most effective when based on an understanding of the underlying physical processes. In addition, the variability and dynamics of the natural ionosphere need to be included in such a model or, at least, be better understood than at present. Active experiments that alter the ionospheric conductivity by the enhancement or depletion of ionization are possible, particularly in the F- region. However, they are generally localized and are effective over regions with horizontal scale-size smaller than 100 km. Because the more significant effects involve an alteration of conductivity over much larger or even global dimensions, it is not expected that small-scale active experiments would be useful for SPS assessment. In addition, charge polarization may occur in small-scale modifications making the results difficult to interpret or to extrapolate correctly to global scale. Natural perturbations of the normal global conductivity are probably the most useful experimental simulation of SPS effects. Examples of such perturbations include particle precipitation and the formation of detached plasma regions during magnetic storms, auroral zone conductivity changes, and relativistic electron precipitation (REP) events. Measuring and understanding the effects of these phenomena would provide data needed for model calibration and development for SPS assessment.
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