The aim of this thesis is to study the effects of particle precipitation on O3 and OH caused by a moderate geomagnetic storm in late July 2009. The basis is data retrieved over Antarctica from the Odin and Aura satellites for an eleven day period. Vertical profiles were extracted from scans at the limb every 165 Km (Aura) and 500 Km (Odin) covering latitudes on the SH extending to 82 S.Geographical variability of O3 and sparse satellite coverage made it difficult to observe storm effects on a latitudinal/longitudinal scale. Zonal means post-storm show a considerable drop of ozone (30-50 %) below 80 Km, and a distinct increase (50 %) at altitudes above 80 Km.Comparison of the zonal means of O3 and OH reveal high concordance between areas of rising hydroxyl and areas of ozone depletion, suggesting that the increase of OH production during the storm is the main cause of O3 depletion. The extent of the O3 and OH changes occur from geomagnetic latitudes greater than 60 S and extend into the polar regions.The zonal means indicate that the OH causes the initial O3 loss, and then hydroxyl disappears rapidly. The O3 remains depleted for four days descending in the polar vortex suggesting contribution from other species (NOx). Time series binned around geomagnetic latitude 62 S were compared with ground-based microwave observations from the Troll station, Antarctica. The two data sets are consistent, showing ozone losses in the order 30-50 % and a gradual poleward descent of this depletion.