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

Abstract

Abstract. Recent experiments carried out at the Arecibo Observatory in Puerto Rico and the magnetically conjugate location in Argentina suggest a connection between the prevalence and intensity of the interhemispheric propagation of VLF 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 transmissions into the affected region of plasma. These signals presumably then entered plasmaspheric ducts more efficiently and led to conjugate measurements 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 extending well into the F region but producing the majority of the total transit delay at plasmaspheric altitudes. It is also suggested that weak long-delay receptions often observed in these data are most probably prolongitudinal whistlers that are not ducted but which represent a potential contaminant in ducted VLF experiments of all types.

Effects of HF heater‐produced ionospheric depletions on the ducting of VLF transmissions: A ray tracing study

“…[36] The computed results provide a reasonable basis for understanding the results of Starks et al [2001], who reported a correlation between O-mode HF heating and enhanced local ducting but in a limited number of cases. They may also explain the results of Wright [1975], who observed the formation of localized sporadic E owing to electron precipitation as a proxy for duct excitation.…”

“…The mechanism explored in this study affects only rays on L $ 1.4, sometimes enhancing ducted transmissions in this region where ducting is naturally inefficient. This could explain the very rapid onset and remission of ducted signals on L $ 1.4 observed in Starks et al [2001] and apparently correlated with HF heating.…”

“…Ducted rays have wave normal angles that oscillate below some fixed value as they propagate. Nonducted rays quickly proceed to large angles to the ambient field and can be easily identified (see Figure 6 of Starks et al [2001]).…”

“…[28] These findings may provide a possible explanation for the difficulties encountered by Starks et al [2001] in consistently reproducing apparent instances of heaterenhanced ducting from night to night. Andrews et al [1978] measured the radial motion of plasma (and hence of ducts) at the equator in the plasmasphere by observing changes in ducted whistler mode signals and found velocities on the order of tens of m s À1 .…”

“…In experiments where HF heater‐produced structures seem to affect the coupling of ground‐based transmissions into plasmaspheric ducts, it has been difficult to reproduce the effect consistently from night to night and even from hour to hour [ Wright , 1975; Starks et al , 2001]. This is probably due to the natural variability of ionospheric structure and plasmaspheric ducts as well as their slow drift across L shells.…”