The structure function of the AE time series shows that the AE time series is self-affine such that the scaling exponent changes at the time scale of approximately 113 (¿9) minutes. Autocorrelation function is shown to have scaling properties similar to those of the structure function. From this result it can be deduced that the time scale at which the scaling properties of the AE data change should equal the typical autocorrelation time of these data. We find the typical autocorrelation time of the AE data is 118 (¿9) minutes. The characteristic time scale of the AE data appears as a spectral break in their power spectrum at a period of about twice the autocorrelation time. ¿ American Geophysical Union 1994