POAM III data show unusually large increases in stratospheric NO2 throughout the late winter and spring at high southern latitudes during 2000. Using HALOE CH4 data as a tracer of vertical descent, we conclude that excess NOx was created by particle impacts in the upper atmosphere and descended in the polar vortex during the winter. We speculate that these NOx enhancements were due to the solar proton event that occurred on 14--15 July 2000, and show that they caused reductions of up to ~45% in middle stratospheric ozone mixing ratios. Comparison of HALOE and POAM data in 2000 to data from 1991--1999 suggests that the 2000 NOx enhancements were the largest ever documented by satellite in the southern hemisphere middle stratosphere. Also, based on H2O data, we conclude that NOx-enriched air observed in the south polar vortex from 1991--1999 originated in the mesosphere, not the thermosphere as is often assumed. ¿ 2001 American Geophysical Union