[P-EM08_2PM1] Space Weather and Space Climate

[PEM08-27] 南極昭和基地で観測された中間圏・下部熱圏のNOの変動

礒野 靖子¹、*水野 亮¹、長浜 智生¹、三好 由純¹、中村 卓司²、片岡 龍峰²、堤 雅基²、江尻 省²、藤原 均³、前澤 裕之⁴、上村 美久¹ (1.名古屋大学　太陽地球環境研究所、2.国立極地研究所、3.成蹊大学、4.大阪府立大学)

キーワード：Nitric oxide, mesosphere and lower thermosphere, energetic particle precipitation, geomagnetic storm, solar proton event, Antarctica

Energetic particle precipitation (EPP) related to solar proton events or geomagnetic storms induce ion-neutral reactions and change abundance of some minor molecules such as NO_x and HO_x in the mesosphere and lower thermosphere. To investigate the temporal variations of NO by EPP, we installed a millimeter-wave spectroscopic radiometer at Syowa Station (69.00S, 39.85E), and we have carried out ground-based observations of spectral line of nitric oxide (NO) at 250.796 GHz since January 2012.We obtained 197 and 172 daily averaged NO spectra in 2012 and 2013 (until 30 September; DOY 273), respectively. The daily NO spectra are characterized by narrow line width with a Full-Width-at-Half-Maximum (FWHM) of about 0.5 MHz. These NO spectra are well fitted by a single Gauss function or by a single Lorenz function. From the spectral line shape, we conclude that the NO emitting region is between 75 and 100 km.We found two temporal variation patterns of NO column density. One is a seasonal variation with a maximum in the winter and a minimum in the summer. The column density of NO during the winter was about 4 times larger than that during the summer. This seasonal variation is considered to be related to the atmospheric transport and the NO dissociation by solar radiation. The other is the short-term variation in a timeframe of 5-10 days associated with EPP events such as solar proton events and geomagnetic storms. At Syowa Station, short-term variations were caused mainly by the precipitation of electrons rather than that of protons. In the electron precipitation events, the column density of NO gradually increases just after the main phase of the geomagnetic storm and gradually decreases soon after its peak.One of the short-term events related to a large geomagnetic storm in April 2012 was the most prominent single event among those observed at Syowa Station since January 2012. From the high time resolution (∼ 3-hour) data, we revealed a diurnal tendency that NO column density increased about twice at UT 0, which is interpreted to be caused by the dawn-dusk asymmetry of the precipitated electrons with energies 30-300 keV.