The 3-h K p index is widely used as a measure of geomagnetic activity for ionospheric studies. Speci®cally, it is the planetary index used to determine the geomagnetic dependence of statistical auroral patterns and the convection electric ®eld for certain models. Its quasi anti-logarithm, the A p index, is similarly used in statistical models of the neutral atmosphere and neutral wind. Physics-based ionospheric models, such as the Utah State University (USU) Time-Dependent Ionospheric Model (TDIM), use these statistical models as magnetospheric and thermospheric inputs. However, the 3-h time interval between index computations is now considered a shortfall with regard to specifying and forecasting phenomena known to have faster time constants, e.g., auroral electrojet variations during a substorm. Therefore, these indices have been targeted for high-time resolution development; we have developed such indices in Della-Rose et al. (1999). We now use our 15-minute station K-like'' index to determine the eect of introducing high-time resolution magnetic ¯uctuations into the TDIM inputs. This study represented the highlatitude ionosphere by a grid of 1484 locations, and was carried out for a geomagnetic storm period during solar maximum and simulated'' winter solstice conditions. We found that, for ®xed Interplanetary Magnetic Field (IMF) B y aB z ratio, driving the TDIM with our 15-minute ``K-like'' index altered the average high-latitude N m F 2 value by as much as 8% (vs. the average N m F 2 obtained using a 3-h index to drive the TDIM). More signi®cantly, the standard deviation of the N m F 2 variations was up to 35%. Under some conditions, the average N m F 2 was changed by up to 30% with a standard deviation of over 60%. However, the eect of selecting dierent convection patterns that represented three southward IMF B z orientations led to larger eects. The high-latitude average N m F 2 changed by 10% or less, but the spread in the distribution always ranged from standard deviations of 29 to 68%. These results indicate that there is a substantial need to consider both short-term magnetic ¯uctuations and inclusion of real-time IMF data in the inputs to ionospheric models.