The polar cap (PC) index measures the level of geomagnetic activity in the polar cap based on magnetic perturbations from overhead ionospheric currents and distant field-aligned currents on the poleward edge of the nightside auroral oval. Because PC essentially measures the main sources of energy input into the polar cap, we propose to use PC as a proxy for the hemispheric Joule heat production rate (JH). In this study, JH is estimated from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) procedure. We fit hourly PC values to hourly averages of JH. Using a data base approximately three times larger than studies, we find a quadratic relationship between JH and PC, differentiated by season. A comparison during the November 1993 storm interval with earlier reported methods using the AE index and the cross polar cap potential, shows that the PC-based Joule heating estimate is as equally accurate. Thus the single station PC index appears to provide a quick estimate of, and is an appropriate proxy for, the hemispheric Joule heating rate. ¿ 1999 American Geophysical Union