HF excited instabilities in space plasmas

Radio waves delivering sufficiently strong electric field strengths to a plasma can excite parametric instabilities in the plasma for properly matched conditions. Technology had advanced by 1970 to the stage where this could be effected in the ionospheric plasma by ground‐based installations. High‐power HF transmitters, in the nominal 3–12 MHz range, deposited substantial energy in the ionospheric plasma, raising thermal plasma temperatures, exciting a rich spectrum of waves and irregularities in the plasma, and leading to extrathermal electron fluxes sensible to energies of the order of 10 eV. The results of experiments utilizing such controlled effects on the ionospheric plasma as diagnosed by remote sensors in a very broad RF band as well as in the optical band, have been followed with great interest by workers in fields as diverse as nuclear fusion, plasma physics, communications, and aeronomy from ionospheric thermal balance to spread‐ F . Significant checks on and advances in relevant theory have already accrued, but much of the potential is yet untapped. Experiments of this nature can also be performed from space vehicles. These could not only supplement studies possible from ground‐based sites, but also permit some qualitative advances by virtue of the spacial trajectory and in situ nature unique to the vehicle. A number of specific features of the phenomena bear importantly on the practicality of such an experiment, including: instability rise times and decay times, irregularity scale sizes, power dependence, frequency spectra and implied physics, thermal plasma time constants, and field geometry dependences. The results of ground‐based “HF heating experiments,” particularly as diagnosed by incoherent scatter techniques, are reviewed with this in mind.