Energetic particle precipitation, in which high-energy electrons and protons from the Sun and magnetosphere hit Earth's upper atmosphere, results in the production of nitrogen oxides (NOx) in the mesosphere and lower thermosphere. Under the right weather conditions, NOx can descend into the stratosphere, where it destroys ozone. The authors note that 2009 is the second time on record in recent years when abnormal weather conditions led to increased descent of NOx in the polar region. They suggest that this may indicate that changes are occurring in the atmosphere and point to a need for better understanding of the interaction between meteorology and space weather.