The large solar storm in October--November 2003 produced enormous amounts of high-energy protons which reached the Earth and penetrated into the middle atmosphere in the polar regions. At this time, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the Environmental Satellite (ENVISAT) was observing the atmosphere in the 6--68 km altitude range. MIPAS observed significant enhancements of the NOy components HNO3, N2O5, and ClONO2 in the northern polar stratosphere after the intense solar proton events. Two distinct HNO3 enhancements were observed. An instantaneous increase of 1--2 ppbv was observed immediately after the SPEs and is attributed to gas-phase chemistry: NO2 + OH + M → HNO3 + M, accelerated by SPE-produced excess OH. A very large second increase of 1--5 ppbv started around 10 November and lasted until the end of December. It is attributed to NOx (NO + NO2) produced in the mesosphere during the major SPEs in late October/early November and then transported downward during November and December, partially converted to N2O5 in the upper stratosphere, which finally formed HNO3 via ion cluster reactions. N2O5 was observed to increase by 0.1--0.4 ppbv 1--3 days after the major SPEs and reached down to 30 km altitude. A second, more pronounced N2O5 enhancement of up to 1.2 ppbv at 40 km appeared about 12--13 days after the major SPEs. With a delay of 1--2 days after the major SPEs ClONO2 increased by up to 0.4 ppbv (40%) at 32 km altitude. NOy enhancements in the Southern Hemisphere were generally less pronounced.