HNO₃, N₂O₅, and ClONO₂ enhancements after the October-November 2003 solar proton events

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 NO_y components HNO ₃, N₂O₅, and ClONO₂ in the northern polar stratosphere after the intense solar proton events. Two distinct HNO ₃ 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 NO _x (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. NO_y enhancements in the Southern Hemisphere were generally less pronounced.