Observations of O3 and NO2 made by the GOMOS instrument on board European Space Agency's Envisat satellite have been used to monitor the increase of NO2 and depletion of ozone due to the solar proton events of October--November 2003. For the first time this phenomenon was measured in polar winter conditions by a satellite instrument. Results show NO2 enhancement of several hundred per cent and tens of per cent ozone depletion between 36 and 60 km, an effect which lasts several months after the events. A comparison of the after-event concentrations of NO2 and ozone reveals a strong negative correlation.