Nitric acid enhancements in the mesosphere during the January 2005 and December 2006 solar proton events

We investigate enhancements of mesospheric nitric acid (HNO3) in the Northern Hemisphere polar night regions during the January 2005 and December 2006 solar proton events (SPEs). The enhancements are caused by ionization due to proton precipitation, followed by ionic reactions that convert NO and NO2 to HNO3. We utilize mesospheric observations of HNO3 from the Microwave Limb Sounder (MLS/Aura). Although in general MLS HNO3 data above 50 km (1.5 hPa) are outside the standard recommended altitude range, we show that in these special conditions, when SPEs produce order-of-magnitude enhancements in HNO3, it is possible to monitor altitudes up to 70 km (0.0464 hPa) reliably. MLS observations show HNO3 enhancements of about 4 ppbv and 2 ppbv around 60 km in January 2005 and December 2006, respectively. The highest mixing ratios are observed inside the polar vortex north of 75 degrees N latitude, right after the main peak of SPE forcing. These measurements are compared with results from the one-dimensional Sodankyla Ion and Neutral Chemistry (SIC) model. The model has been recently revised in terms of rate coefficients of ionic reactions, so that at 50-80 km it produces about 40% less HNO3 during SPEs compared to the earlier version. This is a significant improvement that results in better agreement with the MLS observations. By a few days after the SPEs, HNO3 is heavily influenced by horizontal transport and mixing, leading to its redistribution and decrease of the SPE-enhanced mixing ratios in the polar regions.