In this paper we examine the claim that the power law distribution of burst lifetimes in the AE index is evidence that the magnetosphere is a Self-Organized Critical (SOC) system. To do this we compare the burst lifetime distributions of the AU and |AL| indices with those of the vBs and ϵ solar wind input functions. We show for the first time that both the vBs and ϵ burst lifetime distributions are of power law form with an exponential cut-off, consistent with the solar wind being an SOC system. Furthermore, the power law of the ϵ burst lifetime distribution is not significantly different to that of the AU and |AL| indices, indicating that this scale-free property of the AE indices could arise from the solar wind input and may not be an intrinsic property of the magnetospheric system. We discuss the implications of this result for SOC theories of the magnetosphere. ¿ 2000 American Geophysical Union