On the Cascade Spectrum of Langmuir Waves in HAARP Heating Experiments

Summary form only given. A major facility for conducting ionospheric heating experiments is under development in Gakona, Alaska, as part of the high frequency active auroral research program (HAARP). Currently, HAARP HF transmitting system is being expanded from a phased-array antenna of 48 elements to one with 180 elements. After completion of this upgrading, its maximum effective radiated power (ERP) will exceed 1 GW. A backscatter radar (450 MHz) will also be installed soon near the heating site. Therefore, it is relevant to consider what the expected spectral features in the "HF wave enhanced plasma lines (HFPLs)" will be, which helps to plan the forthcoming heating experiments. The most recognizable one is to observe discrete cascade lines appearing in the spectra of HFPLs. The number and frequency separation of the cascade lines signify the cascade feature of HFPLs. In the present work, the cascade features in the HFPLs to be observed in HAARP heating experiments are studied by analyzing two parametric instabilities based on previously suggested mechanisms. The first one decays a Langmuir pump wave into a Langmuir sideband and an ion acoustic wave, which is heavily damped by the ion Landau damping mechanism. Resonant cascade taking place in different resonant locations to minimize the frequency mismatch effect is considered, but this cascade process has to overcome the propagation loss of the mother Langmuir wave in each cascade step. The cascade lines spread over a range of altitude. The dominant factors, which determine the number of cascade lines in the radar-detected spectrum of HFPLs, include the ion to electron temperature ratio, T_i/T_e, the background plasma inhomogeneity scale length, and the heating wave field intensity. The second one is a three-dimensional coupling process that employs the lower hybrid wave as the decay mode. Through this process, nonresonant cascade of Langmuir waves produce a broad spectrum of HFPLs in a narr- w altitude range locating slightly below the O-mode HF reflection height