Data from the Halogen Occultation Experiment (HALOE) are used to evaluate the contribution of upper atmospheric NOx to the stratospheric polar vortex. Using CH4 and potential vorticity as tracers, an isolated region of enhanced NOx is shown to occur in the Southern Hemisphere (SH) polar vortex almost every spring from 1991--1996. The magnitude of this enhancement varies according to the Ap auroral activity index. Up to half of the NOx in the mid-stratospheric SH polar vortex may be due to particle precipitation. The peak enhancement occurred in 1991 with a magnitude of 3--5% of the NOy source due to N2O oxidation. ¿ 2000 American Geophysical Union