Interannual variation of NOx from the lower thermosphere to the upper stratosphere in the years 1991-2005

The interannual variation of NOx throughout the year is investigated for the period 1991-2005 at middle to high latitudes using Halogen Occultation Experiment (HALOE) on UARS measurements. We find a clear correlation of NOx between 80 and 130 km with the auroral electrojet index in both hemispheres, which is fairly independent of season, indicating a relatively frequent NOx source from precipitating auroral electrons of energies ranging from about 1 keV to several tens of keV. Between 80 and 100 km, NOx is also highly correlated to fluxes of higher-energy electrons as measured by the Space Environment Monitor (SEM) and its successor, the SEM-2, instruments on POES mostly during autumn and spring, indicating a strong impact of 10-100 keV electrons, which, however, precipitate less frequently than the auroral electrons. Electrons with energies of several MeV were investigated also, and a significant correlation was found with NOx during some periods. The correlation is smaller and less stable than for the lower-energetic auroral and POES electrons, indicating that the contribution of MeV electrons to the overall NOy budget is small, at least in the latitude range considered. Also, the altitude range affected, above 60 km, indicates that this impact is probably due to electrons of lower energies ( several hundreds of keV instead of several MeV) than the GOES electrons used for the investigation, to which they must be closely related, however. Downward propagation of NOx is observed in both hemispheres during winter but continues to lower altitudes and lasts longer in the Southern Hemisphere, where the signal can be followed to altitudes around 40 km.