Ground-based observations of nitric oxide in the mesosphere and lower thermosphere over Antarctica in 2012-2013

We report temporal variations of the partial column density of nitric oxide (NO) in an altitude range 75–105 km at Syowa Station, Antarctica, from January 2012 to September 2013. We found two patterns of NO temporal variation: (1) a seasonal cycle with a maximum in winter and a minimum in summer, and (2) short‐term enhancements on a time frame of 5–10 days associated with solar activities. In the seasonal cycle, the variation pattern of NO showed good agreement with scotoperiod of solar radiation rather than the downwelling atmospheric transport, suggesting that photodissociation is the main driver of the seasonal variation. To study the short‐term enhancements, we compared the NO column density with the proton and electron fluxes obtained by the POES/METOP (Polar Orbiting Environmental Satellite/Meteorological Operational) satellites. There is a weak but significant correlation between the NO and the electron flux, but no correlation between the NO and the proton flux. We also made a detailed comparison of the time series of NO and proton/electron fluxes for 12 selected 25‐day time frames, and found that at least two remarkable NO enhancements occurred without any solar proton events (SPEs). During electron precipitation, the NO column density peaked 1–5 days after the commencement of geomagnetic storms, whereas the relationship between NO and the solar proton is not clear because the electron flux also increased at the same time. These results suggest that energetic electron precipitation may be a major cause of NO enhancements above Syowa Station in the auroral region, even during SPEs. Key Points Seasonal cycle and short‐term variations in several days of NO was detectedSeasonal cycle was caused by the descent of air and solar radiationShort‐term variations were caused by electron precipitation rather than by SPE