Abstract. Large-scale ionospheric plasma irregularities, generated by O-mode heater waves at Arecibo, are shown for the first time to have "sheet-like" structures. The irregularities are aligned with the magnetic meridional plane and have scale sizes ranging t¾om a t•w hundred meters to a t•w kilometers. This interpretation is based on detailed considerations of sequential measurements of radar backscatter power, the controlling magnetic field geometry, and the deduced E x B plasma drift. The aligmnent of ()-mode-generated irregularities with the magnetic meridional plane, and their disappearance during X-mode heating intervals are consistent with predictions of the thermal filamentation instability model.

[1] Analyzed in this paper is the cascade spectrum of Langmuir waves, which are the high-frequency sideband of the parametric decay instability or oscillating two-stream instability excited by an O mode HF heating wave. It involves the decay of a Langmuir pump wave into a Langmuir sideband and an ion acoustic wave, which is heavily damped by ion Landau damping. Both resonant and nonresonant cascade processes are considered. The nonresonant cascade of Langmuir waves proceeds at the same location and is severely hampered by the frequency mismatch effect, because the decay wave is a driven ion mode oscillating at considerably lower frequency than that of the ion acoustic wave. In contrast, the resonant cascade, which takes place at different resonant locations to minimize the frequency mismatch effect, has to overcome the propagation loss of the mother Langmuir wave in each cascade step. The resonant cascade has a lower threshold, but 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 HF enhanced plasma lines (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 proposed process can be a reasonable basis for explaining the radar-measured cascade spectrum of Langmuir waves (i.e., HFPLs) in Arecibo and Tromso heating experiments.Citation: Kuo, S. P., and M. C. Lee (2005), Cascade spectrum of HF enhanced plasma lines generated in HF heating experiments,

Abstract.During recent experiments ionospheric plasma bubbles were excited by the upgraded HF heater at Arecibo. These plasma bubbles were observed by radar in the midnight sector with the entire flux tube in darkness. A simple model is outlined to explain the dynamics of density depletions generated during O-mode wave heating of the F layer. We suggest that thermal expansion of plasma away from the heated volume leads to enhanced recombination along the flux tube. In the absence of photoionization sources, density depletions develop along the excited flux tube. The discontinuity of gravity-driven currents at the walls of the depleted region requires development of polarization electric fields. Eastward polarization electric fields of •,2.5 mV/m within the flux tube caused an observed plasma bubble to drift vertically at a speed of 70 m/s.