The effects of hard-spectra solar proton events on the middle atmosphere

The stratospheric and mesospheric impacts of the solar proton events of January 2005 are studied here using ion and neutral chemistry modeling and subionospheric radio wave propagation observations and modeling. This period includes three SPEs, among them an extraordinary solar proton storm on 20 January, during which the >100 MeV proton fluxes were unusually high, making this event the hardest in solar cycle 23. The radio wave results show a significant impact to the lower ionosphere/middle atmosphere from the hard spectrum event of 20 January with a sudden radio wave amplitude decrease of about 10 dB. Results from the Sodankyla Ion and Neutral Chemistry model predict large impacts on the mesospheric NOx (400-500%) and ozone (-30 to -40% NH, -15% SH) in both the northern (winter) and the southern (summer) polar regions. The direct stratospheric effects, however, are only about 10- 20% enhancement in NOx, which result in -1% change in O-3. Imposing a much larger extreme SPE lasting 24 hours rather than just 1 hour produced only about 5% ozone depletion in the stratosphere. Only a massive hard-spectra SPE with high-energy fluxes over ten times larger than observed here (>30 MeV fluence of 1.0 x 10(9) protons/cm(2)), as, e. g., the Carrington event of 1859 (>30 MeV fluence of 1.9 x 10(10) protons/cm(2)), could presumably produce significant in situ impacts on stratospheric ozone.