ronment. The above estimate of I and P(I > I0) are based on a highly idealized model of radiation-belt dynamics. Other sou[r]ces of fluctuation are quite likely to broaden the probability distribution. For example, one might expect both [[spi:math]] and (especially) S in (51) to depend on geomagnetic activity and therefore on time. Moreover, the magnetosphere is subject to reversible (adiabatic) compressions that change the scales of energy and distance. Such compressions (caused by slow variations of solar-wind speed) are not of dynamical interest and so are omitted from (51)-(53), but such adiabatic processes would have to be considered in formulating a model of the detailed time history of radiation dosage impacting a hardened spacecraft. The expected relativistic electron dosage increase estimated here is not sufficiently hazardous to preclude activity in space. However, increased dosage over long durations will undoubtedly impact space-borne equipment lifetimes. This is especially true for space systems operating in the regions of the radiation belts (earthward of GEO); an example among these is the Global Positioning System of twenty-four navigation satellites in 20,000 km orbits. Ironically, radiation belt modification is unlikely to impact SPS construction and operation at GEO because the radiation environment at GEO is considerably more benign. Nevertheless, it is rather likely that mitigation measures in the design of future space activities may have to be introduced to cope with radiation belt modification if SPS construction and operations become reality. G. Plasma Instabilities and Effects on Space Communications As we have alluded to in previous Sections, we expect the injection of the ion engine beam into the magnetosphere would cause turbulence in the form 56
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