Reliable HF communications along short‐, medium‐ and long‐range paths require propagation assessment. Such assessment could be facilitated with the monitoring of ionospheric characteristics by continuously available passive means, i.e., measurement of the total electron content (TEC) using satellite‐emitted signals without a need for burdening the electromagnetic spectrum. With ubiquitous Global Positioning System (GPS) providing instantaneous time delay, or equivalently, TEC, values when needed, an assessment of HF propagation conditions may be available on a near‐real‐time basis. Both TEC and the peak electron density of the ionosphere, which determines the ordinary upper frequency limit (ƒ0F2( for HF sky wave vertical propagation, vary strongly with solar and geomagnetic parameters. Their ratio, the equivalent slab thickness, may vary to a lesser degree and hence be modeled with greater accuracy. A slab thickness model combined with real‐time TEC measurement anywhere on the globe may possibly yield an improved HF parameter prediction algorithm. To test the efficacy of the hypothesis, one has to ascertain the correlation, as exhibited by the correlation coefficient, between the TEC daily variability about the monthly mean and the ƒ0F2 variability. To determine such correlation, a study compared Faraday TEC data as well as GPS‐generated TEC data collected in Israel and with corresponding ƒ0F2 values obtained from vertical sounder measurements near the appropriate subionospheric location in Cyprus. The analysis shows that for large percentages of the time, very good correlation exists between TEC and ƒ0F2 short‐term variations. The correlation coefficient varies between 0.7 or better during winter and summer months to about 0.5–0.6 during equinox months. A study of the diurnal dependence of the correlation indicates that a better correlation exists during daytime than nighttime. There was no indication that the coefficient is dependent on geomagnetic activity or on protonospheric electron content during the period of this study.
By a sample record we show that the radio beacon experiments which utilize the Faraday rotation technique and the scintillation observations can be used to observe equatorial ionization bubbles. In a two‐hour period 5 isolated bubbles have been identified. The depleted total electron content for one such bubble is 2.2×1016 electrons/m² and the east‐west dimension is about 72 km. This translates to a total depletion of 1.6×1021 electron per meter in the north‐south direction.
Recent data collected near the magnetic equator depict one kind of ionospheric perturbation in the nighttime hours variously as bubbles or plumes. Theoretical studies show that the underside of the ionosphere is subjected to Rayleigh‐Taylor instabilities which, when they are triggered, will cause a region of depleted ionization to rise as bubbles. When such regions are traversed by a probing radio wave, the associated Faraday effect is expected to show depletions of the electron content. This paper describes some experimental results obtained at Natal, Brazil (35.23°W, 5.85°S, dip −9.6°), by monitoring radio signals transmitted by the geostationary satellites Marisat 1 and SMS 1. Using ionization depletions as indications of bubbles, statistical studies of occurrence, size, and magnitude of perturbations are carried out. The most probable depletions for the propagation path under study have values in the range 1–4 × 1016 el/m², but depletions as large as 1.2 × 1017 el/m² have also been observed. The average durations for each observed bubble may vary from less than 2 to over 30 min with an average of 8 min. The experimental data further show that the scintillation rate may increase suddenly when these bubbles either form along or drift across the propagation path.
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