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similar to those reached by solar protons. These energetic electrons are accelerated primarily within the magnetosphere (rather than on the Sun) and occur much more frequently than major flare events. The presence or absence of the relativistic electron fluxes appears to be strongly controlled by the existence of high-speed solar-wind streams emanating from persistent coronal holes, which in turn are strongly controlled by the solar activity cycle. The extent to which precipitating relativistic electrons actually enter the atmosphere is currently uncertain. As noted in Chapter 3, if a significant amount of these ionizing particles reaches the middle atmosphere, the long term effects on ozone concentrations in the stratosphere could be significant (Callis et al., 1991).