Variability in the high latitude electric field has been identified as a major contributor to global Joule heating. Electric field patterns from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure are used to characterize the E-field temporal variability over the course of 18 hours. The standard deviation of the E-field magnitude on May 4, 1998 often exceeds the average value of the E-field magnitude. A significant fraction of this variability arises from oscillations with period less than one hour. This confirms that Joule heating calculations based on time-averaged E-fields may significantly under-predict the heating.