Interhemispheric propagation of VLF transmissions in the presence of ionospheric HF heating

Recent experiments carried out at the Arecibo Observatory in Puerto Rico an d the magnetically conjugate location in Argentina suggest a connection bet ween the Prevalence and intensity of the interhemispheric propagation of VL F transmissions and HF heating of the ionosphere. During several nights of the study the O-mode CW heater appeared to enhance the coupling of VLF tran smissions into the affected region of plasma. These signals presumably then entered plasmaspheric ducts more efficiently and led to conjugate measurem ents of relatively high amplitude. No such effects were seen with pulsed or X-mode heating. Short time resolution measurements of fading transmissions were marked by transit delays and amplitudes that remained nearly constant as the interhemispheric VLF signals diminished, consistent with the model of a plasmaspheric duct complemented by favorable plasma structures extendi ng well into the F region but producing the majority of the total transit d elay at plasmaspheric altitudes. It is also suggested that weak long-delay receptions often observed in these data are most probably prolongitudinal w histlers that are not ducted but which represent a potential contaminant in ducted VLF experiments of all types.