1. A regular natural perturbation feature of the nighttime ionosphere at lower altitudes and mid-latitude leads to a factor of three to ten decrease in F-region electron concentration over a time scale of about one hour, corresponding to a displacement of about 1500 km in longitude. In the case of this natural event, the perturbation is due to an interruption or sometimes reversal of the component of neutral wind upward along the direction of the geomagnetic field lines. The interruption of this transport term, which initially maintains the ionospheric plasma in a region of very low recombination rate, allows the plasma to descend in altitude by one to two neutral scale heights so that it recombines rapidly. In the case of SPS operations it is a perturbation of the plasma recombination rate at a given altitude, rather than a transport perturbation bringing the plasma to an altitude of much higher recombination coefficient (see Hanson & Carlson, 1977, p. 94). 2. What is the impact of a major ionospheric F-region depletion on the ion concentration in the conjugate hemisphere? Under typical nighttime solstice conditions, the trans-equatorial, F-region, interhemispheric winds tend to push the ionospheric plasma upward along geomagnetic field lines to regions of low recombination in the summer hemisphere, and downward to regions of rapid recombination in the winter hemisphere. This leads to strong asymmetries of plasma concentration for a given F-region and topside altitude at conjugate ends of the field lines. At latitudes within about 20° of the magnetic equator this pressure asymmetry is believed to cause strong ion temperature, T^, asymmetries (cold T. near and above say 1000 km altitude on the summer side and hot T^ in the conjugate region). The argument is in terms of interhemispheric plasma transport upward along the field lines on the summer high plasma pressure end, involving cooling by non-adiabatic expansion conductive heating while the plasma is at high altitudes, and compressional heating while the plasma is descending along the winter end of the field line. An absence of observation of this effect at greater latitudes, and the interpretation of such a cutoff, would directly relate to the parallel conjugate plasma pressure asymmetry problem for the SPS case (Hanson & Carlson, 1977, p. 93). 3.2.6 Effects of the Reduced Ionization of HF Propagation (Bauer) The potential effect of reduced ionization in the F-region on global skywave HF communication is very significant. A reduction in F-region electron density by a factor of two reduces the maximum usable frequency (MUF) by a factor /2~ , so that in the relevant geographic region the available frequency band is reduced from 3-30 MHz to (say) 3-21 MHz. However, for long- range communication the upper portion of the band, say 15-30 MHz, is optimal, so that a reduction of MUF from 30 to 21 MHz reduces the available band from 15 to 6 MHz. This is a very serious effect because: 1. the HF band is very heavily used, internationally; 2. lesser developed countries (LDCs) in particular depend heavily on this cheap and simple communication medium; 3. a reduction in F-region ionization near latitude 30°N or even closer to the equator would be particularly significant for many LDCs, that are located at these low latitudes;
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