We are grateful to Benno Blumenthal, director of the IRI Data Library, for his assistance with some of the computations and to the reviewers for their helpful comments. This work was supported by National Oceanic and Atmospheric Administration Grant NA08OAR4310622.
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The AC in any given synthetic time series may not be identically zero because of sampling. For this reason, 100 synthetic time series were generated to be able to place confidence limits on the results obtained.