Responses of Nitrogen Oxide to High-Speed Solar Wind Stream in the Polar Middle Atmosphere

Abstract

During high-speed solar wind stream (HSS) events, energetic electrons from the Earth's inner magnetosphere transfer solar wind energy to the high-latitude upper atmosphere, which may affect chemical compositions in the region. We conduct a study on the production of nitrogen oxides (NOx) in the polar middle atmosphere by energetic electron precipitation (EEP) during HSS events in the period of international polar year 2007–2008 northern winter. During this period, the geomagnetic activity was generally quiet and there were no major solar events, which indicates that the EEPs were mostly associated with HSS events. The electron flux immediately increases with the onset of HSS events and remains elevated during the passage of the events. The estimation of the directly produced NOx by EEPs was attempted by using the correlation between NOx and dynamic tracers such as CO and CH4. It was found that the direct effect of EEPs on NOx reaches down to about 55-km altitude and the amount is estimated to be about 2 ppbv. This result indicates that the variations of polar stratospheric NOx in winter are mostly associated with dynamical processes such as vertical transport and horizontal mixing. We also found that the middle atmospheric O3 depletion during HSS events seems to be related to the EEP-induced NOx at least in the uppermost stratosphere in the polar region.