Due to the systematic diurnal motion of the auroral oval, the Q values of magnetic activity derived from a single station are not directly usable to determine the expected degree of auroral activity. To remove the diurnal effects from a single station data base, which leads in daytime to an under-estimation of the Q index, an empirical set of detrending factors was determined for the Sodankyla Q data base. These factors show that the probability that a given level of magnetic activity will be exceeded for a given duration is underestimated by up to a factor of 2 in the least sensitive daytime sector. The detrended data base has been analyzed to determine the probability that a given Q value is exceeded for a selected duration. This analysis has been done for durations up to 12 hr separately for the night and daytime detrended data bases. The results are very similar and confirm that the detrending has been successfully accomplished. Finally the individual and cumulative probabilities that a given Q is exceeded for a selected duration (periods from 0.25 to 24 hr) have been established for the fully detrended data base. To interpret the results in their effect on the Over-the-Horizon Experimental Radar System, the distribution of probabilities has been modulated with the dynamics of the auroral oval by use of the Starkov equation (1969). This permits the assessments of the probability that a fixed location in the radar coverage area is under the auroral oval.