We use atmospheric ozone density profiles between 35 and 65 km altitude derived from SCIAMACHY limb measurements to quantify the ozone changes caused by the solar proton events from 26 October to 6 November 2003, known as the Halloween storm. Detailed maps and daily resolved time series up to 5 weeks after the first event are compared with the results from a chemistry, transport, and photolysis model of the middle atmosphere that includes NOx and HOx production due to energetic particle precipitation. The general features of the ozone loss are captured by the model fairly well. A strong ozone depletion of more than 50% even deep into the stratosphere is observed at high geomagnetic latitudes in the Northern Hemisphere, whereas the observed ozone depletion in the more sunlit Southern Hemisphere is much weaker. Reasons for these interhemispheric differences are given. Two regimes can be distinguished, one above about 50 km dominated by HOx (H, OH, HO2) driven ozone loss, one below about 50 km, dominated by NOx (NO, NO2) driven ozone loss. The regimes display a different temporal evolution of ozone depletion and recovery. We observe for the first time an establishment of two contemporaneous maxima of ozone depletion at different altitudes, which solely can be explained by these regimes.