation of horizontally stratified density depletions due to the ponderomotive and thermal pressure of the hf standing wave and the induced turbulence, and the relationship between the Langmuir turbulence excited near reflection density and the formation of striations. There are also other ways to excite the turbulence which might now be feasible such as the two plasmon decay instability, at 1/4 of reflection density, and by beat wave heating.

In the past few years we have been studying kinetic effects in Langmuir turbulence including the generation of fast electrons. A new reduced particle-in-cell code has been developed at Los Alamos by Hoanh Vu which is the first such code capable of resolving the Langmuir decay cascade and collapse. For weak driving regimes, which seem to include most of the hf heating applications, there is excellent agreement between the full kinetic code and the Zakharov modeling. For stronger drive the agreement is still good provided the Zakharov modeling is augmented by the quasilinear evolution of the electron distribution function. However for kle>0.2 ,where k is the wave number of the primary PDI unstable Langmuir wave and le is the electron Debye length, the Zakharov model begins to fail and we enter a regime where electron trapping appears to become the dominant nonlinearity and stronger excitation is produced than predicted by linear theory. It is interesting to speculate whether this regime has been or can be reached in hf heating say with the UHF radar and low heating frequency.