A study of the short-term (12-hour) variation of the linear correlation between the solar windmagnetosphere energy coupling function &egr; and the electrojet index AE revealed unexpectedly large variations of the correlation coefficient R and of the ratio between &egr; and AE. About half of all R values fell below the 95% confidence level, and the ratio between &egr; and AE varied by more than 100% of the average value, even when R was large. This result reveals that the &egr; function is not a very good predictor of substorms and their intensities. Several possible causes for the variations are discussed. It is shown that some of the variations of the correlation coefficient occur because of a variable temporal relationship between &egr; and AE.