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Abstract:
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It is well-known that the generation of equatorial, F-region plasma density irregularities, via the Generalized Rayleigh-Taylor instability mechanism is critically dependent on the magnitude of the pre-reversal enhancement (PRE) in upward ExB drift velocity after sunset. These plasma density “bubbles” that are generated after sunset lead to the “scintillation” of trans-ionospheric radio wave signals that pass through these bubbles and is commonly referred to as “scintillation activity”. Communication and Navigation systems can be severely disrupted by these plasma density irregularities. A measure of scintillation activity is given by the “S4 Index” and a network of Air Force, ground-based UHF and L-band receivers measuring the S4 Index is called the SCIntillation Network Decision Aid (SCINDA) network. This paper describes a technique for automatically forecasting, in real-time, the occurrence or non-occurrence of scintillation activity that relies on real-time data from a ground-based ionospheric sounder at or near the geomagnetic equator. After sunset, the height-rise with time of the bottom-side of the F-layer reflects the magnitude of the upward ExB drift velocity. The value of the ionospheric parameter, h’F (the virtual height of the bottom-side F-layer) at 1930 LT reflects the integrated ExB drift effect on lifting the F-layer to an altitude where the Rayleigh-Taylor instability mechanism becomes important. It is found that there exists a linear relationship (R2 = 0.99) between h’F values at 1930 LT and the magnitude of the pre-reversal enhancement (PRE) in upward ExB drift which means that the real-time observations of h’F at 1930 LT would provide real-time PRE values as input to future IRI models that calculate equatorial, post-sunset ionospheric parameters. We describe how FIRST has been developed into a real-time capability for automatically forecasting scintillation activity that is available on Google Earth to all interested parties. |
