MASAKI EJIRI, MASAKI OKADA, and SHOICHI OKANO, National Institute of Polar Research, Tokyo, Japan
Since 1957, when Amundsen-Scott South Pole Station was opened as one of the key stations for the International Geophysical Year (IGY), the National Institute of Polar Research (NIPR) has processed auroral image data from South Pole Station; data have been provided to NIPR by the University of Alaska and Utah State University (NIPR 1995). NIPR has acted as the World Data Center (WDC) C2 for aurora since 1980.
Until now, image gathering has been done using an all-sky-camera and 35-millimeter (mm) film. We have developed an all-sky optical imager (ASI), which was installed at South Pole Station during the 1996-1997 austral summer season. ASI is a highly sensitive (monochromatic and panchromatic) optical imager with high spatial and time resolutions. ASI is a digital CCD (charge coupled device) imager. Its digital image data can be stored and easily archived in a computer, so that users can access the data through a computer network. The ASI instrument at South Pole Station itself is being monitored and controlled via satellite by an NIPR computer in Japan.
The South Pole is a unique place for an auroral observation during austral winter season. We can observe
It remains an open question how the polar cap aurora is causally connected to the night-side high-latitude aurora, although various studies have been done since the IGY in 1957. Furthermore, the South Pole is a singular point of the Earth's rotation, so its location also provides us with a unique opportunity to observe the airglow and, thus, to study effective multiwavelength (i.e., at different altitudes) characteristics of acoustic gravity waves at the polar region. With a recent improvement in the CCD imager sensitivity, the optical observations of aurora have been noticeably developed. These kinds of observational investigations cannot be performed in the Northern Hemisphere because there is no observational site on land. It is evident that the ground-based optical imagings of aurora/airglow can distinguish temporal and spatial changes of the phenomena; these two types of changes cannot be separated in the in situ observations by the rockets and satellites. To investigate the physical causalities of these geophysical phenomena, however, the data analysis of ASI at South Pole Station will be closely coordinated with the satellite experiments. High-frequency radars at Halley Bay, Sanae, and Syowa Station also give us the velocity vector of ionospheric plasma over the South Pole. Automatic geophysical observatories (AGOs), developed in the polar cap region of Antarctica, will be coordinated with ASI data gathered at South Pole Station. NIPR has installed an all-sky camera at Chinese Zhongshan Station, Antarctica, located on the polar cap. These international collaborations will contribute to greater understanding of the magnetosphere, ionosphere, and upper/middle atmosphere physics.
The ASI is equipped with interference filters for auroral emissions of N2+ (nitrogen molecule ion first negative band) 427.8 nanometers (nm), OI (oxygen green line) 557.7 nm, and OI (oxygen red line) 630.0 nm. An OH (730 nm) filter was also assembled, and a panchromatic image can be obtained without the filter. An objective lens is a Fisheye Nikkon with F 1.4 and f=6 mm. The image sensor is a back-illuminated air-cooled CCD camera with 512 x 512 pixels. The time required to read out a full frame is only 1.2 seconds. The field test of the ASI was carried out at the Zao Observatory of Tohoku University on the night of 10 October 1996. The OI 557.7 nm airflow emissions were imaged every 2 minutes at an exposure time of 60 seconds. On that night, the ASI could detect a moving wave structure on the OI 557.7-nm images with a spatial extent of as small as about 4 kilometers, in addition to large-scale structures of a few tens of kilometer. This detection level is a new finding of the atmospheric gravity-wave phenomena.
After this field test, the ASI was sent to the South Pole. S. Okano and M. Okada had been at the Amundsen-Scott South Pole Station in November 1996. They installed the ASI with the glass radome on the highest roof of the Sky Tower.
With help from Chris Cleavelin of Antarctic Support Associates, who took care of our instrument at South Pole Station, we started the operation (initial performance tests) of the ASI in April 1997. We found some malfunctions of the workstation/digital linear tape drive. After we fixed this problem, normal observations have been carried out from June, and all operations of the ASI were shut down on 2 September 1997. The special campaign in cooperation with high-frequency radar network (called SuperDARN) were successfully carried out 16-20 June, 15-19 July, and 1-5 and 14-17 August 1997.
The initial scientific results have been reported at the Sapporo Meeting in October 1997 and also at the American Geophysical Union Fall Meeting in December 1997. Sample data are already available on the Web site ( http://www.nipr.ac.jp/~asi-dp ), which will be updated after receiving all the data (digital linear tapes).
The authors would like to express their thanks to F.T. Berkey and M.J. Taylor for their discussions and advice. Special thanks are also due to Chris Cleavelin, Antarctic Support Associates, who operated our ASI at South Pole. This project is carried out under cooperation with the Office of Polar Programs of the U.S. National Science Foundation and NIPR, the Grant-in-Aid for Scientific Research (No. 0904115) of Monbusho (the Ministry of Education, Science, Sports and Culture, Japan).
NIPR. 1995 (February). Data catalogue (No. 5). Tokyo: National Institute of Polar Research.