Solar proton events (SPEs) are known to have caused changes in constituents in the Earth's polar neutral middle atmosphere. The past four years, 2000 2003, have been replete with SPEs. Huge fluxes of high energy protons entered the Earth's atmosphere in periods lasting 2 3 days in July and November 2000, September and November 2001 and October 2003. The highly energetic protons produce ionizations, excitations, dissociations and dissociative ionizations of the background constituents, which lead to the production of HO, (H, OH, HO 2) and NO, (N, NO, NO2 , NO 3, N 2 0 5 , HNO 3 , H0 2NO 2 , ClONO 2 , BrONO 2). The HO1 increases lead to short-lived ozone decreases in the polar mesosphere and upper stratosphere due to the short lifetimes of the HO, constituents. Large mesospheric ozone depletions (>70%) due to the HO, enhancements were observed and modeled as a result of the very large July 2000 SPE. The NOY increases lead to long-lived stratospheric ozone changes because of the long lifetime of the NOY family in this region. Polar total ozone depletions >1% were simulated in both hemispheres for extended periods of time (several months) as a result of the NOY enhancements due to the very large SPEs.