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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.

We report extreme enhancements of the nitric oxide (NO) column density observed with the ground-based millimeter-wave spectroscopic radiometer installed at Syowa Station, Antarctica, during a large geomagnetic storm in April 2012. From the NO spectrum line shape and NO column density relationship with solar radiation, we concluded that the NO was emitted in the altitude range between 75 km and 100 km. The column density of NO gradually increased during the recovery phase. In addition to variations on a time frame of several days, we found diurnal variations. The increase of NO was related to precipitated electrons in the energy range of 30–300 keV observed by Polar-orbiting Operational Environmental Satellite (POES)/The Meteorological Operational (METOP). We found a rapid response (within 1 h) and a one-to-one correspondence between them. For the first time, we show that a remarkable increase of the column density of NO is caused by dawn-dusk asymmetry of the plasma sheet electrons.

Accepted: 2011-08-01, 資料番号: SA1003137000

Accepted: 2010-12-16, 資料番号: SA1002907000

第１回極域科学シンポジウム「極域大気圏を通して探る地球規模環境変動」12月2日（木） 国立極地研究所 ２階大会議室

We present the results of daily monitoring of 6.7 GHz methanol maser in Cepheus A (Cep A) using Yamaguchi 32-m radio telescope as well as the results of imaging observations conducted with the JVN (Japanese VLBI Network). We indentified five spectral features, which are grouped into red-shifted (-1.9 and -2.7 km/s) and blue-shifted (-3.8, -4.2, and -4.9 km/s), and we detected rapid variabilities of these maser features within a monitoring period of 81 days. The red-shifted features decreased in flux density to 50% of its initial value, while the flux density of the blue-shifted features rapidly increased within a 30 days. The time variation of these maser features showed two remarkable properties; synchronization and anti-correlation between the red-shifted and the blue-shifted. The spatial distribution of the maser spots obtained by the JVN observation showed an arclike structure with a scale of $\sim$1400 AU, and separations of the five maser features were found to be larger than 100 AU. The absolute position of the methanol maser was also obtained based on the phase-referencing observations, and the arclike structure were found to be associated with the Cep A-HW2 object, with the elongation of the arclike structure nearly perpendicularly to the radio continuum jet from the Cep A-HW2 object. These properties of the masers, namely, the synchronization of flux variation, and the spectral and spatial isolation of features, suggest that the collisional excitation by shock wave from a common exciting source is unlikely. Instead, the synchronized time variation of the masers can be explained if all the maser features are excited by infrared radiation from dust which is heated by a common exciting source with a rapid variability., 7 pages, 3 figures, 2 tables, accepted for publication in PASJ

The Japanese VLBI network (JVN) has begun observations of 6.7-GHz methanol masers associated with massive star-forming regions. The JVN is a newly-established VLBI array with baselines ranging from 50 to 2560 km spread across the Japanese islands. Three observing bands of 6.7, 8.4, and 22 GHz are now available. The array consists of ten antennas: VERA Mizusawa 20 m, VERA Ishigaki 20 m, VERA Iriki 20 m, Usuda 64 m, Yamaguchi 32 m, Tomakomai 11 m, Tsukuba 32 m, Kashima 34 m, VERA Ogasawara 20 m, and Gifu 11 m, the first five of which have 6.7-GHz receiving systems. In summer 2005, we obtained the first fringes at 6.7 GHz, and VLBI images of 12 methanol maser sites including seven that had not previously been imaged with VLBI at this band. In 2006 summer, we obtained phase-reference observations toward several methanol maser sites.

We are using a Japanese VLBI network (JVN) for VLBI observations of 6.7-GHz methanol masers associated with massive star-forming regions. Here we present results for Cepheus A (Cep A) from observations taken on September 9, 2006. The distribution of the maser spots indicates either a spherical bubble or a disk having an inclination of several tens degree. We construct a disk model with an inclination of ~70° and a radius of ~700 AU. From a luminosity of 1.7 × 104L⊙ for a source observed in this region, the excitation of the maser is probably radiative.